reflect: record PkgPath of StructOf arguments

Fixes #36190
Fixes #36191

Change-Id: I1213ef37b6595af63dbe202a8ade65741caf1356
Reviewed-on: https://go-review.googlesource.com/c/go/+/212001Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/reflect/all_test.go

@@ -4853,6 +4853,9 @@ func TestStructOfExportRules(t *testing.T) {
 			if exported != test.exported {
 				t.Errorf("test-%d: got exported=%v want exported=%v", i, exported, test.exported)
 			}
+			if field.PkgPath != test.field.PkgPath {
+				t.Errorf("test-%d: got PkgPath=%q want pkgPath=%q", i, field.PkgPath, test.field.PkgPath)
+			}
 		})
 	}
 }
@@ -5308,6 +5311,24 @@ func TestStructOfTooManyFields(t *testing.T) {
 	}
 }
 
+func TestStructOfDifferentPkgPath(t *testing.T) {
+	fields := []StructField{
+		{
+			Name:    "f1",
+			PkgPath: "p1",
+			Type:    TypeOf(int(0)),
+		},
+		{
+			Name:    "f2",
+			PkgPath: "p2",
+			Type:    TypeOf(int(0)),
+		},
+	}
+	shouldPanic(func() {
+		StructOf(fields)
+	})
+}
+
 func TestChanOf(t *testing.T) {
 	// check construction and use of type not in binary
 	type T string

src/reflect/type.go

@@ -2371,6 +2371,7 @@ func StructOf(fields []StructField) Type {
 
 	lastzero := uintptr(0)
 	repr = append(repr, "struct {"...)
+	pkgpath := ""
 	for i, field := range fields {
 		if field.Name == "" {
 			panic("reflect.StructOf: field " + strconv.Itoa(i) + " has no name")
@@ -2381,11 +2382,18 @@ func StructOf(fields []StructField) Type {
 		if field.Type == nil {
 			panic("reflect.StructOf: field " + strconv.Itoa(i) + " has no type")
 		}
-		f := runtimeStructField(field)
+		f, fpkgpath := runtimeStructField(field)
 		ft := f.typ
 		if ft.kind&kindGCProg != 0 {
 			hasGCProg = true
 		}
+		if fpkgpath != "" {
+			if pkgpath == "" {
+				pkgpath = fpkgpath
+			} else if pkgpath != fpkgpath {
+				panic("reflect.Struct: fields with different PkgPath " + pkgpath + " and " + fpkgpath)
+			}
+		}
 
 		// Update string and hash
 		name := f.name.name()
@@ -2617,6 +2625,9 @@ func StructOf(fields []StructField) Type {
 	prototype := *(**structType)(unsafe.Pointer(&istruct))
 	*typ = *prototype
 	typ.fields = fs
+	if pkgpath != "" {
+		typ.pkgPath = newName(pkgpath, "", false)
+	}
 
 	// Look in cache.
 	if ts, ok := structLookupCache.m.Load(hash); ok {
@@ -2741,7 +2752,10 @@ func StructOf(fields []StructField) Type {
 	return addToCache(&typ.rtype)
 }
 
-func runtimeStructField(field StructField) structField {
+// runtimeStructField takes a StructField value passed to StructOf and
+// returns both the corresponding internal representation, of type
+// structField, and the pkgpath value to use for this field.
+func runtimeStructField(field StructField) (structField, string) {
 	if field.Anonymous && field.PkgPath != "" {
 		panic("reflect.StructOf: field \"" + field.Name + "\" is anonymous but has PkgPath set")
 	}
@@ -2762,11 +2776,12 @@ func runtimeStructField(field StructField) structField {
 	}
 
 	resolveReflectType(field.Type.common()) // install in runtime
-	return structField{
+	f := structField{
 		name:        newName(field.Name, string(field.Tag), exported),
 		typ:         field.Type.common(),
 		offsetEmbed: offsetEmbed,
 	}
+	return f, field.PkgPath
 }
 
 // typeptrdata returns the length in bytes of the prefix of t