internal/analysisinternal: ZeroValue: support materialized aliases

My previous audit of this file contained a mistake in the order
in which constructors were tested; it was revealed by testing
with gotypesalias=1.

Updates golang/go#65294

Change-Id: I80f6f0bb24bceb2c69b9919e9c482b780c0d546a
Reviewed-on: https://go-review.googlesource.com/c/tools/+/575699
Reviewed-by: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Alan Donovan <adonovan@google.com>

internal/analysisinternal/analysis.go

diff --git a/internal/analysisinternal/analysis.go b/internal/analysisinternal/analysis.go
index c3022a2..eb83088 100644
--- a/internal/analysisinternal/analysis.go
+++ b/internal/analysisinternal/analysis.go
@@ -32,22 +32,22 @@
 func ZeroValue(f *ast.File, pkg *types.Package, typ types.Type) ast.Expr {
 	// TODO(adonovan): think about generics, and also generic aliases.
 	under := aliases.Unalias(typ)
-	// Don't call Underlying unconditionally: although it removed
+	// Don't call Underlying unconditionally: although it removes
 	// Named and Alias, it also removes TypeParam.
-	if n, ok := typ.(*types.Named); ok {
+	if n, ok := under.(*types.Named); ok {
 		under = n.Underlying()
 	}
-	switch u := under.(type) {
+	switch under := under.(type) {
 	case *types.Basic:
 		switch {
-		case u.Info()&types.IsNumeric != 0:
+		case under.Info()&types.IsNumeric != 0:
 			return &ast.BasicLit{Kind: token.INT, Value: "0"}
-		case u.Info()&types.IsBoolean != 0:
+		case under.Info()&types.IsBoolean != 0:
 			return &ast.Ident{Name: "false"}
-		case u.Info()&types.IsString != 0:
+		case under.Info()&types.IsString != 0:
 			return &ast.BasicLit{Kind: token.STRING, Value: `""`}
 		default:
-			panic(fmt.Sprintf("unknown basic type %v", u))
+			panic(fmt.Sprintf("unknown basic type %v", under))
 		}
 	case *types.Chan, *types.Interface, *types.Map, *types.Pointer, *types.Signature, *types.Slice, *types.Array:
 		return ast.NewIdent("nil")
@@ -178,7 +178,7 @@
 				List: returns,
 			},
 		}
-	case *types.Named:
+	case interface{ Obj() *types.TypeName }: // *types.{Alias,Named,TypeParam}
 		if t.Obj().Pkg() == nil {
 			return ast.NewIdent(t.Obj().Name())
 		}