feat(genapi): derive discriminators and decode RichText alternates

The generator gated all union decoder emission behind knownDiscriminators, a
hand-curated map of 13 entries. Any sealed union absent from that map received
no decoder, so decoding must fail. Bot API v10 added RichBlock and RichText,
which were absent. Roughly 8.6 percent of Telegram updates failed with HTTP 400.

The generator now derives discriminators from the specification instead of a
fixed list. A gate check fails the build when a union resolves no discriminator.
Eleven unions gain decoders that never had one: BotCommandScope, InputMedia,
InputPaidMedia, InputPollMedia, InputPollOptionMedia, InputProfilePhoto,
InputRichBlock, InputStoryContent, PassportElementError, RichBlock and RichText.

RichText can also appear as a bare string or an array of RichText. The scraper
now extracts these alternate shapes into the intermediate representation. The
emitter adds RichTextPlain and RichTextSequence. The decoder dispatches on the
first JSON byte before it reads any discriminator.

Unions without alternates must regenerate byte for byte. The golden test proves
this.

Claude-Session: https://claude.ai/code/session_01Fu2wXqGUrfVBi5aLafKk8U

cmd/genapi/alternates_test.go

@@ -0,0 +1,144 @@
+package main
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"github.com/lukaszraczylo/go-telegram/internal/spec"
+	"github.com/stretchr/testify/require"
+)
+
+// TestUnionAlternates_NoAlternates confirms a union with no Alternates
+// (the common case — every union except RichText) produces no synthetic
+// variants.
+func TestUnionAlternates_NoAlternates(t *testing.T) {
+	td := spec.TypeDecl{Name: "ChatMember", OneOf: []string{"ChatMemberOwner"}}
+	require.Nil(t, unionAlternates(td))
+}
+
+// TestUnionAlternates_StringAndArray confirms both alternate shapes map
+// to the expected synthetic variant names, in declaration order.
+func TestUnionAlternates_StringAndArray(t *testing.T) {
+	td := spec.TypeDecl{
+		Name:  "RichText",
+		OneOf: []string{"RichTextBold"},
+		Alternates: []spec.Alternate{
+			{Shape: spec.AlternateString},
+			{Shape: spec.AlternateArray},
+		},
+	}
+	got := unionAlternates(td)
+	require.Equal(t, []alternateVariant{
+		{Name: "RichTextPlain", IsString: true},
+		{Name: "RichTextSequence", IsArray: true},
+	}, got)
+}
+
+// TestEmitTypes_UnionWithAlternates_EmitsShapeDispatcher builds a minimal
+// synthetic API with one discriminated union carrying Alternates (mirrors
+// RichText's shape without depending on the full real spec) and confirms
+// genapi emits: the two synthetic variant structs, their marker methods
+// and MarshalJSON, and a shape-dispatching Unmarshal<Union> that branches
+// on the first non-whitespace byte before falling back to the existing
+// discriminator switch.
+func TestEmitTypes_UnionWithAlternates_EmitsShapeDispatcher(t *testing.T) {
+	api := &spec.API{
+		Types: []spec.TypeDecl{
+			{
+				Name:  "Widget",
+				OneOf: []string{"WidgetBold"},
+				Alternates: []spec.Alternate{
+					{Shape: spec.AlternateString},
+					{Shape: spec.AlternateArray},
+				},
+			},
+			{
+				Name: "WidgetBold",
+				Fields: []spec.Field{
+					{Name: "Type", JSONName: "type", Required: true,
+						Type: spec.TypeRef{Kind: spec.KindPrimitive, Name: "string"}, EnumValues: []string{"bold"}},
+				},
+			},
+		},
+	}
+
+	tmp := t.TempDir()
+	e := newEmitter(api, tmp)
+	require.NoError(t, e.emitTypes())
+
+	got, err := os.ReadFile(filepath.Join(tmp, "types.gen.go"))
+	require.NoError(t, err)
+	src := string(got)
+
+	// Synthetic variants exist and implement the sealed interface.
+	require.Contains(t, src, "type WidgetPlain struct")
+	require.Contains(t, src, "Text string")
+	require.Contains(t, src, "func (*WidgetPlain) isWidget() {}")
+	require.Contains(t, src, "type WidgetSequence struct")
+	require.Contains(t, src, "Items []Widget")
+	require.Contains(t, src, "func (*WidgetSequence) isWidget() {}")
+
+	// Round-trip MarshalJSON for both.
+	require.Contains(t, src, "func (v *WidgetPlain) MarshalJSON() ([]byte, error) {")
+	require.Contains(t, src, "return json.Marshal(v.Text)")
+	require.Contains(t, src, "func (v *WidgetSequence) MarshalJSON() ([]byte, error) {")
+	require.Contains(t, src, "return json.Marshal(v.Items)")
+
+	// Shape-dispatching UnmarshalWidget: trims, branches on '"', '[', '{'.
+	require.Contains(t, src, "func UnmarshalWidget(data []byte) (Widget, error) {")
+	require.Contains(t, src, `trimmed := bytes.TrimLeft(data, " \t\r\n")`)
+	require.Contains(t, src, "case '\"':")
+	require.Contains(t, src, "return &WidgetPlain{Text: s}, nil")
+	require.Contains(t, src, "case '[':")
+	require.Contains(t, src, "return &WidgetSequence{Items: items}, nil")
+	require.Contains(t, src, "case '{':")
+	// The pre-existing discriminator switch is preserved beneath the
+	// shape dispatch, unrecognised discriminator still a clear error.
+	require.Contains(t, src, `fmt.Errorf("Widget: unknown type %q", probe.V)`)
+
+	// The struct-holding-the-union case inherits shape dispatch for free:
+	// WidgetBold has no union-typed field itself, so nothing further to
+	// assert there beyond successful compilation-shaped source (covered
+	// by TestEmit_Types_FixtureGolden's gofmt round trip for the real
+	// fixture, and by the api-package tests for full decode behaviour).
+	require.False(t, strings.Contains(src, "WidgetPlainSequence"), "sanity: no accidental name collision")
+}
+
+// TestEmitTypes_UnionWithoutAlternates_Unaffected confirms a discriminated
+// union with no Alternates emits the original (pre-alternates) shape:
+// no synthetic variants, no shape dispatch, and the same "decodes a X
+// from JSON by inspecting" doc comment wording as before this feature.
+func TestEmitTypes_UnionWithoutAlternates_Unaffected(t *testing.T) {
+	api := &spec.API{
+		Types: []spec.TypeDecl{
+			{Name: "Widget", OneOf: []string{"WidgetBold"}},
+			{
+				Name: "WidgetBold",
+				Fields: []spec.Field{
+					{Name: "Type", JSONName: "type", Required: true,
+						Type: spec.TypeRef{Kind: spec.KindPrimitive, Name: "string"}, EnumValues: []string{"bold"}},
+				},
+			},
+		},
+	}
+
+	tmp := t.TempDir()
+	e := newEmitter(api, tmp)
+	require.NoError(t, e.emitTypes())
+
+	got, err := os.ReadFile(filepath.Join(tmp, "types.gen.go"))
+	require.NoError(t, err)
+	src := string(got)
+
+	require.NotContains(t, src, "WidgetPlain")
+	require.NotContains(t, src, "WidgetSequence")
+	// The package-level `var _ = bytes.TrimLeft` import guard is always
+	// present (it keeps the "bytes" import valid even when no union in
+	// this API has alternates); what must NOT appear is the per-union
+	// shape-dispatch call site.
+	require.NotContains(t, src, "trimmed := bytes.TrimLeft(data,")
+	require.Contains(t, src, "// UnmarshalWidget decodes a Widget from JSON by inspecting the")
+	require.Contains(t, src, `// "type" field and dispatching to the correct concrete type.`)
+}

