cmd/compile: support type C comparable

Support 'type C comparable' properly by using the same logic as for
'type T error', since ErrorType and ComparableType are entirely
analogous.

Added support for 'any' type as well, as requested by Robert. (For the
future - we can't currently have 'any' anywhere other than in a
constraint.)

Fixes #47966

Change-Id: I68bd284ced9a8bfca7d2339cd576f3cb909b1b83
Reviewed-on: https://go-review.googlesource.com/c/go/+/345174
Trust: Dan Scales <danscales@google.com>
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

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

@@ -99,6 +99,9 @@ func predeclared() []*types.Type {
 
 			// comparable
 			types.ComparableType,
+
+			// any
+			types.AnyType,
 		}
 	}
 	return predecl

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

@@ -566,6 +566,14 @@ func (p *iexporter) doDecl(n *ir.Name) {
 			// for predeclared objects).
 			underlying = types.ErrorType
 		}
+		if underlying == types.ComparableType.Underlying() {
+			// Do same for ComparableType as for ErrorType.
+			underlying = types.ComparableType
+		}
+		if base.Flag.G > 0 && underlying == types.AnyType.Underlying() {
+			// Do same for AnyType as for ErrorType.
+			underlying = types.AnyType
+		}
 		w.typ(underlying)
 
 		t := n.Type()

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

@@ -119,6 +119,8 @@ var (
 	ErrorType *Type
 	// Predeclared comparable interface type.
 	ComparableType *Type
+	// Predeclared any interface type.
+	AnyType *Type
 
 	// Types to represent untyped string and boolean constants.
 	UntypedString = newType(TSTRING)

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

@@ -107,6 +107,14 @@ func InitTypes(defTypeName func(sym *Sym, typ *Type) Object) {
 	ComparableType.SetUnderlying(makeComparableInterface())
 	ResumeCheckSize()
 
+	// any type (interface)
+	if base.Flag.G > 0 {
+		DeferCheckSize()
+		AnyType = defBasic(TFORW, BuiltinPkg, "any")
+		AnyType.SetUnderlying(NewInterface(NoPkg, []*Field{}))
+		ResumeCheckSize()
+	}
+
 	Types[TUNSAFEPTR] = defBasic(TUNSAFEPTR, UnsafePkg, "Pointer")
 
 	Types[TBLANK] = newType(TBLANK)

test/typeparam/issue47966.go

@@ -0,0 +1,9 @@
+// compile -G=3
+
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type C comparable

test/typeparam/subdict.go

@@ -14,7 +14,9 @@ import (
 	"fmt"
 )
 
-type value[T comparable] struct {
+type C comparable
+
+type value[T C] struct {
 	val T
 }