fmt: fix %d and other non-string verbs on errors

When formatting an error with a non-string formatting verb such as %d,
use the default formatting behavior rather than treating this as a bad
verb.

For example, this should print 42, not %!d(main.E=42):

  var E int
  func (E) Error() string { return "error" }
  fmt.Printf("%d", E(42))

Fixes #30472

Change-Id: I62fd309c8ee9839a69052b0ec7f1808449dcee8e
Reviewed-on: https://go-review.googlesource.com/c/164557
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

2 files changed, 19 insertions, 3 deletions

diff --git a/src/fmt/errors.go b/src/fmt/errors.go
index 0fd3e83814..7506b6a20b 100644
--- a/src/fmt/errors.go
+++ b/src/fmt/errors.go
@@ -130,8 +130,7 @@ func fmtError(p *pp, verb rune, err error) (handled bool) {
 			w = newPrinter()
 			defer w.free()
 		default:
-			w.badVerb(verb)
-			return true
+			return false
 		}
 	}
 
diff --git a/src/fmt/errors_test.go b/src/fmt/errors_test.go
index 9e6ad74697..d2957e675b 100644
--- a/src/fmt/errors_test.go
+++ b/src/fmt/errors_test.go
@@ -327,7 +327,7 @@ func TestErrorFormatter(t *testing.T) {
 	}, {
 		err:  &wrapped{"simple", nil},
 		fmt:  "%🤪",
-		want: "%!🤪(*fmt_test.wrapped=&{simple <nil>})",
+		want: "&{%!🤪(string=simple) <nil>}",
 	}, {
 		err:  formatError("use fmt.Formatter"),
 		fmt:  "%#v",
@@ -345,6 +345,14 @@ func TestErrorFormatter(t *testing.T) {
 		err:  fmtTwice("%o %s", panicValue{}, "ok"),
 		fmt:  "%s",
 		want: "{} ok/{} ok",
+	}, {
+		err:  intError(4),
+		fmt:  "%v",
+		want: "error 4",
+	}, {
+		err:  intError(4),
+		fmt:  "%d",
+		want: "4",
 	}}
 	for i, tc := range testCases {
 		t.Run(fmt.Sprintf("%d/%s", i, tc.fmt), func(t *testing.T) {
@@ -434,6 +442,15 @@ func (e detail) FormatError(p errors.Printer) (next error) {
 	return e.next
 }
 
+type intError int
+
+func (e intError) Error() string { return fmt.Sprint(e) }
+
+func (e intError) FormatError(p errors.Printer) (next error) {
+	p.Printf("error %d", e)
+	return nil
+}
+
 // formatError is an error implementing Format instead of errors.Formatter.
 // The implementation mimics the implementation of github.com/pkg/errors.
 type formatError string