cmd/genapi/derive_discriminators_test.go

@@ -0,0 +1,148 @@
+package main
+
+import (
+	"testing"
+
+	"github.com/lukaszraczylo/go-telegram/internal/spec"
+	"github.com/stretchr/testify/require"
+)
+
+// TestDeriveDiscriminators_RichBlockAndRichText_FromRealSpec is a
+// regression test for the Bot API v10 rich_message decode failure:
+// RichBlock and RichText had no knownDiscriminators entry, so genapi
+// never emitted UnmarshalRichBlock / UnmarshalRichText and any update
+// whose message carried rich_message failed to decode. Derivation must
+// pick "type" for both and cover every variant the spec declares.
+//
+// The expected variant counts are read from the spec rather than pinned
+// as literals. Telegram adds variants in most releases - v10.3 took
+// RichBlock from 21 to 24 and RichText from 25 to 26 - so a frozen
+// number turns a routine regeneration into a spurious failure while
+// testing nothing the coverage assertion does not already cover.
+func TestDeriveDiscriminators_RichBlockAndRichText_FromRealSpec(t *testing.T) {
+	api, err := loadAPI("../../internal/spec/api.json")
+	require.NoError(t, err)
+
+	// declaredVariants returns how many concrete variants the spec lists
+	// for a union, so the assertions below track the spec instead of a
+	// hardcoded count.
+	declaredVariants := func(union string) int {
+		for i := range api.Types {
+			if api.Types[i].Name == union {
+				return len(api.Types[i].OneOf)
+			}
+		}
+		t.Fatalf("union %q not found in spec", union)
+		return 0
+	}
+
+	derived, _ := deriveDiscriminators(api)
+
+	rb, ok := derived["RichBlock"]
+	require.True(t, ok, "RichBlock must derive a discriminator")
+	require.Equal(t, "type", rb.Field)
+	require.Len(t, rb.Variants, declaredVariants("RichBlock"),
+		"every RichBlock variant declared in the spec must be covered")
+	require.Equal(t, "RichBlockParagraph", rb.Variants["paragraph"])
+	require.Equal(t, "RichBlockTable", rb.Variants["table"])
+	require.Equal(t, "RichBlockThinking", rb.Variants["thinking"])
+
+	rt, ok := derived["RichText"]
+	require.True(t, ok, "RichText must derive a discriminator")
+	require.Equal(t, "type", rt.Field)
+	require.Len(t, rt.Variants, declaredVariants("RichText"),
+		"every RichText variant declared in the spec must be covered")
+	require.Equal(t, "RichTextBold", rt.Variants["bold"])
+	require.Equal(t, "RichTextAnchorLink", rt.Variants["anchor_link"])
+	require.Equal(t, "RichTextReferenceLink", rt.Variants["reference_link"])
+}
+
+// TestMergeDiscriminators_HandCuratedOverridesDerived confirms a
+// knownDiscriminators entry always wins the merge even when derivation
+// would also have produced a (different) answer for the same union —
+// hand-curation is the escape hatch for a union the generic rule gets
+// wrong.
+func TestMergeDiscriminators_HandCuratedOverridesDerived(t *testing.T) {
+	api := &spec.API{
+		Types: []spec.TypeDecl{
+			{Name: "Widget", OneOf: []string{"WidgetA", "WidgetB"}},
+			{Name: "WidgetA", Fields: []spec.Field{
+				{Name: "Type", JSONName: "type", Required: true,
+					Type: spec.TypeRef{Kind: spec.KindPrimitive, Name: "string"}, EnumValues: []string{"a"}},
+			}},
+			{Name: "WidgetB", Fields: []spec.Field{
+				{Name: "Type", JSONName: "type", Required: true,
+					Type: spec.TypeRef{Kind: spec.KindPrimitive, Name: "string"}, EnumValues: []string{"b"}},
+			}},
+		},
+	}
+
+	derived, ungated := deriveDiscriminators(api)
+	require.Empty(t, ungated)
+	require.Equal(t, discriminatorSpec{
+		Field:    "type",
+		Variants: map[string]string{"a": "WidgetA", "b": "WidgetB"},
+	}, derived["Widget"])
+
+	// Inject a hand-curated override that disagrees with the derived
+	// spec, merge, and confirm the override wins outright.
+	override := discriminatorSpec{Field: "kind", Variants: map[string]string{"custom": "WidgetA"}}
+	knownDiscriminators["Widget"] = override
+	t.Cleanup(func() { delete(knownDiscriminators, "Widget") })
+
+	merged := mergeDiscriminators(derived)
+	require.Equal(t, override, merged["Widget"])
+}
+
+// TestCheckDiscriminatorGate_FailsOnUngatedInboundUnion confirms the
+// build gate fails generation when a union Telegram can actually send
+// has no derivable discriminator: the variants share no common
+// Type/Status/Source field at all.
+func TestCheckDiscriminatorGate_FailsOnUngatedInboundUnion(t *testing.T) {
+	api := &spec.API{
+		Types: []spec.TypeDecl{
+			{Name: "Widget", OneOf: []string{"WidgetA", "WidgetB"}},
+			{Name: "WidgetA", Fields: []spec.Field{
+				{Name: "Kind", JSONName: "kind", Required: true, Type: spec.TypeRef{Kind: spec.KindPrimitive, Name: "string"}},
+			}},
+			{Name: "WidgetB", Fields: []spec.Field{
+				{Name: "Flavor", JSONName: "flavor", Required: true, Type: spec.TypeRef{Kind: spec.KindPrimitive, Name: "string"}},
+			}},
+			// Container is a plain (non-Input) type with a field of the
+			// undiscriminated union — this is what makes Widget reachable
+			// from real inbound Telegram data.
+			{Name: "Container", Fields: []spec.Field{
+				{Name: "W", JSONName: "w", Type: spec.TypeRef{Kind: spec.KindNamed, Name: "Widget"}},
+			}},
+		},
+	}
+
+	err := checkDiscriminatorGate(api)
+	require.Error(t, err)
+	require.ErrorContains(t, err, "Widget")
+	require.ErrorContains(t, err, "WidgetA")
+	require.ErrorContains(t, err, "WidgetB")
+}
+
+// TestCheckDiscriminatorGate_SilentForInputOnlyUnion confirms the same
+// undiscriminated union does NOT fail the build when its only reference
+// is a field on an Input*-prefixed type — those are always constructed
+// and marshalled by the caller, never decoded from a Telegram response.
+func TestCheckDiscriminatorGate_SilentForInputOnlyUnion(t *testing.T) {
+	api := &spec.API{
+		Types: []spec.TypeDecl{
+			{Name: "Widget", OneOf: []string{"WidgetA", "WidgetB"}},
+			{Name: "WidgetA", Fields: []spec.Field{
+				{Name: "Kind", JSONName: "kind", Required: true, Type: spec.TypeRef{Kind: spec.KindPrimitive, Name: "string"}},
+			}},
+			{Name: "WidgetB", Fields: []spec.Field{
+				{Name: "Flavor", JSONName: "flavor", Required: true, Type: spec.TypeRef{Kind: spec.KindPrimitive, Name: "string"}},
+			}},
+			{Name: "InputContainer", Fields: []spec.Field{
+				{Name: "W", JSONName: "w", Type: spec.TypeRef{Kind: spec.KindNamed, Name: "Widget"}},
+			}},
+		},
+	}
+
+	require.NoError(t, checkDiscriminatorGate(api))
+}

