go/types: disallow aliases for generic types

This is a port of CL 346294 to go/types.

Change-Id: Ib70541a92e352c8df8123c8b82bb4eeedce3b89f
Reviewed-on: https://go-review.googlesource.com/c/go/+/346560
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>

src/go/types/decl.go

@@ -603,7 +603,7 @@ func (check *Checker) typeDecl(obj *TypeName, tdecl *ast.TypeSpec, def *Named) {
 		}
 
 		obj.typ = Typ[Invalid]
-		rhs = check.anyType(tdecl.Type)
+		rhs = check.varType(tdecl.Type)
 		obj.typ = rhs
 		return
 	}

src/go/types/testdata/check/typeinst.go2

@@ -17,13 +17,15 @@ type T2[P any] struct {
 
 type List[P any] []P
 
-// Alias type declarations cannot have type parameters. Syntax error.
+// Alias type declarations cannot have type parameters.
+// Issue #46477 proposses to change that.
 type A1[P any] = /* ERROR cannot be alias */ P
 
-// But an alias may refer to a generic, uninstantiated type.
+// Pending clarification of #46477 we disallow aliases
-type A2 = List
+// of generic types.
+type A2 = List // ERROR cannot use generic type
 var _ A2[int]
-var _ A2 /* ERROR without instantiation */
+var _ A2
 
 type A3 = List[int]
 var _ A3

src/go/types/testdata/fixedbugs/issue39768.go2

@@ -5,9 +5,9 @@
 package p
 
 type T[P any] P
-type A = T
+type A = T  // ERROR cannot use generic type
 var x A[int]
-var _ A /* ERROR cannot use generic type */
+var _ A
 
 type B = T[int]
 var y B = x
@@ -16,5 +16,5 @@ var _ B /* ERROR not a generic type */ [int]
 // test case from issue
 
 type Vector[T any] []T
-type VectorAlias = Vector
+type VectorAlias = Vector // ERROR cannot use generic type
 var v Vector[int]

src/go/types/testdata/fixedbugs/issue47968.go2

@@ -8,7 +8,7 @@ type T[P any] struct{}
 
 func (T[P]) m1()
 
-type A1 = T
+type A1 = T // ERROR cannot use generic type
 
 func (A1[P]) m2() {}
 

src/go/types/typexpr.go

@@ -36,12 +36,15 @@ func (check *Checker) ident(x *operand, e *ast.Ident, def *Named, wantType bool)
 		}
 		return
 	case universeAny, universeComparable:
-		// complain if necessary but keep going
+		// complain if necessary
 		if !check.allowVersion(check.pkg, 1, 18) {
-			check.softErrorf(e, _UndeclaredName, "undeclared name: %s (requires version go1.18 or later)", e.Name)
+			check.errorf(e, _UndeclaredName, "undeclared name: %s (requires version go1.18 or later)", e.Name)
-		} else if obj == universeAny {
+			return // avoid follow-on errors
+		}
+		if obj == universeAny {
 			// If we allow "any" for general use, this if-statement can be removed (issue #33232).
 			check.softErrorf(e, _Todo, "cannot use any outside constraint position")
+			// ok to continue
 		}
 	}
 	check.recordUse(e, obj)
@@ -155,15 +158,6 @@ func (check *Checker) varType(e ast.Expr) Type {
 	return typ
 }
 
-// anyType type-checks the type expression e and returns its type, or Typ[Invalid].
-// The type may be generic or instantiated.
-func (check *Checker) anyType(e ast.Expr) Type {
-	typ := check.typInternal(e, nil)
-	assert(isTyped(typ))
-	check.recordTypeAndValue(e, typexpr, typ, nil)
-	return typ
-}
-
 // definedType is like typ but also accepts a type name def.
 // If def != nil, e is the type specification for the defined type def, declared
 // in a type declaration, and def.underlying will be set to the type of e before