cmd/compile/internal/types: don't change outer formatting mode when recursing

Change-Id: I2a27c32284c996128a27f61d1587ab4c67f0bd0c
GitHub-Last-Rev: 2f5e1a9b51
GitHub-Pull-Request: golang/go#78951
Reviewed-on: https://go-review.googlesource.com/c/go/+/770641
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>

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

@@ -431,10 +431,11 @@ func tconv2(b *bytes.Buffer, t *Type, verb rune, mode fmtMode, visited map[*Type
 			case IsExported(f.Sym.Name):
 				sconv2(b, f.Sym, 'S', mode)
 			default:
+				smode := mode
 				if mode != fmtTypeIDName {
-					mode = fmtTypeID
+					smode = fmtTypeID
 				}
-				sconv2(b, f.Sym, 'v', mode)
+				sconv2(b, f.Sym, 'v', smode)
 			}
 			tconv2(b, f.Type, 'S', mode, visited)
 		}

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

@@ -5,9 +5,62 @@
 package types
 
 import (
+	"bytes"
+	"fmt"
+	"strings"
 	"testing"
+
+	"cmd/internal/src"
 )
 
+// testTypeName is a minimal types.Object for NewNamed in tests.
+type testTypeName struct {
+	sym *Sym
+}
+
+func (o *testTypeName) Sym() *Sym     { return o.sym }
+func (o *testTypeName) Pos() src.XPos { return src.NoXPos }
+func (o *testTypeName) Type() *Type   { return nil }
+
+// If the printer mistakenly switches to fmtTypeID while formatting an
+// unexported interface method name, the method's signature is then
+// printed in fmtTypeID as well, so package-local names use Pkg.Prefix
+// (import path) instead of Pkg.Name. Regression test for that mistake.
+func TestFormatInterfaceUnexportedMethodUsesGoModeForSignature(t *testing.T) {
+	oldLocal := LocalPkg
+	oldPtr, oldReg, oldMax := PtrSize, RegSize, MaxWidth
+	t.Cleanup(func() {
+		LocalPkg = oldLocal
+		PtrSize, RegSize, MaxWidth = oldPtr, oldReg, oldMax
+	})
+	LocalPkg = NewPkg("main", "main")
+	PtrSize = 8
+	RegSize = 8
+	MaxWidth = 1 << 50
+
+	lib := NewPkg("example.com/lib", "lib")
+	namedT := NewNamed(&testTypeName{sym: lib.Lookup("T")})
+	// Underlying must not rely on Types[…]; InitTypes is not run in this package's tests.
+	namedT.SetUnderlying(NewStruct(nil))
+
+	sig := NewSignature(nil, nil, []*Field{NewField(src.NoXPos, nil, namedT)})
+	iface := NewInterface([]*Field{NewField(src.NoXPos, LocalPkg.Lookup("unexported"), sig)})
+	CalcSize(iface) // populate interface method set for printing (expandiface)
+
+	var buf bytes.Buffer
+	fmt.Fprintf(&buf, "%v", iface)
+	got := buf.String()
+	// Go syntax should qualify T with the package name, not the linker prefix / path.
+	const want = "lib.T"
+	if !strings.Contains(got, want) {
+		t.Fatalf("formatted type missing %q:\n%s", want, got)
+	}
+	// Stale fmtTypeID mode used Prefix ("example.com/lib") instead of Name ("lib").
+	if strings.Contains(got, "example.com/lib.T") {
+		t.Fatalf("formatted type should not use import path to qualify T (found example.com/lib.T):\n%s", got)
+	}
+}
+
 func TestSSACompare(t *testing.T) {
 	a := []*Type{
 		TypeInvalid,