cmd/genapi/emitter.go

@@ -3,6 +3,7 @@ package main
 import (
 	"bytes"
 	_ "embed"
+	"errors"
 	"fmt"
 	"github.com/goccy/go-json"
 	"go/format"
@@ -10,6 +11,7 @@ import (
 	"path/filepath"
 	"regexp"
 	"sort"
+	"strings"
 	"text/template"
 
 	"github.com/lukaszraczylo/go-telegram/internal/spec"
@@ -77,9 +79,18 @@ type discriminatorSpec struct {
 	Variants map[string]string // discriminator value → concrete Go type name
 }
 
-// knownDiscriminators maps parent union name → discriminator spec.
-// Used by the template helpers hasDiscriminator / discriminatorField /
-// discriminatorMap to emit UnmarshalXxx helpers.
+// knownDiscriminators is the hand-curated OVERRIDE table for sealed-union
+// discriminators. deriveDiscriminators computes the same kind of map
+// automatically from the IR for every union that has one (see
+// mergeDiscriminators); an entry here always wins the merge regardless of
+// what derivation would have produced. Use this table only for unions the
+// generic rule can't handle — e.g. MaybeInaccessibleMessage's integer
+// discriminator, which needs bespoke decode logic no discriminatorSpec
+// can express (Field is "" and Variants is nil; the template emits a
+// hand-coded Unmarshal via isMaybeInaccessibleMessage instead). Every
+// other entry here exists only because it predates derivation — removing
+// one and re-running `go test -run TestEmit -update ./cmd/genapi/...`
+// should reproduce it byte-for-byte from deriveDiscriminators.
 var knownDiscriminators = map[string]discriminatorSpec{
 	"ChatMember": {
 		Field: "status",
@@ -205,15 +216,24 @@ type emitter struct {
 	// template to emit per-variant MarshalJSON that hardcodes the
 	// discriminator so callers don't have to set it by hand.
 	variantDiscs map[string]variantDiscriminator
+	// discs is the merged union-level discriminator table: derivation
+	// output with knownDiscriminators layered on top as overrides (see
+	// mergeDiscriminators). Every codegen path that decides whether a
+	// union auto-decodes — and how — consults this instead of the
+	// package-level knownDiscriminators map, so a union qualifies the
+	// moment deriveDiscriminators can resolve it, hand-curated or not.
+	discs map[string]discriminatorSpec
 }
 
 func newEmitter(api *spec.API, outDir string) *emitter {
 	knownInterfaceTypes = buildUnionTypeSet(api)
+	derived, _ := deriveDiscriminators(api)
 	return &emitter{
 		api:          api,
 		outDir:       outDir,
 		enums:        planEnums(api),
 		variantDiscs: variantDiscriminators(api),
+		discs:        mergeDiscriminators(derived),
 	}
 }
 
@@ -349,6 +369,262 @@ func parseDiscriminatorDoc(doc string) string {
 	return ""
 }
 
