build: separate compiler and libs

compiler/internal/aliases/aliases_go121.go

@@ -0,0 +1,31 @@
+// Copyright 2024 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !go1.22
+// +build !go1.22
+
+package aliases
+
+import (
+	"go/types"
+)
+
+// Alias is a placeholder for a go/types.Alias for <=1.21.
+// It will never be created by go/types.
+type Alias struct{}
+
+func (*Alias) String() string         { panic("unreachable") }
+func (*Alias) Underlying() types.Type { panic("unreachable") }
+func (*Alias) Obj() *types.TypeName   { panic("unreachable") }
+func Rhs(alias *Alias) types.Type     { panic("unreachable") }
+
+// Unalias returns the type t for go <=1.21.
+func Unalias(t types.Type) types.Type { return t }
+
+func newAlias(name *types.TypeName, rhs types.Type) *Alias { panic("unreachable") }
+
+// Enabled reports whether [NewAlias] should create [types.Alias] types.
+//
+// Before go1.22, this function always returns false.
+func Enabled() bool { return false }