cmd/compile/internal/types2: disallow aliases for generic types

The existing approach (alias name stands for generic type name)
is an exception: it's the only place where a generic type could
be used without explicit instantiation. The correct solution is
currently under discussion (see proposal issue #46477).

This CL requires that the RHS of an alias type declaration be
an instantiated non-generic type. If #46477 is accepted, the
implementation will require proper representation of alias
types.

Change-Id: Ie85b923213a64f39837e56e38e14757458272b93
Reviewed-on: https://go-review.googlesource.com/c/go/+/346294
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

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

@@ -555,7 +555,7 @@ func (check *Checker) typeDecl(obj *TypeName, tdecl *syntax.TypeDecl, def *Named
 		}
 
 		obj.typ = Typ[Invalid]
-		rhs = check.anyType(tdecl.Type)
+		rhs = check.varType(tdecl.Type)
 		obj.typ = rhs
 		return
 	}

src/cmd/compile/internal/types2/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.
-type A2 = List
+// Pending clarification of #46477 we disallow aliases
+// 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/cmd/compile/internal/types2/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/cmd/compile/internal/types2/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/cmd/compile/internal/types2/typexpr.go

@@ -38,12 +38,15 @@ func (check *Checker) ident(x *operand, e *syntax.Name, def *Named, wantType boo
 		}
 		return
 	case universeAny, universeComparable:
-		// complain if necessary but keep going
+		// complain if necessary
 		if !check.allowVersion(check.pkg, 1, 18) {
-			check.softErrorf(e, "undeclared name: %s (requires version go1.18 or later)", e.Value)
-		} else if obj == universeAny {
+			check.errorf(e, "undeclared name: %s (requires version go1.18 or later)", e.Value)
+			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, "cannot use any outside constraint position")
+			// ok to continue
 		}
 	}
 	check.recordUse(e, obj)
@@ -159,15 +162,6 @@ func (check *Checker) varType(e syntax.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 syntax.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

test/typeparam/aliasimp.dir/main.go

@@ -10,16 +10,18 @@ type R[T any] struct {
 	F T
 }
 
-type S = R
+// type S = R // disallowed for now
 
 type Sint = R[int]
 
-type Simp = a.Rimp
+// type Simp = a.Rimp // disallowed for now
 
-type SimpString Simp[string]
+// type SimpString Simp[string] // disallowed for now
+type SimpString a.Rimp[string]
 
 func main() {
-	var s S[int]
+	// var s S[int] // disallowed for now
+	var s R[int]
 	if s.F != 0 {
 		panic(s.F)
 	}
@@ -27,7 +29,8 @@ func main() {
 	if s2.F != 0 {
 		panic(s2.F)
 	}
-	var s3 Simp[string]
+	// var s3 Simp[string] // disallowed for now
+	var s3 a.Rimp[string]
 	if s3.F != "" {
 		panic(s3.F)
 	}