+// discriminatorFieldCandidates returns the Go field names to try, in
+// priority order, when looking for a discriminator shared by every
+// variant of a union: the hand-curated field from knownDiscriminators
+// (resolved to its Go name via the first variant that declares it) when
+// the union already has an override entry, then "Type", "Status",
+// "Source" — the three names Telegram's docs consistently use for this
+// purpose. Shared by planUnifiedUnionEnums (enum naming, enums.go) and
+// deriveDiscriminators (decode dispatch, below) so the two never disagree
+// on where to look.
+func discriminatorFieldCandidates(unionName string, variants []*spec.TypeDecl) []string {
+	var candidates []string
+	seen := map[string]bool{}
+	add := func(name string) {
+		if name == "" || seen[name] {
+			return
+		}
+		seen[name] = true
+		candidates = append(candidates, name)
+	}
+	if ds, ok := knownDiscriminators[unionName]; ok && ds.Field != "" {
+		for _, v := range variants {
+			for _, f := range v.Fields {
+				if f.JSONName == ds.Field {
+					add(f.Name)
+					break
+				}
+			}
+		}
+	}
+	add("Type")
+	add("Status")
+	add("Source")
+	return candidates
+}
+
+// fieldByGoName returns the field named name on fields, or nil if none matches.
+func fieldByGoName(fields []spec.Field, name string) *spec.Field {
+	for i := range fields {
+		if fields[i].Name == name {
+			return &fields[i]
+		}
+	}
+	return nil
+}
+
+// discriminatorValue extracts the wire-level value a variant field
+// contributes to a discriminator: a single-entry EnumValues list (the
+// scraper's detected value), or a doc-parsed "must be X" / "always “X”"
+// literal via parseDiscriminatorDoc (the same extraction
+// extractVariantDiscriminator uses for the first-field case). Returns ""
+// when the field isn't a required string or carries no resolvable value.
+func discriminatorValue(f spec.Field) string {
+	if !f.Required || f.Type.Kind != spec.KindPrimitive || f.Type.Name != "string" {
+		return ""
+	}
+	if len(f.EnumValues) == 1 {
+		return f.EnumValues[0]
+	}
+	return parseDiscriminatorDoc(f.Doc)
+}
+
+// deriveDiscriminators inspects every union TypeDecl (len(OneOf) > 0) in
+// api and tries to pick a discriminator JSON field automatically, so the
+// hand-maintained knownDiscriminators map no longer has to name every
+// union that qualifies. A union qualifies when, for some candidate field
+// (see discriminatorFieldCandidates), EVERY variant declares that field
+// and discriminatorValue resolves a non-empty wire value for it, and
+// those values are unique across variants.
+//
+// Returns the derived map plus the names of unions that did not qualify,
+// in declaration order. Callers merge the derived map with
+// knownDiscriminators (mergeDiscriminators) and decide what to do about
+// the leftovers — rescue via a hand-curated override, or fail the build
+// if the union is reachable from real Telegram data (checkDiscriminatorGate).
+func deriveDiscriminators(api *spec.API) (map[string]discriminatorSpec, []string) {
+	typeByName := make(map[string]*spec.TypeDecl, len(api.Types))
+	for i := range api.Types {
+		typeByName[api.Types[i].Name] = &api.Types[i]
+	}
+
+	derived := make(map[string]discriminatorSpec, 32)
+	var ungated []string
+
+	for i := range api.Types {
+		u := &api.Types[i]
+		if len(u.OneOf) == 0 {
+			continue
+		}
+
+		variants := make([]*spec.TypeDecl, 0, len(u.OneOf))
+		resolved := true
+		for _, vName := range u.OneOf {
+			v, found := typeByName[vName]
+			if !found {
+				resolved = false
+				break
+			}
+			variants = append(variants, v)
+		}
+		if !resolved {
+			ungated = append(ungated, u.Name)
+			continue
+		}
+
+		ds, qualified := deriveUnionDiscriminator(u.Name, variants)
+		if !qualified {
+			ungated = append(ungated, u.Name)
+			continue
+		}
+		derived[u.Name] = ds
+	}
+	return derived, ungated
+}
+
+// deriveUnionDiscriminator tries each candidate Go field name in
+// priority order and returns the first one where every variant declares
+// that field with a resolvable, unique wire value.
+func deriveUnionDiscriminator(unionName string, variants []*spec.TypeDecl) (discriminatorSpec, bool) {
+	for _, goName := range discriminatorFieldCandidates(unionName, variants) {
+		jsonField := ""
+		values := make(map[string]string, len(variants)) // wire value -> variant Go type name
+		qualifies := true
+		for _, v := range variants {
+			f := fieldByGoName(v.Fields, goName)
+			if f == nil {
+				qualifies = false
+				break
+			}
+			value := discriminatorValue(*f)
+			if value == "" {
+				qualifies = false
+				break
+			}
+			if jsonField == "" {
+				jsonField = f.JSONName
+			} else if f.JSONName != jsonField {
+				qualifies = false
+				break
+			}
+			if _, dup := values[value]; dup {
+				qualifies = false
+				break
+			}
+			values[value] = v.Name
+		}
+		if qualifies {
+			return discriminatorSpec{Field: jsonField, Variants: values}, true
+		}
+	}
+	return discriminatorSpec{}, false
+}
+
+// mergeDiscriminators layers the hand-curated knownDiscriminators table
+// on top of derived: every entry deriveDiscriminators produces is used
+// as-is unless knownDiscriminators also names that union, in which case
+// the hand-curated entry wins outright (the whole discriminatorSpec, not
+// a per-key merge).
+func mergeDiscriminators(derived map[string]discriminatorSpec) map[string]discriminatorSpec {
+	merged := make(map[string]discriminatorSpec, len(derived)+len(knownDiscriminators))
+	for name, ds := range derived {
+		merged[name] = ds
+	}
+	for name, ds := range knownDiscriminators {
+		merged[name] = ds
+	}
+	return merged
+}
+
+// isInboundReachable reports whether Telegram can actually send data
+// typed as the named union to the bot: as a field on some type that
+// isn't itself send-only, or as a method's return value. "Input*"-named
+// unions are excluded outright regardless of where they're referenced —
+// by Telegram's own naming convention in this API, every such union is
+// something the bot constructs and marshals, never something it decodes.
+func isInboundReachable(api *spec.API, unionName string) bool {
+	if strings.HasPrefix(unionName, "Input") {
+		return false
+	}
+	for _, m := range api.Methods {
+		if typeRefIsUnion(m.Returns, api, unionName) {
+			return true
+		}
+	}
+	for _, t := range api.Types {
+		if strings.HasPrefix(t.Name, "Input") {
+			continue // send-only struct; its fields are never decoded either.
+		}
+		for _, f := range t.Fields {
+			if typeRefIsUnion(f.Type, api, unionName) {
+				return true
+			}
+		}
+	}
+	return false
+}
+
+// typeRefIsUnion reports whether tr resolves to the named union: directly,
+// as an array element, or via an inline oneOf variant-set match against
+// unionName's own OneOf list.
+func typeRefIsUnion(tr spec.TypeRef, api *spec.API, unionName string) bool {
+	switch tr.Kind {
+	case spec.KindNamed:
+		return tr.Name == unionName
+	case spec.KindArray:
+		if tr.ElemType != nil {
+			return typeRefIsUnion(*tr.ElemType, api, unionName)
+		}
+	case spec.KindOneOf:
+		for i := range api.Types {
+			if api.Types[i].Name == unionName && matchesVariants(tr.Variants, api.Types[i].OneOf...) {
+				return true
+			}
+		}
+	}
+	return false
+}
+
+// checkDiscriminatorGate fails codegen when a union Telegram can actually
+// send to the bot has no usable discriminator — neither derived nor
+// hand-curated. Such a union decodes to a bare interface field that no
+// JSON codec can ever fill (the RichBlock/RichText Bot API v10
+// regression this generalisation fixes: both had 20+ variants and no
+// knownDiscriminators entry, so genapi silently emitted no UnmarshalXxx
+// helper at all). Send-only "Input*" unions are exempt — see
+// isInboundReachable.
+func checkDiscriminatorGate(api *spec.API) error {
+	_, ungated := deriveDiscriminators(api)
+
+	typeByName := make(map[string]*spec.TypeDecl, len(api.Types))
+	for i := range api.Types {
+		typeByName[api.Types[i].Name] = &api.Types[i]
+	}
+
+	var offending []string
+	for _, name := range ungated {
+		if _, overridden := knownDiscriminators[name]; overridden {
+			continue // hand-curated rescue, e.g. MaybeInaccessibleMessage.
+		}
+		if isInboundReachable(api, name) {
+			offending = append(offending, name)
+		}
+	}
+	if len(offending) == 0 {
+		return nil
+	}
+
+	var b strings.Builder
+	fmt.Fprintf(&b, "genapi: %d inbound union(s) have no derivable or hand-curated discriminator "+
+		"and would decode as an unfillable interface field:", len(offending))
+	for _, name := range offending {
+		fmt.Fprintf(&b, "\n  - %s (variants: %s): no single field is present, required, and holds a unique non-empty wire value on every variant",
+			name, strings.Join(typeByName[name].OneOf, ", "))
+	}
+	return errors.New(b.String())
+}
+
 // knownInterfaceTypes is the full set of sealed-interface union type names
 // (both auto-decoded ones in knownDiscriminators and marker-only ones from
 // types with OneOf). Populated at emitter construction. goType and
@@ -359,7 +635,7 @@ var knownInterfaceTypes = map[string]bool{}
 
