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/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