[dev.typeparams] go/types: disallow "free" type parameter as RHS of a type declaration

This is a port of CL 332411 to go/types. methodset_test.go is similarly
updated.

Change-Id: I332b1837a954acc9d3b7e0e2ad2bec3425f088f7
Reviewed-on: https://go-review.googlesource.com/c/go/+/335109
Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Robert Findley <rfindley@google.com>

src/go/types/decl.go

@@ -680,18 +680,18 @@ func (check *Checker) typeDecl(obj *TypeName, tdecl *ast.TypeSpec, def *Named) {
 		alias = false
 	}
 
+	// alias declaration
 	if alias {
-		// type alias declaration
 		if !check.allowVersion(check.pkg, 1, 9) {
 			check.errorf(atPos(tdecl.Assign), _BadDecl, "type aliases requires go1.9 or later")
 		}
 
 		obj.typ = Typ[Invalid]
 		obj.typ = check.anyType(tdecl.Type)
+		return
+	}
 
-	} else {
-		// defined type declaration
-
+	// type definition or generic type declaration
 	named := check.newNamed(obj, nil, nil, nil, nil)
 	def.setUnderlying(named)
 
@@ -720,8 +720,12 @@ func (check *Checker) typeDecl(obj *TypeName, tdecl *ast.TypeSpec, def *Named) {
 	// TODO(gri) Investigate if we can just use named.fromRHS here
 	//           and rely on lazy computation of the underlying type.
 	named.underlying = under(named)
-	}
 
+	// If the RHS is a type parameter, it must be from this type declaration.
+	if tpar, _ := named.underlying.(*TypeParam); tpar != nil && tparamIndex(named.tparams, tpar) < 0 {
+		check.errorf(tdecl.Type, _Todo, "cannot use function type parameter %s as RHS in type declaration", tpar)
+		named.underlying = Typ[Invalid]
+	}
 }
 
 func (check *Checker) collectTypeParams(list *ast.FieldList) []*TypeName {

src/go/types/methodset_test.go

@@ -50,8 +50,9 @@ func TestNewMethodSet(t *testing.T) {
 		"type C interface{ f() }; func g[T C]() { var a T; _ = a }":         {{"f", []int{0}, true}},
 		"type C interface{ f() }; func g[T C]() { var a struct{T}; _ = a }": {{"f", []int{0, 0}, true}},
 
-		// Issue #45639.
-		"type C interface{ f() }; func g[T C]() { type Y T; var a Y; _ = a }": {},
+		// Issue #45639: We don't allow this anymore. Keep this code in case we
+		//               decide to revisit this decision.
+		// "type C interface{ f() }; func g[T C]() { type Y T; var a Y; _ = a }": {},
 	}
 
 	check := func(src string, methods []method, generic bool) {

src/go/types/testdata/examples/types.go2

@@ -161,30 +161,40 @@ type _ struct {
 	* /* ERROR List redeclared */ List[int]
 }
 
+// Issue #45639: We don't allow this anymore. Keep this code
+//               in case we decide to revisit this decision.
+//
 // It's possible to declare local types whose underlying types
 // are type parameters. As with ordinary type definitions, the
 // types underlying properties are "inherited" but the methods
 // are not.
-func _[T interface{ m(); ~int }]() {
-	type L T
-	var x L
+//func _[T interface{ m(); ~int }]() {
+//	type L T
+//	var x L
+//
+//	// m is not defined on L (it is not "inherited" from
+//	// its underlying type).
+//	x.m /* ERROR x.m undefined */ ()
+//
+//	// But the properties of T, such that as that it supports
+//	// the operations of the types given by its type bound,
+//	// are also the properties of L.
+//	x++
+//	_ = x - x
+//
+//	// On the other hand, if we define a local alias for T,
+//	// that alias stands for T as expected.
+//	type A = T
+//	var y A
+//	y.m()
+//	_ = y < 0
+//}
 
-	// m is not defined on L (it is not "inherited" from
-	// its underlying type).
-	x.m /* ERROR x.m undefined */ ()
-
-	// But the properties of T, such that as that it supports
-	// the operations of the types given by its type bound,
-	// are also the properties of L.
-	x++
-	_ = x - x
-
-	// On the other hand, if we define a local alias for T,
-	// that alias stands for T as expected.
-	type A = T
-	var y A
-	y.m()
-	_ = y < 0
+// It is not permitted to declare a local type whose underlying
+// type is a type parameters not declared by that type declaration.
+func _[T any]() {
+	type _ T         // ERROR cannot use function type parameter T as RHS in type declaration
+	type _ [_ any] T // ERROR cannot use function type parameter T as RHS in type declaration
 }
 
 // As a special case, an explicit type argument may be omitted

src/go/types/testdata/fixedbugs/issue45639.go2

@@ -0,0 +1,12 @@
+// Copyright 2021 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
+
+// It is not permitted to declare a local type whose underlying
+// type is a type parameters not declared by that type declaration.
+func _[T any]() {
+	type _ T         // ERROR cannot use function type parameter T as RHS in type declaration
+	type _ [_ any] T // ERROR cannot use function type parameter T as RHS in type declaration
+}

src/go/types/unify.go

@@ -147,10 +147,17 @@ func (u *unifier) join(i, j int) bool {
 // If typ is a type parameter of d, index returns the type parameter index.
 // Otherwise, the result is < 0.
 func (d *tparamsList) index(typ Type) int {
-	if t, ok := typ.(*TypeParam); ok {
-		if i := t.index; i < len(d.tparams) && d.tparams[i].typ == t {
-			return i
+	if tpar, ok := typ.(*TypeParam); ok {
+		return tparamIndex(d.tparams, tpar)
 	}
+	return -1
+}
+
+// If tpar is a type parameter in list, tparamIndex returns the type parameter index.
+// Otherwise, the result is < 0. tpar must not be nil.
+func tparamIndex(list []*TypeName, tpar *TypeParam) int {
+	if i := tpar.index; i < len(list) && list[i].typ == tpar {
+		return i
 	}
 	return -1
 }