 // emitTypes renders types.gen.go.
 func (e *emitter) emitTypes() error {
-	t, err := template.New("types").Funcs(funcsWithDiscs(e.enums, e.variantDiscs)).Parse(typesTmpl)
+	t, err := template.New("types").Funcs(funcsWithDiscs(e.enums, e.variantDiscs, e.discs)).Parse(typesTmpl)
 	if err != nil {
 		return fmt.Errorf("parse types.tmpl: %w", err)
 	}
@@ -396,24 +672,28 @@ func loadAPI(path string) (*spec.API, error) {
 }
 
 // funcsWithDiscs returns the shared FuncMap with the variant
-// discriminator helpers bound to discs. types.tmpl uses
+// discriminator helpers bound to variantDiscs. types.tmpl uses
 // variantDiscFor/variantHasDisc to emit per-variant MarshalJSON that
-// hardcodes the wire discriminator value.
-func funcsWithDiscs(plan *enumPlan, discs map[string]variantDiscriminator) template.FuncMap {
-	fm := funcs(plan)
+// hardcodes the wire discriminator value. unionDiscs is the merged
+// union-level table (see mergeDiscriminators), threaded into funcs.
+func funcsWithDiscs(plan *enumPlan, variantDiscs map[string]variantDiscriminator, unionDiscs map[string]discriminatorSpec) template.FuncMap {
+	fm := funcs(plan, unionDiscs)
 	fm["variantHasDisc"] = func(name string) bool {
-		_, ok := discs[name]
+		_, ok := variantDiscs[name]
 		return ok
 	}
-	fm["variantDiscField"] = func(name string) string { return discs[name].JSONField }
-	fm["variantDiscGoField"] = func(name string) string { return discs[name].GoField }
-	fm["variantDiscValue"] = func(name string) string { return discs[name].Value }
+	fm["variantDiscField"] = func(name string) string { return variantDiscs[name].JSONField }
+	fm["variantDiscGoField"] = func(name string) string { return variantDiscs[name].GoField }
+	fm["variantDiscValue"] = func(name string) string { return variantDiscs[name].Value }
 	return fm
 }
 
 // funcs is the FuncMap shared across templates. plan is the resolved
-// enum plan; pass nil only in unit tests that don't exercise enums.
-func funcs(plan *enumPlan) template.FuncMap {
+// enum plan; pass nil only in unit tests that don't exercise enums. discs
+// is the merged union-level discriminator table (see
+// mergeDiscriminators); pass nil (or knownDiscriminators directly) in
+// unit tests that only exercise the hand-curated unions.
+func funcs(plan *enumPlan, discs map[string]discriminatorSpec) template.FuncMap {
 	return template.FuncMap{
 		"goType": goType,
 		"goField": func(parent string, f spec.Field) string {
@@ -444,20 +724,22 @@ func funcs(plan *enumPlan) template.FuncMap {
 			return plan.All()
 		},
 		"enumConstName": constName,
-		// discriminator helpers for types.tmpl
-		"hasDiscriminator": func(name string) bool { s, ok := knownDiscriminators[name]; return ok && len(s.Variants) > 0 },
+		// discriminator helpers for types.tmpl — all consult the merged
+		// table (discs), not the package-level knownDiscriminators, so a
+		// derived-only union gates exactly like a hand-curated one.
+		"hasDiscriminator": func(name string) bool { s, ok := discs[name]; return ok && len(s.Variants) > 0 },
 		"isSealedUnionReturn": func(tr spec.TypeRef) bool {
 			if tr.Kind != spec.KindNamed {
 				return false
 			}
-			s, ok := knownDiscriminators[tr.Name]
+			s, ok := discs[tr.Name]
 			return ok && len(s.Variants) > 0
 		},
 		"isSealedUnionArrayReturn": func(tr spec.TypeRef) bool {
 			if tr.Kind != spec.KindArray || tr.ElemType == nil || tr.ElemType.Kind != spec.KindNamed {
 				return false
 			}
-			s, ok := knownDiscriminators[tr.ElemType.Name]
+			s, ok := discs[tr.ElemType.Name]
 			return ok && len(s.Variants) > 0
 		},
 		"sealedUnionElemName": func(tr spec.TypeRef) string {
@@ -467,12 +749,13 @@ func funcs(plan *enumPlan) template.FuncMap {
 			return ""
 		},
 		"isMaybeInaccessibleMessage": func(name string) bool { return name == "MaybeInaccessibleMessage" },
-		"discriminatorField":         func(name string) string { return knownDiscriminators[name].Field },
-		"discriminatorMap":           func(name string) map[string]string { return knownDiscriminators[name].Variants },
+		"discriminatorField":         func(name string) string { return discs[name].Field },
+		"discriminatorMap":           func(name string) map[string]string { return discs[name].Variants },
 		// union-field helpers for per-struct UnmarshalJSON emission
-		"unionFields":   unionFieldsOf,
-		"isArrayUnion":  func(tr spec.TypeRef) bool { return hasUnionElem(tr) },
-		"unionTypeName": func(tr spec.TypeRef) string { name, _ := unionTypeFor(tr); return name },
+		"unionFields":     func(t spec.TypeDecl) []unionField { return unionFieldsOf(t, discs) },
+		"isArrayUnion":    func(tr spec.TypeRef) bool { return hasUnionElem(tr, discs) },
+		"unionTypeName":   func(tr spec.TypeRef) string { name, _ := unionTypeFor(tr, discs); return name },
+		"unionAlternates": unionAlternates,
 	}
 }
 
