[dev.typeparams] cmd/compile/internal/types2: more consistent handling of predeclared "any"

Rather than removing "any" from the universe scope, keep it predeclared
but provide a better error message.

While at it, remove some unnecessary type assertions.

Change-Id: I10603274282ea6afc107f703ab194f32bd334dd1
Reviewed-on: https://go-review.googlesource.com/c/go/+/334911
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

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

@@ -617,8 +617,9 @@ func (check *Checker) declareTypeParam(index int, name *syntax.Name) *TypeName {
 // The type must be an interface, including the predeclared type "any".
 func (check *Checker) boundType(e syntax.Expr) Type {
 	// The predeclared identifier "any" is visible only as a type bound in a type parameter list.
-	if name, _ := unparen(e).(*syntax.Name); name != nil && name.Value == "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).(*syntax.Name); name != nil && name.Value == "any" && check.lookup("any") == universeAny {
+		return universeAny.Type()
 	}
 
 	bound := check.typ(e)

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

@@ -25,7 +25,7 @@ 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/cmd/compile/internal/types2/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/cmd/compile/internal/types2/typexpr.go

@@ -25,7 +25,8 @@ func (check *Checker) ident(x *operand, e *syntax.Name, def *Named, wantType boo
 	// 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.Value, check.pos)
-	if obj == nil || obj == universeComparable && !check.allowVersion(check.pkg, 1, 18) {
+	switch obj {
+	case nil:
 		if e.Value == "_" {
 			check.error(e, "cannot use _ as value or type")
 		} else {
@@ -36,6 +37,16 @@ func (check *Checker) ident(x *operand, e *syntax.Name, def *Named, wantType boo
 			}
 		}
 		return
+	case universeAny, universeComparable:
+		if !check.allowVersion(check.pkg, 1, 18) {
+			check.errorf(e, "undeclared name: %s (requires version go1.18 or later)", e.Value)
+			return
+		}
+		// If we allow "any" for general use, this if-statement can be removed (issue #33232).
+		if obj == universeAny {
+			check.error(e, "cannot use any outside constraint position")
+			return
+		}
 	}
 	check.recordUse(e, obj)
 

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

@@ -20,11 +20,11 @@ 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
 )
 
@@ -79,9 +79,6 @@ 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(nopos, nil, "any", &emptyInterface))
 
 	// type error interface{ Error() string }
@@ -224,15 +221,12 @@ 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

test/fixedbugs/issue14652.go

@@ -6,4 +6,4 @@
 
 package p
 
-var x any // ERROR "undefined: any|undefined type .*any.*"
+var x any // ERROR "undefined: any|undefined type .*any.*|cannot use any outside constraint position"

test/typeparam/tparam1.go

@@ -10,8 +10,8 @@ package tparam1
 
 // The predeclared identifier "any" is only visible as a constraint
 // in a type parameter list.
-var _ any // ERROR "undefined"
-func _(_ any) // ERROR "undefined"
+var _ any // ERROR "cannot use any outside constraint position"
+func _(_ any) // ERROR "cannot use any outside constraint position"
 type _[_ any /* ok here */ ] struct{}
 
 const N = 10