cmd/api: fix panic on exported basic type aliases

The order of emitting named type and type aliases in the `Walker`'s
`emitType` function is inverted. When the type alias references a basic
type, this causes a panic as the type assertion on `*types.Named` fails.
This change reorders the logic such that type aliases are emitted prior
to this type assertion.

Fixes #64958

Change-Id: I52dbe13999978912ded788d9cf4948103869bcfa
Reviewed-on: https://go-review.googlesource.com/c/go/+/554076
Reviewed-by: Bryan Mills <bcmills@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Bryan Mills <bcmills@google.com>

src/cmd/api/api_test.go

@@ -285,6 +285,25 @@ func TestIssue41358(t *testing.T) {
 	}
 }
 
+func TestIssue64958(t *testing.T) {
+	defer func() {
+		if x := recover(); x != nil {
+			t.Errorf("expected no panic; recovered %v", x)
+		}
+	}()
+
+	testenv.MustHaveGoBuild(t)
+
+	for _, context := range contexts {
+		w := NewWalker(context, "testdata/src/issue64958")
+		pkg, err := w.importFrom("p", "", 0)
+		if err != nil {
+			t.Errorf("expected no error importing; got %T", err)
+		}
+		w.export(pkg)
+	}
+}
+
 func TestCheck(t *testing.T) {
 	if !*flagCheck {
 		t.Skip("-check not specified")

src/cmd/api/main_test.go

@@ -957,17 +957,17 @@ func (w *Walker) emitType(obj *types.TypeName) {
 	if w.isDeprecated(obj) {
 		w.emitf("type %s //deprecated", name)
 	}
+	typ := obj.Type()
+	if obj.IsAlias() {
+		w.emitf("type %s = %s", name, w.typeString(typ))
+		return
+	}
 	if tparams := obj.Type().(*types.Named).TypeParams(); tparams != nil {
 		var buf bytes.Buffer
 		buf.WriteString(name)
 		w.writeTypeParams(&buf, tparams, true)
 		name = buf.String()
 	}
-	typ := obj.Type()
-	if obj.IsAlias() {
-		w.emitf("type %s = %s", name, w.typeString(typ))
-		return
-	}
 	switch typ := typ.Underlying().(type) {
 	case *types.Struct:
 		w.emitStructType(name, typ)

src/cmd/api/testdata/src/issue64958/p/p.go

@@ -0,0 +1,3 @@
+package p
+
+type BasicAlias = uint8