@@ -645,7 +928,7 @@ func returnGoElem(tr spec.TypeRef) string {
 
 // emitMethods renders methods.gen.go.
 func (e *emitter) emitMethods() error {
-	t, err := template.New("methods").Funcs(funcs(e.enums)).Parse(methodsTmpl)
+	t, err := template.New("methods").Funcs(funcs(e.enums, e.discs)).Parse(methodsTmpl)
 	if err != nil {
 		return fmt.Errorf("parse methods.tmpl: %w", err)
 	}
@@ -662,7 +945,7 @@ func (e *emitter) emitMethods() error {
 
 // emitEnums renders enums.gen.go.
 func (e *emitter) emitEnums() error {
-	t, err := template.New("enums").Funcs(funcs(e.enums)).Parse(enumsTmpl)
+	t, err := template.New("enums").Funcs(funcs(e.enums, e.discs)).Parse(enumsTmpl)
 	if err != nil {
 		return fmt.Errorf("parse enums.tmpl: %w", err)
 	}
@@ -735,11 +1018,12 @@ type unionField struct {
 }
 
 // unionFieldsOf returns the subset of t.Fields whose type is a known
-// discriminated union (directly or as array element).
-func unionFieldsOf(t spec.TypeDecl) []unionField {
+// discriminated union (directly or as array element), per discs — the
+// merged union-level discriminator table (see mergeDiscriminators).
+func unionFieldsOf(t spec.TypeDecl, discs map[string]discriminatorSpec) []unionField {
 	var out []unionField
 	for _, f := range t.Fields {
-		if u, ok := unionTypeFor(f.Type); ok {
+		if u, ok := unionTypeFor(f.Type, discs); ok {
 			out = append(out, unionField{Field: f, UnionName: u})
 		}
 	}
@@ -747,19 +1031,20 @@ func unionFieldsOf(t spec.TypeDecl) []unionField {
 }
 
 // unionTypeFor inspects a TypeRef and reports whether it (or its array
-// element) is a known discriminated union. Returns the union name and true.
-func unionTypeFor(tr spec.TypeRef) (string, bool) {
+// element) is a known discriminated union in discs. Returns the union
+// name and true.
+func unionTypeFor(tr spec.TypeRef, discs map[string]discriminatorSpec) (string, bool) {
 	switch tr.Kind {
 	case spec.KindNamed:
-		if _, ok := knownDiscriminators[tr.Name]; ok {
+		if _, ok := discs[tr.Name]; ok {
 			return tr.Name, true
 		}
 	case spec.KindArray:
 		if tr.ElemType != nil {
-			return unionTypeFor(*tr.ElemType)
+			return unionTypeFor(*tr.ElemType, discs)
 		}
 	case spec.KindOneOf:
-		if u := unionNameByVariants(tr.Variants); u != "" {
+		if u := unionNameByVariants(tr.Variants, discs); u != "" {
 			return u, true
 		}
 	}
@@ -768,8 +1053,8 @@ func unionTypeFor(tr spec.TypeRef) (string, bool) {
 
 // unionNameByVariants finds the parent union whose variant type names exactly
 // match the given variant set (order-insensitive).
-func unionNameByVariants(variants []string) string {
-	for parentName, ds := range knownDiscriminators {
+func unionNameByVariants(variants []string, discs map[string]discriminatorSpec) string {
+	for parentName, ds := range discs {
 		wanted := make([]string, 0, len(ds.Variants))
 		for _, vt := range ds.Variants {
 			wanted = append(wanted, vt)
@@ -782,14 +1067,50 @@ func unionNameByVariants(variants []string) string {
 }
 
 // hasUnionElem reports whether tr is an array whose element type is a known union.
-func hasUnionElem(tr spec.TypeRef) bool {
+func hasUnionElem(tr spec.TypeRef, discs map[string]discriminatorSpec) bool {
 	if tr.Kind != spec.KindArray || tr.ElemType == nil {
 		return false
 	}
-	_, ok := unionTypeFor(*tr.ElemType)
+	_, ok := unionTypeFor(*tr.ElemType, discs)
 	return ok
 }
 
+// alternateVariant describes one synthetic sealed-interface variant the
+// generator emits for a union's non-object wire shape (see
+// spec.Alternate): a bare-JSON-string variant, or a variant holding a
+// JSON array of the union itself. Exactly one of IsString / IsArray is
+// set. types.tmpl emits the struct, marker method, and MarshalJSON for
+// each, and Unmarshal<Union> dispatches to it by inspecting the first
+// non-whitespace byte of the wire value.
+type alternateVariant struct {
+	Name     string // synthetic Go type name, e.g. "RichTextPlain"
+	IsString bool
+	IsArray  bool
+}
+
+// unionAlternates maps a union TypeDecl's spec.Alternate entries (see
+// cmd/scrape's extractAlternates) to the synthetic Go variant
+// descriptors types.tmpl emits alongside the regular OneOf variants:
+// "<Union>Plain" for the bare-string shape, "<Union>Sequence" for the
+// array-of-<Union> shape. Returns nil for a union with no alternates,
+// which is every union except RichText as of Bot API v10 — so this is a
+// no-op for the rest of the type list.
+func unionAlternates(t spec.TypeDecl) []alternateVariant {
+	if len(t.Alternates) == 0 {
+		return nil
+	}
+	out := make([]alternateVariant, 0, len(t.Alternates))
+	for _, a := range t.Alternates {
+		switch a.Shape {
+		case spec.AlternateString:
+			out = append(out, alternateVariant{Name: t.Name + "Plain", IsString: true})
+		case spec.AlternateArray:
+			out = append(out, alternateVariant{Name: t.Name + "Sequence", IsArray: true})
+		}
+	}
+	return out
+}
+
 // matchesVariants reports whether got equals want as a set (order-insensitive).
 func matchesVariants(got []string, want ...string) bool {
 	if len(got) != len(want) {
@@ -1023,7 +1344,7 @@ func (e *emitter) emitTests() error {
 	unionTypes := buildUnionTypeSet(e.api)
 
