fmt: add frame info to Errorf and support %w

Partly implements proposal Issue #29934.

Change-Id: Ibcf12f383158dcfbc313ab29c417a710571d1acb
Reviewed-on: https://go-review.googlesource.com/c/163559
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>

6 files changed, 851 insertions, 26 deletions

diff --git a/src/fmt/doc.go b/src/fmt/doc.go
index a7115809d3..b784399e0d 100644
--- a/src/fmt/doc.go
+++ b/src/fmt/doc.go
@@ -149,20 +149,28 @@
 	1. If the operand is a reflect.Value, the operand is replaced by the
 	concrete value that it holds, and printing continues with the next rule.
 
-	2. If an operand implements the Formatter interface, it will
-	be invoked. Formatter provides fine control of formatting.
+	2. If an operand implements the Formatter interface, and not
+	errors.Formatter, it will be invoked. Formatter provides fine
+	control of formatting.
 
 	3. If the %v verb is used with the # flag (%#v) and the operand
 	implements the GoStringer interface, that will be invoked.
 
 	If the format (which is implicitly %v for Println etc.) is valid
-	for a string (%s %q %v %x %X), the following two rules apply:
+	for a string (%s %q %v %x %X), the following three rules apply:
 
-	4. If an operand implements the error interface, the Error method
+	4. If an operand implements errors.Formatter, the FormatError
+	method will be invoked with an errors.Printer to print the error.
+	If the %v flag is used with the + flag (%+v), the Detail method
+	of the Printer will return true and the error will be formatted
+	as a detailed error message. Otherwise the printed string will
+	be formatted as required by the verb (if any).
+
+	5. If an operand implements the error interface, the Error method
 	will be invoked to convert the object to a string, which will then
 	be formatted as required by the verb (if any).
 
-	5. If an operand implements method String() string, that method
+	6. If an operand implements method String() string, that method
 	will be invoked to convert the object to a string, which will then
 	be formatted as required by the verb (if any).
 
