[release-branch.go1.22] go/types, types2: ensure that Alias.actual is set in NewAlias

Types returned by the go/types API must be immutable (or at least
concurrency safe), but NewAlias returned an alias without actual set.

Ensure that actual is set by unaliasing. Also make some superficial
simplifications to unalias, and avoid indirection where unnecessary.

Fixes golang/go#65728

Change-Id: Ic9a020da5accf9032056a924b65c9e9e08cb2e0a
Reviewed-on: https://go-review.googlesource.com/c/go/+/560915
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Robert Griesemer <gri@google.com>
(cherry picked from commit 10a65649a3b2e34ffe8e4202bfa3df851cea0fb4)
Reviewed-on: https://go-review.googlesource.com/c/go/+/564356

4 files changed, 54 insertions, 32 deletions

diff --git a/src/cmd/compile/internal/types2/alias.go b/src/cmd/compile/internal/types2/alias.go
index 2cc57721f9..06dfba1697 100644
--- a/src/cmd/compile/internal/types2/alias.go
+++ b/src/cmd/compile/internal/types2/alias.go
@@ -21,11 +21,14 @@ type Alias struct {
 // NewAlias creates a new Alias type with the given type name and rhs.
 // rhs must not be nil.
 func NewAlias(obj *TypeName, rhs Type) *Alias {
-	return (*Checker)(nil).newAlias(obj, rhs)
+	alias := (*Checker)(nil).newAlias(obj, rhs)
+	// Ensure that alias.actual is set (#65455).
+	unalias(alias)
+	return alias
 }
 
 func (a *Alias) Obj() *TypeName   { return a.obj }
-func (a *Alias) Underlying() Type { return a.actual.Underlying() }
+func (a *Alias) Underlying() Type { return unalias(a).Underlying() }
 func (a *Alias) String() string   { return TypeString(a, nil) }
 
 // Type accessors
@@ -36,24 +39,26 @@ func (a *Alias) String() string   { return TypeString(a, nil) }
 // Consequently, the result is never an alias type.
 func Unalias(t Type) Type {
 	if a0, _ := t.(*Alias); a0 != nil {
-		if a0.actual != nil {
-			return a0.actual
-		}
-		for a := a0; ; {
-			t = a.fromRHS
-			a, _ = t.(*Alias)
-			if a == nil {
-				break
-			}
-		}
-		if t == nil {
-			panic(fmt.Sprintf("non-terminated alias %s", a0.obj.name))
-		}
-		a0.actual = t
+		return unalias(a0)
 	}
 	return t
 }
 
+func unalias(a0 *Alias) Type {
+	if a0.actual != nil {
+		return a0.actual
+	}
+	var t Type
+	for a := a0; a != nil; a, _ = t.(*Alias) {
+		t = a.fromRHS
+	}
+	if t == nil {
+		panic(fmt.Sprintf("non-terminated alias %s", a0.obj.name))
+	}
+	a0.actual = t
+	return t
+}
+
 // asNamed returns t as *Named if that is t's
 // actual type. It returns nil otherwise.
 func asNamed(t Type) *Named {
diff --git a/src/cmd/compile/internal/types2/api_test.go b/src/cmd/compile/internal/types2/api_test.go
index c70d914453..bacba71955 100644
--- a/src/cmd/compile/internal/types2/api_test.go
+++ b/src/cmd/compile/internal/types2/api_test.go
@@ -2195,6 +2195,12 @@ func TestIssue61737(t *testing.T) {
 	iface.NumMethods() // unlike go/types, there is no Complete() method, so we complete implicitly
 }
 
+func TestNewAlias_Issue65455(t *testing.T) {
+	obj := NewTypeName(nopos, nil, "A", nil)
+	alias := NewAlias(obj, Typ[Int])
+	alias.Underlying() // must not panic
+}
+
 func TestIssue15305(t *testing.T) {
 	const src = "package p; func f() int16; var _ = f(undef)"
 	f := mustParse(src)
diff --git a/src/go/types/alias.go b/src/go/types/alias.go
index 8333a4d9c9..6043c0a984 100644
--- a/src/go/types/alias.go
+++ b/src/go/types/alias.go
@@ -23,11 +23,14 @@ type Alias struct {
 // NewAlias creates a new Alias type with the given type name and rhs.
 // rhs must not be nil.
 func NewAlias(obj *TypeName, rhs Type) *Alias {
-	return (*Checker)(nil).newAlias(obj, rhs)
+	alias := (*Checker)(nil).newAlias(obj, rhs)
+	// Ensure that alias.actual is set (#65455).
+	unalias(alias)
+	return alias
 }
 
 func (a *Alias) Obj() *TypeName   { return a.obj }
-func (a *Alias) Underlying() Type { return a.actual.Underlying() }
+func (a *Alias) Underlying() Type { return unalias(a).Underlying() }
 func (a *Alias) String() string   { return TypeString(a, nil) }
 
 // Type accessors
@@ -38,24 +41,26 @@ func (a *Alias) String() string   { return TypeString(a, nil) }
 // Consequently, the result is never an alias type.
 func Unalias(t Type) Type {
 	if a0, _ := t.(*Alias); a0 != nil {
-		if a0.actual != nil {
-			return a0.actual
-		}
-		for a := a0; ; {
-			t = a.fromRHS
-			a, _ = t.(*Alias)
-			if a == nil {
-				break
-			}
-		}
-		if t == nil {
-			panic(fmt.Sprintf("non-terminated alias %s", a0.obj.name))
-		}
-		a0.actual = t
+		return unalias(a0)
 	}
 	return t
 }
 
+func unalias(a0 *Alias) Type {
+	if a0.actual != nil {
+		return a0.actual
+	}
+	var t Type
+	for a := a0; a != nil; a, _ = t.(*Alias) {
+		t = a.fromRHS
+	}
+	if t == nil {
+		panic(fmt.Sprintf("non-terminated alias %s", a0.obj.name))
+	}
+	a0.actual = t
+	return t
+}
+
 // asNamed returns t as *Named if that is t's
 // actual type. It returns nil otherwise.
 func asNamed(t Type) *Named {
diff --git a/src/go/types/api_test.go b/src/go/types/api_test.go
index 0dc5f35dff..52f0009804 100644
--- a/src/go/types/api_test.go
+++ b/src/go/types/api_test.go
@@ -2196,6 +2196,12 @@ func TestIssue61737(t *testing.T) {
 	iface.Complete()
 }
 
+func TestNewAlias_Issue65455(t *testing.T) {
+	obj := NewTypeName(nopos, nil, "A", nil)
+	alias := NewAlias(obj, Typ[Int])
+	alias.Underlying() // must not panic
+}
+
 func TestIssue15305(t *testing.T) {
 	const src = "package p; func f() int16; var _ = f(undef)"
 	fset := token.NewFileSet()