 	// Add test-specific helpers to the shared func map.
-	fm := funcs(e.enums)
+	fm := funcs(e.enums, e.discs)
 	fm["sentinelValue"] = makeSentinelValue(unionTypes, e.enums)
 	fm["successResp"] = successResp
 

cmd/genapi/enums.go

@@ -271,32 +271,12 @@ func planUnifiedUnionEnums(api *spec.API) (map[string]enumDecl, map[string]strin
 			continue
 		}
 
-		// Build the candidate Go-name list. Priority order:
+		// Build the candidate Go-name list (shared with deriveDiscriminators
+		// in emitter.go so enum naming and decode dispatch never disagree
+		// on where to look). Priority order:
 		//  1. discriminator GoField from knownDiscriminators (resolved via JSONName);
 		//  2. "Type", "Status", "Source".
-		var candidateNames []string
-		seen := map[string]bool{}
-		add := func(name string) {
-			if name == "" || seen[name] {
-				return
-			}
-			seen[name] = true
-			candidateNames = append(candidateNames, name)
-		}
-		if ds, ok := knownDiscriminators[u.Name]; ok && ds.Field != "" {
-			// Resolve Go-name from the first variant whose field matches the JSON name.
-			for _, v := range variants {
-				for _, f := range v.Fields {
-					if f.JSONName == ds.Field {
-						add(f.Name)
-						break
-					}
-				}
-			}
-		}
-		for _, n := range []string{"Type", "Status", "Source"} {
-			add(n)
-		}
+		candidateNames := discriminatorFieldCandidates(u.Name, variants)
 
 		// Find the first candidate Go-name where every variant has a
 		// matching single-value string-enum field.

cmd/genapi/extra_test.go

@@ -280,7 +280,7 @@ func TestMultipartFieldEntry_Named_Optional(t *testing.T) {
 
 func TestUnionTypeFor_DirectNamed(t *testing.T) {
 	tr := spec.TypeRef{Kind: spec.KindNamed, Name: "ChatMember"}
-	name, ok := unionTypeFor(tr)
+	name, ok := unionTypeFor(tr, knownDiscriminators)
 	require.True(t, ok)
 	require.Equal(t, "ChatMember", name)
 }
@@ -288,26 +288,26 @@ func TestUnionTypeFor_DirectNamed(t *testing.T) {
 func TestUnionTypeFor_Array(t *testing.T) {
 	elem := spec.TypeRef{Kind: spec.KindNamed, Name: "ChatMember"}
 	tr := spec.TypeRef{Kind: spec.KindArray, ElemType: &elem}
-	name, ok := unionTypeFor(tr)
+	name, ok := unionTypeFor(tr, knownDiscriminators)
 	require.True(t, ok)
 	require.Equal(t, "ChatMember", name)
 }
 
 func TestUnionTypeFor_ArrayNilElem(t *testing.T) {
 	tr := spec.TypeRef{Kind: spec.KindArray}
-	_, ok := unionTypeFor(tr)
+	_, ok := unionTypeFor(tr, knownDiscriminators)
 	require.False(t, ok)
 }
 
 func TestUnionTypeFor_NotUnion(t *testing.T) {
 	tr := spec.TypeRef{Kind: spec.KindNamed, Name: "Message"}
-	_, ok := unionTypeFor(tr)
+	_, ok := unionTypeFor(tr, knownDiscriminators)
 	require.False(t, ok)
 }
 
 func TestUnionTypeFor_Unknown(t *testing.T) {
 	tr := spec.TypeRef{Kind: spec.Kind(99)}
-	_, ok := unionTypeFor(tr)
+	_, ok := unionTypeFor(tr, knownDiscriminators)
 	require.False(t, ok)
 }
 
@@ -321,12 +321,12 @@ func TestUnionNameByVariants_ChatMember(t *testing.T) {
 		"ChatMemberOwner", "ChatMemberAdministrator", "ChatMemberMember",
 		"ChatMemberRestricted", "ChatMemberLeft", "ChatMemberBanned",
 	}
-	name := unionNameByVariants(variants)
+	name := unionNameByVariants(variants, knownDiscriminators)
 	require.Equal(t, "ChatMember", name)
 }
 
 func TestUnionNameByVariants_Unknown(t *testing.T) {
-	name := unionNameByVariants([]string{"X", "Y", "Z"})
+	name := unionNameByVariants([]string{"X", "Y", "Z"}, knownDiscriminators)
 	require.Equal(t, "", name)
 }
 
@@ -336,24 +336,24 @@ func TestUnionNameByVariants_Unknown(t *testing.T) {
 
 func TestHasUnionElem_NonArray(t *testing.T) {
 	tr := spec.TypeRef{Kind: spec.KindNamed, Name: "ChatMember"}
-	require.False(t, hasUnionElem(tr))
+	require.False(t, hasUnionElem(tr, knownDiscriminators))
 }
 
 func TestHasUnionElem_ArrayNilElem(t *testing.T) {
 	tr := spec.TypeRef{Kind: spec.KindArray}
-	require.False(t, hasUnionElem(tr))
+	require.False(t, hasUnionElem(tr, knownDiscriminators))
 }
 
 func TestHasUnionElem_ArrayUnionElem(t *testing.T) {
 	elem := spec.TypeRef{Kind: spec.KindNamed, Name: "ChatMember"}
 	tr := spec.TypeRef{Kind: spec.KindArray, ElemType: &elem}
-	require.True(t, hasUnionElem(tr))
+	require.True(t, hasUnionElem(tr, knownDiscriminators))
 }
 
 func TestHasUnionElem_ArrayNonUnionElem(t *testing.T) {
 	elem := spec.TypeRef{Kind: spec.KindNamed, Name: "Message"}
 	tr := spec.TypeRef{Kind: spec.KindArray, ElemType: &elem}
-	require.False(t, hasUnionElem(tr))
+	require.False(t, hasUnionElem(tr, knownDiscriminators))
 }
 
 // ---------------------------------------------------------------------------
@@ -369,7 +369,7 @@ func TestUnionFieldsOf_WithUnionField(t *testing.T) {
 			{Name: "Date", JSONName: "date", Type: spec.TypeRef{Kind: spec.KindPrimitive, Name: "int64"}},
 		},
 	}
-	uf := unionFieldsOf(td)
+	uf := unionFieldsOf(td, knownDiscriminators)
 	require.Len(t, uf, 2)
 	require.Equal(t, "ChatMember", uf[0].UnionName)
 }
@@ -621,14 +621,14 @@ func TestUnionTypeFor_OneOfVariants(t *testing.T) {
 			"ChatMemberRestricted", "ChatMemberLeft", "ChatMemberBanned",
 		},
 	}
-	name, ok := unionTypeFor(tr)
+	name, ok := unionTypeFor(tr, knownDiscriminators)
 	require.True(t, ok)
 	require.Equal(t, "ChatMember", name)
 }
 
 func TestUnionTypeFor_OneOfNoMatch(t *testing.T) {
 	tr := spec.TypeRef{Kind: spec.KindOneOf, Variants: []string{"Foo", "Bar"}}
-	_, ok := unionTypeFor(tr)
+	_, ok := unionTypeFor(tr, knownDiscriminators)
 	require.False(t, ok)
 }
 
