[dev.typeparams] cmd/compile: simplify type alias handling for export

Currently the exporter uses types.IsDotAlias(n.Sym()) to recognize
that n is a type alias, but IsDotAlias is actually meant for
recognizing aliases introduced by dot imports. Translated to go/types,
the current logic amounts recognizing type aliases as if by:

	var n *types.TypeName
	typ, ok := n.Pkg().Scope().Lookup(n.Name()).Type().(*types.Named)
	isAlias := !ok || typ.Obj().Pkg() != n.Pkg() || typ.Obj().Name() != n.Name()

But we can instead just check n.Alias() (eqv. n.IsAlias() in
go/types). In addition to being much simpler, this is also actually
correct for recognizing function-scoped type declarations (though we
don't currently support those anyway, nor would they go through this
exact code path).

To avoid possible future misuse of IsDotAlias, this CL also inlines
its trivial definition into its only call site.

Passes toolstash -cmp, also w/ -gcflags=all=-G=3.

Change-Id: I7c6283f4b58d5311aa683f8229bbf62f8bab2ff9
Reviewed-on: https://go-review.googlesource.com/c/go/+/320613
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com>
Trust: Dan Scales <danscales@google.com>
Reviewed-by: Dan Scales <danscales@google.com>

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

@@ -104,13 +104,7 @@ func (g *irgen) typeDecl(out *ir.Nodes, decl *syntax.TypeDecl) {
 	if decl.Alias {
 		name, _ := g.def(decl.Name)
 		g.pragmaFlags(decl.Pragma, 0)
-
+		assert(name.Alias()) // should be set by irgen.obj
-		// TODO(mdempsky): This matches how typecheckdef marks aliases for
-		// export, but this won't generalize to exporting function-scoped
-		// type aliases. We should maybe just use n.Alias() instead.
-		if ir.CurFunc == nil {
-			name.Sym().Def = ir.TypeNode(name.Type())
-		}
 
 		out.Append(ir.NewDecl(g.pos(decl), ir.ODCLTYPE, name))
 		return

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

@@ -431,7 +431,7 @@ func clearImports() {
 			s.Def = nil
 			continue
 		}
-		if types.IsDotAlias(s) {
+		if s.Def != nil && s.Def.Sym() != s {
 			// throw away top-level name left over
 			// from previous import . "x"
 			// We'll report errors after type checking in CheckDotImports.

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

@@ -101,6 +101,7 @@ func (g *irgen) obj(obj types2.Object) *ir.Name {
 	case *types2.TypeName:
 		if obj.IsAlias() {
 			name = g.objCommon(pos, ir.OTYPE, g.sym(obj), class, g.typ(obj.Type()))
+			name.SetAlias(true)
 		} else {
 			name = ir.NewDeclNameAt(pos, ir.OTYPE, g.sym(obj))
 			g.objFinish(name, class, types.NewNamed(name))

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

@@ -479,7 +479,7 @@ func (p *iexporter) doDecl(n *ir.Name) {
 		w.constExt(n)
 
 	case ir.OTYPE:
-		if types.IsDotAlias(n.Sym()) {
+		if n.Alias() {
 			// Alias.
 			w.tag('A')
 			w.pos(n.Pos())

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

@@ -1889,11 +1889,6 @@ func typecheckdef(n *ir.Name) {
 					n.SetDiag(true)
 					goto ret
 				}
-				// For package-level type aliases, set n.Sym.Def so we can identify
-				// it as a type alias during export. See also #31959.
-				if n.Curfn == nil {
-					n.Sym().Def = n.Ntype
-				}
 			}
 			break
 		}

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

@@ -137,7 +137,3 @@ func CleanroomDo(f func()) {
 	f()
 	pkgMap = saved
 }
-
-func IsDotAlias(sym *Sym) bool {
-	return sym.Def != nil && sym.Def.Sym() != sym
-}