go/types, types2: don't implicitly modify an argument function's type

See the comment in the (very small) fix for a detailed description.
Use the opportunity to introduce a generic clone function which may
be useful elsewhere.

Fixes #63260.

Change-Id: Ic63c6b8bc443011b1a201908254f7d062e1aec71
Reviewed-on: https://go-review.googlesource.com/c/go/+/532157
Run-TryBot: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/cmd/compile/internal/types2/call.go

@@ -569,6 +569,13 @@ func (check *Checker) arguments(call *syntax.CallExpr, sig *Signature, targs []T
 		for i, arg := range args {
 			// generic arguments cannot have a defined (*Named) type - no need for underlying type below
 			if asig, _ := arg.typ.(*Signature); asig != nil && asig.TypeParams().Len() > 0 {
+				// The argument type is a generic function signature. This type is
+				// pointer-identical with (it's copied from) the type of the generic
+				// function argument and thus the function object.
+				// Before we change the type (type parameter renaming, below), make
+				// a clone of it as otherwise we implicitly modify the object's type
+				// (go.dev/issues/63260).
+				asig = clone(asig)
 				// Rename type parameters for cases like f(g, g); this gives each
 				// generic function argument a unique type identity (go.dev/issues/59956).
 				// TODO(gri) Consider only doing this if a function argument appears

src/cmd/compile/internal/types2/issues_test.go

@@ -936,3 +936,47 @@ func _()        { f() }
 	conf.Error = func(error) {}
 	typecheck(src, &conf, nil) // must not panic
 }
+
+func TestIssue63260(t *testing.T) {
+	const src = `
+package p
+
+func _() {
+        use(f[*string])
+}
+
+func use(func()) {}
+
+func f[I *T, T any]() {
+        var v T
+        _ = v
+}`
+
+	info := Info{
+		Defs: make(map[*syntax.Name]Object),
+	}
+	pkg := mustTypecheck(src, nil, &info)
+
+	// get type parameter T in signature of f
+	T := pkg.Scope().Lookup("f").Type().(*Signature).TypeParams().At(1)
+	if T.Obj().Name() != "T" {
+		t.Fatalf("got type parameter %s, want T", T)
+	}
+
+	// get type of variable v in body of f
+	var v Object
+	for name, obj := range info.Defs {
+		if name.Value == "v" {
+			v = obj
+			break
+		}
+	}
+	if v == nil {
+		t.Fatal("variable v not found")
+	}
+
+	// type of v and T must be pointer-identical
+	if v.Type() != T {
+		t.Fatalf("types of v and T are not pointer-identical: %p != %p", v.Type().(*TypeParam), T)
+	}
+}

src/cmd/compile/internal/types2/predicates.go

@@ -533,3 +533,9 @@ func maxType(x, y Type) Type {
 	}
 	return nil
 }
+
+// clone makes a "flat copy" of *p and returns a pointer to the copy.
+func clone[P *T, T any](p P) P {
+	c := *p
+	return &c
+}

src/go/types/call.go

@@ -571,6 +571,13 @@ func (check *Checker) arguments(call *ast.CallExpr, sig *Signature, targs []Type
 		for i, arg := range args {
 			// generic arguments cannot have a defined (*Named) type - no need for underlying type below
 			if asig, _ := arg.typ.(*Signature); asig != nil && asig.TypeParams().Len() > 0 {
+				// The argument type is a generic function signature. This type is
+				// pointer-identical with (it's copied from) the type of the generic
+				// function argument and thus the function object.
+				// Before we change the type (type parameter renaming, below), make
+				// a clone of it as otherwise we implicitly modify the object's type
+				// (go.dev/issues/63260).
+				asig = clone(asig)
 				// Rename type parameters for cases like f(g, g); this gives each
 				// generic function argument a unique type identity (go.dev/issues/59956).
 				// TODO(gri) Consider only doing this if a function argument appears

src/go/types/issues_test.go

@@ -946,3 +946,47 @@ func _()        { f() }
 	conf.Error = func(error) {}
 	typecheck(src, &conf, nil) // must not panic
 }
+
+func TestIssue63260(t *testing.T) {
+	const src = `
+package p
+
+func _() {
+        use(f[*string])
+}
+
+func use(func()) {}
+
+func f[I *T, T any]() {
+        var v T
+        _ = v
+}`
+
+	info := Info{
+		Defs: make(map[*ast.Ident]Object),
+	}
+	pkg := mustTypecheck(src, nil, &info)
+
+	// get type parameter T in signature of f
+	T := pkg.Scope().Lookup("f").Type().(*Signature).TypeParams().At(1)
+	if T.Obj().Name() != "T" {
+		t.Fatalf("got type parameter %s, want T", T)
+	}
+
+	// get type of variable v in body of f
+	var v Object
+	for name, obj := range info.Defs {
+		if name.Name == "v" {
+			v = obj
+			break
+		}
+	}
+	if v == nil {
+		t.Fatal("variable v not found")
+	}
+
+	// type of v and T must be pointer-identical
+	if v.Type() != T {
+		t.Fatalf("types of v and T are not pointer-identical: %p != %p", v.Type().(*TypeParam), T)
+	}
+}

src/go/types/predicates.go

@@ -535,3 +535,9 @@ func maxType(x, y Type) Type {
 	}
 	return nil
 }
+
+// clone makes a "flat copy" of *p and returns a pointer to the copy.
+func clone[P *T, T any](p P) P {
+	c := *p
+	return &c
+}