go/types, types2: alias type declarations may refer to constraint types

Fixes #51616.

Change-Id: I388a6d91d9bfe5410b5eb32e1606257ec668d618
Reviewed-on: https://go-review.googlesource.com/c/go/+/392715
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

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

@@ -502,7 +502,7 @@ func (check *Checker) typeDecl(obj *TypeName, tdecl *syntax.TypeDecl, def *Named
 		}
 
 		check.brokenAlias(obj)
-		rhs = check.varType(tdecl.Type)
+		rhs = check.typ(tdecl.Type)
 		check.validAlias(obj, rhs)
 		return
 	}

src/cmd/compile/internal/types2/testdata/fixedbugs/issue51616.go2

@@ -0,0 +1,19 @@
+// Copyright 2022 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.
+
+package p
+
+type (
+        C[T any] interface{~int; M() T}
+
+        _ C[bool]
+        _ comparable
+        _ interface {~[]byte | ~string}
+
+        // Alias type declarations may refer to "constraint" types
+        // like ordinary type declarations.
+        _ = C[bool]
+        _ = comparable
+        _ = interface {~[]byte | ~string}
+)

src/go/types/decl.go

@@ -559,7 +559,7 @@ func (check *Checker) typeDecl(obj *TypeName, tdecl *ast.TypeSpec, def *Named) {
 		}
 
 		check.brokenAlias(obj)
-		rhs = check.varType(tdecl.Type)
+		rhs = check.typ(tdecl.Type)
 		check.validAlias(obj, rhs)
 		return
 	}

src/go/types/testdata/fixedbugs/issue51616.go2

@@ -0,0 +1,19 @@
+// Copyright 2022 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.
+
+package p
+
+type (
+        C[T any] interface{~int; M() T}
+
+        _ C[bool]
+        _ comparable
+        _ interface {~[]byte | ~string}
+
+        // Alias type declarations may refer to "constraint" types
+        // like ordinary type declarations.
+        _ = C[bool]
+        _ = comparable
+        _ = interface {~[]byte | ~string}
+)