@@ -637,7 +637,7 @@ func TestUnionTypeFor_OneOfNoMatch(t *testing.T) {
 // ---------------------------------------------------------------------------
 
 func TestFuncs_HasExpectedKeys(t *testing.T) {
-	fm := funcs(nil)
+	fm := funcs(nil, nil)
 	require.NotNil(t, fm)
 	for _, key := range []string{"goType", "docComment", "returnGoType", "unionFields"} {
 		require.NotNil(t, fm[key], "funcs() missing key %q", key)

cmd/genapi/main.go

@@ -29,6 +29,9 @@ func run(input, outdir string) error {
 	if err != nil {
 		return fmt.Errorf("load api: %w", err)
 	}
+	if err := checkDiscriminatorGate(api); err != nil {
+		return err
+	}
 	if err := os.MkdirAll(outdir, 0o750); err != nil {
 		return err
 	}

cmd/genapi/types.tmpl

@@ -7,14 +7,16 @@
 package api
 
 import (
+	"bytes"
 	"github.com/goccy/go-json"
 	"fmt"
 	"io"
 )
 
-var _ = io.Discard   // keep import even if no fields use io
-var _ = json.Marshal // keep import for UnmarshalXxx helpers
-var _ = fmt.Errorf   // keep import for UnmarshalXxx helpers
+var _ = io.Discard     // keep import even if no fields use io
+var _ = json.Marshal   // keep import for UnmarshalXxx helpers
+var _ = fmt.Errorf     // keep import for UnmarshalXxx helpers
+var _ = bytes.TrimLeft // keep import for shape-dispatching UnmarshalXxx helpers (unions with alternates)
 
 {{range .Types}}
 {{- $td := . -}}
@@ -22,6 +24,7 @@ var _ = fmt.Errorf   // keep import for UnmarshalXxx helpers
 // {{.Name}} is a union type. The following concrete variants implement
 // it:
 {{range .OneOf}}//   - {{.}}
+{{end}}{{range unionAlternates .}}//   - {{.Name}} ({{if .IsString}}bare JSON string{{else}}JSON array of {{$td.Name}}{{end}})
 {{end}}//
 {{docComment .Doc -}}
 type {{.Name}} interface{ is{{.Name}}() }
@@ -30,11 +33,67 @@ type {{.Name}} interface{ is{{.Name}}() }
 // is{{$td.Name}} is the marker method that makes {{.}} implement {{$td.Name}}.
 func (*{{.}}) is{{$td.Name}}() {}
 {{end}}
+{{range unionAlternates .}}
+// {{.Name}} implements {{$td.Name}} for the {{if .IsString}}bare JSON string{{else}}JSON array of {{$td.Name}}{{end}} wire
+// shape Telegram documents for this union, alongside its object variants.
+type {{.Name}} struct {
+{{if .IsString}}	Text string
+{{else}}	Items []{{$td.Name}}
+{{end}}}
+
+// is{{$td.Name}} is the marker method that makes {{.Name}} implement {{$td.Name}}.
+func (*{{.Name}}) is{{$td.Name}}() {}
+
+{{if .IsString}}// MarshalJSON encodes {{.Name}} back to its bare JSON string wire shape.
+func (v *{{.Name}}) MarshalJSON() ([]byte, error) {
+	return json.Marshal(v.Text)
+}
+{{else}}// MarshalJSON encodes {{.Name}} back to its bare JSON array wire shape.
+func (v *{{.Name}}) MarshalJSON() ([]byte, error) {
+	return json.Marshal(v.Items)
+}
+{{end}}
+{{end}}
 {{if hasDiscriminator .Name}}
-// Unmarshal{{.Name}} decodes a {{.Name}} from JSON by inspecting the
+{{$alts := unionAlternates .}}{{if $alts}}// Unmarshal{{.Name}} decodes a {{.Name}} from JSON. Telegram documents
+// this type as either a bare JSON string, a JSON array of {{.Name}}, or
+// an object dispatched by its "{{discriminatorField .Name}}" field;
+// leading whitespace is trimmed and the first remaining byte selects
+// the shape.
+{{else}}// Unmarshal{{.Name}} decodes a {{.Name}} from JSON by inspecting the
 // "{{discriminatorField .Name}}" field and dispatching to the correct concrete type.
-func Unmarshal{{.Name}}(data []byte) ({{.Name}}, error) {
-	var probe struct {
+{{end}}func Unmarshal{{.Name}}(data []byte) ({{.Name}}, error) {
+{{if $alts}}	trimmed := bytes.TrimLeft(data, " \t\r\n")
+	if len(trimmed) == 0 {
+		return nil, fmt.Errorf("{{.Name}}: empty JSON value")
+	}
+	switch trimmed[0] {
+{{range $alts}}{{if .IsString}}	case '"':
+		var s string
+		if err := json.Unmarshal(data, &s); err != nil {
+			return nil, fmt.Errorf("{{$td.Name}}: decoding string: %w", err)
+		}
+		return &{{.Name}}{Text: s}, nil
+{{else}}	case '[':
+		var raws []json.RawMessage
+		if err := json.Unmarshal(data, &raws); err != nil {
+			return nil, fmt.Errorf("{{$td.Name}}: decoding array: %w", err)
+		}
+		items := make([]{{$td.Name}}, 0, len(raws))
+		for i, r := range raws {
+			v, err := Unmarshal{{$td.Name}}(r)
+			if err != nil {
+				return nil, fmt.Errorf("{{$td.Name}}: decoding array[%d]: %w", i, err)
+			}
+			items = append(items, v)
+		}
+		return &{{.Name}}{Items: items}, nil
+{{end}}{{end}}	case '{':
+		// fall through to the discriminator switch below.
+	default:
+		return nil, fmt.Errorf("{{.Name}}: unrecognised JSON value starting with %q", trimmed[0])
+	}
+{{end}}	var probe struct {
 		V string `json:"{{discriminatorField .Name}}"`
 	}
 	if err := json.Unmarshal(data, &probe); err != nil {