fmt: undo clearflags in catchPanic after error message has been printed

Fixes #18282

Change-Id: I024ca4a03bbbcccd48a0a6245bc3ec22c6a90288
Reviewed-on: https://go-review.googlesource.com/34254
TryBot-Result: Gobot Gobot <gobot@golang.org>
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
Reviewed-by: Rob Pike <r@golang.org>

src/fmt/fmt_test.go

@@ -1561,18 +1561,23 @@ func TestWidthAndPrecision(t *testing.T) {
 	}
 }
 
-// Panic is a type that panics in String.
-type Panic struct {
+// PanicS is a type that panics in String.
+type PanicS struct {
 	message interface{}
 }
 
 // Value receiver.
-func (p Panic) GoString() string {
+func (p PanicS) String() string {
 	panic(p.message)
 }
 
+// PanicGo is a type that panics in GoString.
+type PanicGo struct {
+	message interface{}
+}
+
 // Value receiver.
-func (p Panic) String() string {
+func (p PanicGo) GoString() string {
 	panic(p.message)
 }
 
@@ -1592,13 +1597,15 @@ var panictests = []struct {
 	out string
 }{
 	// String
-	{"%s", (*Panic)(nil), "<nil>"}, // nil pointer special case
-	{"%s", Panic{io.ErrUnexpectedEOF}, "%!s(PANIC=unexpected EOF)"},
-	{"%s", Panic{3}, "%!s(PANIC=3)"},
+	{"%s", (*PanicS)(nil), "<nil>"}, // nil pointer special case
+	{"%s", PanicS{io.ErrUnexpectedEOF}, "%!s(PANIC=unexpected EOF)"},
+	{"%s", PanicS{3}, "%!s(PANIC=3)"},
 	// GoString
-	{"%#v", (*Panic)(nil), "<nil>"}, // nil pointer special case
-	{"%#v", Panic{io.ErrUnexpectedEOF}, "%!v(PANIC=unexpected EOF)"},
-	{"%#v", Panic{3}, "%!v(PANIC=3)"},
+	{"%#v", (*PanicGo)(nil), "<nil>"}, // nil pointer special case
+	{"%#v", PanicGo{io.ErrUnexpectedEOF}, "%!v(PANIC=unexpected EOF)"},
+	{"%#v", PanicGo{3}, "%!v(PANIC=3)"},
+	// Issue 18282. catchPanic should not clear fmtFlags permanently.
+	{"%#v", []interface{}{PanicGo{3}, PanicGo{3}}, "[]interface {}{%!v(PANIC=3), %!v(PANIC=3)}"},
 	// Format
 	{"%s", (*PanicF)(nil), "<nil>"}, // nil pointer special case
 	{"%s", PanicF{io.ErrUnexpectedEOF}, "%!s(PANIC=unexpected EOF)"},

src/fmt/print.go

@@ -535,7 +535,11 @@ func (p *pp) catchPanic(arg interface{}, verb rune) {
 			// Nested panics; the recursion in printArg cannot succeed.
 			panic(err)
 		}
-		p.fmt.clearflags() // We are done, and for this output we want default behavior.
+
+		oldFlags := p.fmt.fmtFlags
+		// For this output we want default behavior.
+		p.fmt.clearflags()
+
 		p.buf.WriteString(percentBangString)
 		p.buf.WriteRune(verb)
 		p.buf.WriteString(panicString)
@@ -543,6 +547,8 @@ func (p *pp) catchPanic(arg interface{}, verb rune) {
 		p.printArg(err, 'v')
 		p.panicking = false
 		p.buf.WriteByte(')')
+
+		p.fmt.fmtFlags = oldFlags
 	}
 }