[dev.typeparams] cmd/compile: add built-in name/type "comparable".

This allows exporting comparable type bounds, and importing back into
types2 for typechecking.

Fixes typeparam/mdempsky/8.go

Change-Id: I3ee12433df2ed68ac6ef4cad24be9fcdfaaca4e3
Reviewed-on: https://go-review.googlesource.com/c/go/+/333129
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Trust: Dan Scales <danscales@google.com>

src/cmd/compile/internal/importer/support.go

@@ -101,6 +101,8 @@ var predeclared = []types2.Type{
 
 	// error
 	types2.Universe.Lookup("error").Type(),
+	// comparable
+	types2.Universe.Lookup("comparable").Type(),
 
 	// untyped types
 	types2.Typ[types2.UntypedBool],

src/cmd/compile/internal/noder/types.go

@@ -189,14 +189,6 @@ func (g *irgen) typ0(typ types2.Type) *types.Type {
 
 			// With Go 1.18, an embedded element can be any type, not
 			// just an interface.
-			if t := types2.AsInterface(e); t != nil {
-				if t.IsComparable() {
-					// Ignore predefined type 'comparable', since it
-					// doesn't resolve and it doesn't have any
-					// relevant methods.
-					continue
-				}
-			}
 			embeddeds[j] = types.NewField(src.NoXPos, nil, g.typ1(e))
 			j++
 		}

src/cmd/compile/internal/typecheck/bexport.go

@@ -78,6 +78,8 @@ func predeclared() []*types.Type {
 
 			// error
 			types.ErrorType,
+			// comparable
+			types.ComparableType,
 
 			// untyped types
 			types.UntypedBool,

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

@@ -158,6 +158,15 @@ func InitUniverse() {
 	s.Def = n
 	types.CalcSize(types.ErrorType)
 
+	// comparable type (interface)
+	s = types.BuiltinPkg.Lookup("comparable")
+	n = ir.NewDeclNameAt(src.NoXPos, ir.OTYPE, s)
+	types.ComparableType = types.NewNamed(n)
+	types.ComparableType.SetUnderlying(makeComparableInterface())
+	n.SetType(types.ComparableType)
+	s.Def = n
+	types.CalcSize(types.ComparableType)
+
 	types.Types[types.TUNSAFEPTR] = defBasic(types.TUNSAFEPTR, ir.Pkgs.Unsafe, "Pointer")
 
 	// simple aliases
@@ -338,6 +347,12 @@ func makeErrorInterface() *types.Type {
 	return types.NewInterface(types.NoPkg, []*types.Field{method})
 }
 
+func makeComparableInterface() *types.Type {
+	sig := types.NewSignature(types.NoPkg, fakeRecvField(), nil, nil, nil)
+	method := types.NewField(src.NoXPos, Lookup("=="), sig)
+	return types.NewInterface(types.NoPkg, []*types.Field{method})
+}
+
 // DeclareUniverse makes the universe block visible within the current package.
 func DeclareUniverse() {
 	// Operationally, this is similar to a dot import of builtinpkg, except

src/cmd/compile/internal/types/type.go

@@ -123,6 +123,8 @@ var (
 
 	// Predeclared error interface type.
 	ErrorType *Type
+	// Predeclared comparable interface type.
+	ComparableType *Type
 
 	// Types to represent untyped string and boolean constants.
 	UntypedString = New(TSTRING)

src/go/internal/gcimporter/support.go

@@ -104,6 +104,8 @@ var predeclared = []types.Type{
 
 	// error
 	types.Universe.Lookup("error").Type(),
+	// comparable
+	types.Universe.Lookup("comparable").Type(),
 
 	// untyped types
 	types.Typ[types.UntypedBool],

test/run.go

@@ -2210,7 +2210,6 @@ var g3Failures = setOf(
 	"typeparam/mdempsky/4.go",
 	"typeparam/mdempsky/5.go",
 	"typeparam/mdempsky/7.go",
-	"typeparam/mdempsky/8.go",
 	"typeparam/mdempsky/9.go",
 	"typeparam/mdempsky/11.go",
 	"typeparam/mdempsky/12.go",