[dev.typeparams] go/types: more consistent handling of predeclared "any"

This is a port of CL 334911 to go/types.

Change-Id: I2cafdc76cb4d06ba82188c530f35952c1f77d292
Reviewed-on: https://go-review.googlesource.com/c/go/+/335569
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/types/decl.go

@@ -678,8 +678,9 @@ func (check *Checker) declareTypeParams(tparams []*TypeName, names []*ast.Ident)
 // The type must be an interface, including the predeclared type "any".
 func (check *Checker) boundType(e ast.Expr) Type {
 	// The predeclared identifier "any" is visible only as a type bound in a type parameter list.
-	if name, _ := unparen(e).(*ast.Ident); name != nil && name.Name == "any" && check.lookup("any") == nil {
-		return universeAny
+	// If we allow "any" for general use, this if-statement can be removed (issue #33232).
+	if name, _ := unparen(e).(*ast.Ident); name != nil && name.Name == "any" && check.lookup("any") == universeAny {
+		return universeAny.Type()
 	}
 
 	bound := check.typ(e)

src/go/types/object_test.go

@@ -22,7 +22,7 @@ func TestIsAlias(t *testing.T) {
 	check(Unsafe.Scope().Lookup("Pointer").(*TypeName), false)
 	for _, name := range Universe.Names() {
 		if obj, _ := Universe.Lookup(name).(*TypeName); obj != nil {
-			check(obj, name == "byte" || name == "rune")
+			check(obj, name == "any" || name == "byte" || name == "rune")
 		}
 	}
 

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

@@ -6,11 +6,11 @@ package p
 
 // import "io" // for type assertion tests
 
-// The predeclared identifier "any" is only visible as a constraint
+// The predeclared identifier "any" can only be used as a constraint
 // in a type parameter list.
-var _ any // ERROR undeclared
-func _[_ any /* ok here */ , _ interface{any /* ERROR undeclared */ }](any /* ERROR undeclared */ ) {
-        var _ any /* ERROR undeclared */
+var _ any // ERROR cannot use any outside constraint position
+func _[_ any /* ok here */ , _ interface{any /* ERROR constraint */ }](any /* ERROR constraint */ ) {
+        var _ any /* ERROR constraint */
 }
 
 func identity[T any](x T) T { return x }

src/go/types/typexpr.go

@@ -27,13 +27,24 @@ func (check *Checker) ident(x *operand, e *ast.Ident, def *Named, wantType bool)
 	// Note that we cannot use check.lookup here because the returned scope
 	// may be different from obj.Parent(). See also Scope.LookupParent doc.
 	scope, obj := check.scope.LookupParent(e.Name, check.pos)
-	if obj == nil || obj == universeComparable && !check.allowVersion(check.pkg, 1, 18) {
+	switch obj {
+	case nil:
 		if e.Name == "_" {
-			check.errorf(e, _InvalidBlank, "cannot use _ as value or type")
+			check.error(e, _InvalidBlank, "cannot use _ as value or type")
 		} else {
 			check.errorf(e, _UndeclaredName, "undeclared name: %s", e.Name)
 		}
 		return
+	case universeAny, universeComparable:
+		if !check.allowVersion(check.pkg, 1, 18) {
+			check.errorf(e, _UndeclaredName, "undeclared name: %s (requires version go1.18 or later)", e.Name)
+			return
+		}
+		// If we allow "any" for general use, this if-statement can be removed (issue #33232).
+		if obj == universeAny {
+			check.error(e, _Todo, "cannot use any outside constraint position")
+			return
+		}
 	}
 	check.recordUse(e, obj)
 

src/go/types/universe.go

@@ -21,11 +21,11 @@ var Universe *Scope
 var Unsafe *Package
 
 var (
-	universeIota       *Const
-	universeByte       *Basic // uint8 alias, but has name "byte"
-	universeRune       *Basic // int32 alias, but has name "rune"
-	universeAny        *Interface
-	universeError      *Named
+	universeIota       Object
+	universeByte       Type // uint8 alias, but has name "byte"
+	universeRune       Type // int32 alias, but has name "rune"
+	universeAny        Object
+	universeError      Type
 	universeComparable Object
 )
 
@@ -80,9 +80,6 @@ func defPredeclaredTypes() {
 	}
 
 	// type any = interface{}
-	// Entered into universe scope so we do all the usual checks;
-	// but removed again from scope later since it's only visible
-	// as constraint in a type parameter list.
 	def(NewTypeName(token.NoPos, nil, "any", &emptyInterface))
 
 	// type error interface{ Error() string }
@@ -225,15 +222,12 @@ func init() {
 	defPredeclaredNil()
 	defPredeclaredFuncs()
 
-	universeIota = Universe.Lookup("iota").(*Const)
-	universeByte = Universe.Lookup("byte").(*TypeName).typ.(*Basic)
-	universeRune = Universe.Lookup("rune").(*TypeName).typ.(*Basic)
-	universeAny = Universe.Lookup("any").(*TypeName).typ.(*Interface)
-	universeError = Universe.Lookup("error").(*TypeName).typ.(*Named)
+	universeIota = Universe.Lookup("iota")
+	universeByte = Universe.Lookup("byte").Type()
+	universeRune = Universe.Lookup("rune").Type()
+	universeAny = Universe.Lookup("any")
+	universeError = Universe.Lookup("error").Type()
 	universeComparable = Universe.Lookup("comparable")
-
-	// "any" is only visible as constraint in a type parameter list
-	delete(Universe.elems, "any")
 }
 
 // Objects with names containing blanks are internal and not entered into