seatgen writes a scalar typed [T, null] as *T in Go and T | null in TypeScript, so a declared zero survives with its type (ADR 0634; issue 1966)

Claude-Session: https://claude.ai/code/session_01NcbgfFSnn24M3GcqttoU7i

modules/mesh-build-go/buildstep/seatgen/gogen.go

@@ -292,6 +292,11 @@ func goEnum(b *bytes.Buffer, what, prefix string, t *typ) {
 }
 
 func goType(t *typ, kinds map[string]*typ, required bool) string {
+	if t.null {
+		n := *t
+		n.null = false
+		return "*" + goType(&n, kinds, true)
+	}
 	switch t.kind {
 	case "string":
 		return "string"

modules/mesh-build-go/buildstep/seatgen/layout_test.go

@@ -81,6 +81,15 @@ func TestEachSeatDefinedWholeIsAGoModuleOfItsOwn(t *testing.T) {
 		if ts := string(files["ts/seats/"+seat+"/src/parts/registry.ts"]); !strings.Contains(ts, `"pushed": string;`) || !strings.Contains(ts, `"at"?: string;`) {
 			t.Errorf("%s's TypeScript does not write a date-time as a string:\n%s", seat, ts)
 		}
+		// A scalar typed ["T","null"] is a pointer in Go and `T | null` in TypeScript, so a declared zero survives with its type.
+		for _, w := range []string{`Floor\s+\*float64\s+` + "`json:\"floor,omitempty\"`", `Limit\s+\*int64\s+` + "`json:\"limit,omitempty\"`", `Label\s+\*string\s+` + "`json:\"label,omitempty\"`", `On\s+\*bool\s+` + "`json:\"on,omitempty\"`"} {
+			if !re.MustCompile(w).MatchString(src) {
+				t.Errorf("%s does not write a nullable scalar as a pointer (%s):\n%s", seat, w, src)
+			}
+		}
+		if ts := string(files["ts/seats/"+seat+"/src/parts/registry.ts"]); !strings.Contains(ts, `"floor"?: number | null;`) || !strings.Contains(ts, `"on"?: boolean | null;`) || !strings.Contains(ts, `"label"?: string | null;`) {
+			t.Errorf("%s's TypeScript does not write a nullable scalar as T | null:\n%s", seat, ts)
+		}
 		// A string's enum is a constant per value, so a caller names the definition's value (A3t: the removal reason).
 		if !re.MustCompile(`RefReasonRetention\s+= "retention"`).MatchString(src) || !re.MustCompile(`RefReasonTagGone\s+= "tag-gone"`).MatchString(src) {
 			t.Errorf("%s does not name the part's enum values:\n%s", seat, src)

modules/mesh-build-go/buildstep/seatgen/testdata/contracts/parts/registry.json

@@ -1,3 +1,3 @@
 {"format":1,"part":"registry","verbs":[{"name":"publish","input":{"$ref":"#/defs/Ref"},"answer":{"type":"object"}}],
  "events":[{"name":"published","body":{"$ref":"#/defs/Ref"}}],
- "defs":{"Ref":{"type":"object","properties":{"digest":{"type":"string"},"at":{"type":"string","format":"date-time"},"pushed":{"type":"string","format":"date-time"},"reason":{"type":"string","enum":["tag-gone","retention"]}},"required":["digest","pushed"]}}}
+ "defs":{"Ref":{"type":"object","properties":{"digest":{"type":"string"},"at":{"type":"string","format":"date-time"},"pushed":{"type":"string","format":"date-time"},"floor":{"type":["number","null"]},"limit":{"type":["integer","null"]},"label":{"type":["string","null"]},"on":{"type":["boolean","null"]},"reason":{"type":"string","enum":["tag-gone","retention"]}},"required":["digest","pushed"]}}}

modules/mesh-build-go/buildstep/seatgen/tsgen.go

@@ -194,6 +194,11 @@ func tsTypes(b *bytes.Buffer, tp *typer) {
 }
 
 func tsType(t *typ) string {
+	if t.null {
+		n := *t
+		n.null = false
+		return tsType(&n) + " | null"
+	}
 	switch t.kind {
 	case "string":
 		if len(t.enum) > 0 {

modules/mesh-build-go/buildstep/seatgen/types.go

@@ -15,6 +15,7 @@ type typ struct {
 	ref    string // a referenced type's name
 	fields []field
 	enum   []string
+	null   bool // a scalar typed ["T","null"]: Go writes *T, TypeScript T | null, so a declared zero survives
 }
 
 type field struct {
@@ -68,6 +69,41 @@ func (tp *typer) read(where string, s map[string]any, top bool) (*typ, error) {
 		return &typ{kind: "ref", ref: n}, nil
 	}
 	t, _ := s["type"].(string)
+	if list, ok := s["type"].([]any); ok {
+		// ["T","null"] with T a scalar is a nullable T; any other list of types stays `any`.
+		var base string
+		null := false
+		for _, v := range list {
+			switch str, _ := v.(string); str {
+			case "null":
+				null = true
+			case "number", "integer", "string", "boolean":
+				if base != "" {
+					base = "?"
+				} else {
+					base = str
+				}
+			default:
+				base = "?"
+			}
+		}
+		if null && base != "" && base != "?" {
+			inner := make(map[string]any, len(s))
+			for k, v := range s {
+				inner[k] = v
+			}
+			inner["type"] = base
+			out, err := tp.read(where, inner, top)
+			if err != nil {
+				return nil, err
+			}
+			if out.kind == "time" {
+				return out, nil
+			}
+			out.null = true
+			return out, nil
+		}
+	}
 	switch t {
 	case "string":
 		out := &typ{kind: "string"}