[dev.typeparams] go/types: import universe changes from dev.go2go

This is unmodified from the dev.go2go branch, except to remove the aType
embedding.

Change-Id: Id14d97a56d27ea609c43b453e8f9f1cf1a451406
Reviewed-on: https://go-review.googlesource.com/c/go/+/276252
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/universe.go

@@ -24,6 +24,7 @@ var (
 	universeIota  *Const
 	universeByte  *Basic // uint8 alias, but has name "byte"
 	universeRune  *Basic // int32 alias, but has name "rune"
+	universeAny   *Named
 	universeError *Named
 )
 
@@ -77,13 +78,24 @@ func defPredeclaredTypes() {
 		def(NewTypeName(token.NoPos, nil, t.name, t))
 	}
 
+	// any
+	// (Predeclared and 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.)
+	{
+		typ := &Named{underlying: &emptyInterface}
+		def(NewTypeName(token.NoPos, nil, "any", typ))
+	}
+
 	// Error has a nil package in its qualified name since it is in no package
-	res := NewVar(token.NoPos, nil, "", Typ[String])
-	sig := &Signature{results: NewTuple(res)}
-	err := NewFunc(token.NoPos, nil, "Error", sig)
-	typ := &Named{underlying: NewInterfaceType([]*Func{err}, nil).Complete()}
-	sig.recv = NewVar(token.NoPos, nil, "", typ)
-	def(NewTypeName(token.NoPos, nil, "error", typ))
+	{
+		res := NewVar(token.NoPos, nil, "", Typ[String])
+		sig := &Signature{results: NewTuple(res)}
+		err := NewFunc(token.NoPos, nil, "Error", sig)
+		typ := &Named{underlying: NewInterfaceType([]*Func{err}, nil).Complete()}
+		sig.recv = NewVar(token.NoPos, nil, "", typ)
+		def(NewTypeName(token.NoPos, nil, "error", typ))
+	}
 }
 
 var predeclaredConsts = [...]struct {
@@ -188,6 +200,33 @@ func DefPredeclaredTestFuncs() {
 	def(newBuiltin(_Trace))
 }
 
+func defPredeclaredComparable() {
+	// The "comparable" interface can be imagined as defined like
+	//
+	// type comparable interface {
+	//         == () untyped bool
+	//         != () untyped bool
+	// }
+	//
+	// == and != cannot be user-declared but we can declare
+	// a magic method == and check for its presence when needed.
+
+	// Define interface { == () }. We don't care about the signature
+	// for == so leave it empty except for the receiver, which is
+	// set up later to match the usual interface method assumptions.
+	sig := new(Signature)
+	eql := NewFunc(token.NoPos, nil, "==", sig)
+	iface := NewInterfaceType([]*Func{eql}, nil).Complete()
+
+	// set up the defined type for the interface
+	obj := NewTypeName(token.NoPos, nil, "comparable", nil)
+	named := NewNamed(obj, iface, nil)
+	obj.color_ = black
+	sig.recv = NewVar(token.NoPos, nil, "", named) // complete == signature
+
+	def(obj)
+}
+
 func init() {
 	Universe = NewScope(nil, token.NoPos, token.NoPos, "universe")
 	Unsafe = NewPackage("unsafe", "unsafe")
@@ -197,11 +236,16 @@ func init() {
 	defPredeclaredConsts()
 	defPredeclaredNil()
 	defPredeclaredFuncs()
+	defPredeclaredComparable()
 
 	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.(*Named)
 	universeError = Universe.Lookup("error").(*TypeName).typ.(*Named)
+
+	// "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
@@ -215,7 +259,7 @@ func def(obj Object) {
 		return // nothing to do
 	}
 	// fix Obj link for named types
-	if typ, ok := obj.Type().(*Named); ok {
+	if typ := asNamed(obj.Type()); typ != nil {
 		typ.obj = obj.(*TypeName)
 	}
 	// exported identifiers go into package unsafe