diff --git a/src/fmt/errors.go b/src/fmt/errors.go
new file mode 100644
index 0000000000..0fd3e83814
--- /dev/null
+++ b/src/fmt/errors.go
@@ -0,0 +1,239 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fmt
+
+import (
+	"errors"
+	"strings"
+)
+
+// Errorf formats according to a format specifier and returns the string as a
+// value that satisfies error.
+//
+// The returned error includes the file and line number of the caller when
+// formatted with additional detail enabled. If the last argument is an error
+// the returned error's Format method will return it if the format string ends
+// with ": %s", ": %v", or ": %w". If the last argument is an error and the
+// format string ends with ": %w", the returned error implements errors.Wrapper
+// with an Unwrap method returning it.
+func Errorf(format string, a ...interface{}) error {
+	err, wrap := lastError(format, a)
+	if err == nil {
+		return &noWrapError{Sprintf(format, a...), nil, errors.Caller(1)}
+	}
+
+	// TODO: this is not entirely correct. The error value could be
+	// printed elsewhere in format if it mixes numbered with unnumbered
+	// substitutions. With relatively small changes to doPrintf we can
+	// have it optionally ignore extra arguments and pass the argument
+	// list in its entirety.
+	msg := Sprintf(format[:len(format)-len(": %s")], a[:len(a)-1]...)
+	if wrap {
+		return &wrapError{msg, err, errors.Caller(1)}
+	}
+	return &noWrapError{msg, err, errors.Caller(1)}
+}
+
+func lastError(format string, a []interface{}) (err error, wrap bool) {
+	wrap = strings.HasSuffix(format, ": %w")
+	if !wrap &&
+		!strings.HasSuffix(format, ": %s") &&
+		!strings.HasSuffix(format, ": %v") {
+		return nil, false
+	}
+
+	if len(a) == 0 {
+		return nil, false
+	}
+
+	err, ok := a[len(a)-1].(error)
+	if !ok {
+		return nil, false
+	}
+
+	return err, wrap
+}
+
+type noWrapError struct {
+	msg   string
+	err   error
+	frame errors.Frame
+}
+
+func (e *noWrapError) Error() string {
+	return Sprint(e)
+}
+
+func (e *noWrapError) FormatError(p errors.Printer) (next error) {
+	p.Print(e.msg)
+	e.frame.Format(p)
+	return e.err
+}
+
+type wrapError struct {
+	msg   string
+	err   error
+	frame errors.Frame
+}
+
+func (e *wrapError) Error() string {
+	return Sprint(e)
+}
+
+func (e *wrapError) FormatError(p errors.Printer) (next error) {
+	p.Print(e.msg)
+	e.frame.Format(p)
+	return e.err
+}
+
+func (e *wrapError) Unwrap() error {
+	return e.err
+}
+
+func fmtError(p *pp, verb rune, err error) (handled bool) {
+	var (
+		sep = " " // separator before next error
+		w   = p   // print buffer where error text is written
+	)
+	switch {
+	// Note that this switch must match the preference order
+	// for ordinary string printing (%#v before %+v, and so on).
+
+	case p.fmt.sharpV:
+		if stringer, ok := p.arg.(GoStringer); ok {
+			// Print the result of GoString unadorned.
+			p.fmt.fmtS(stringer.GoString())
+			return true
+		}
+		return false
+
+	case p.fmt.plusV:
+		sep = "\n  - "
+		w.fmt.fmtFlags = fmtFlags{plusV: p.fmt.plusV} // only keep detail flag
+
+		// The width or precision of a detailed view could be the number of
+		// errors to print from a list.
+
+	default:
+		// Use an intermediate buffer in the rare cases that precision,
+		// truncation, or one of the alternative verbs (q, x, and X) are
+		// specified.
+		switch verb {
+		case 's', 'v':
+			if (!w.fmt.widPresent || w.fmt.wid == 0) && !w.fmt.precPresent {
+				break
+			}
+			fallthrough
+		case 'q', 'x', 'X':
+			w = newPrinter()
+			defer w.free()
+		default:
+			w.badVerb(verb)
+			return true
+		}
+	}
+
+loop:
+	for {
+		w.fmt.inDetail = false
+		switch v := err.(type) {
+		case errors.Formatter:
+			err = v.FormatError((*errPP)(w))
+		case Formatter:
+			if w.fmt.plusV {
+				v.Format((*errPPState)(w), 'v') // indent new lines
+			} else {
+				v.Format(w, 'v') // do not indent new lines
+			}
+			break loop
+		default:
+			w.fmtString(v.Error(), 's')
+			break loop
+		}
+		if err == nil {
+			break
+		}
+		if w.fmt.needColon || !p.fmt.plusV {
+			w.buf.WriteByte(':')
+			w.fmt.needColon = false
+		}
+		w.buf.WriteString(sep)
+		w.fmt.inDetail = false
+		w.fmt.needNewline = false
+	}
+
+	if w != p {
+		p.fmtString(string(w.buf), verb)
+	}
+	return true
+}
+
+var detailSep = []byte("\n    ")
+
+// errPPState wraps a pp to implement State with indentation. It is used
+// for errors implementing fmt.Formatter.
+type errPPState pp
+
+func (p *errPPState) Width() (wid int, ok bool)      { return (*pp)(p).Width() }
+func (p *errPPState) Precision() (prec int, ok bool) { return (*pp)(p).Precision() }
+func (p *errPPState) Flag(c int) bool                { return (*pp)(p).Flag(c) }
+
+func (p *errPPState) Write(b []byte) (n int, err error) {
+	if p.fmt.plusV {
+		if len(b) == 0 {
+			return 0, nil
+		}
+		if p.fmt.inDetail && p.fmt.needColon {
+			p.fmt.needNewline = true
+			if b[0] == '\n' {
+				b = b[1:]
+			}
+		}
+		k := 0
+		for i, c := range b {
+			if p.fmt.needNewline {
+				if p.fmt.inDetail && p.fmt.needColon {
+					p.buf.WriteByte(':')
+					p.fmt.needColon = false
+				}
+				p.buf.Write(detailSep)
+				p.fmt.needNewline = false
+			}
+			if c == '\n' {
+				p.buf.Write(b[k:i])
+				k = i + 1
+				p.fmt.needNewline = true
+			}
+		}
+		p.buf.Write(b[k:])
+		if !p.fmt.inDetail {
+			p.fmt.needColon = true
+		}
+	} else if !p.fmt.inDetail {
+		p.buf.Write(b)
+	}
+	return len(b), nil
+
+}
+
+// errPP wraps a pp to implement an errors.Printer.
+type errPP pp
+
+func (p *errPP) Print(args ...interface{}) {
+	if !p.fmt.inDetail || p.fmt.plusV {
+		Fprint((*errPPState)(p), args...)
+	}
+}
+
+func (p *errPP) Printf(format string, args ...interface{}) {
+	if !p.fmt.inDetail || p.fmt.plusV {
+		Fprintf((*errPPState)(p), format, args...)
+	}
+}
+
+func (p *errPP) Detail() bool {
+	p.fmt.inDetail = true
+	return p.fmt.plusV
+}
diff --git a/src/fmt/errors_test.go b/src/fmt/errors_test.go
new file mode 100644
index 0000000000..9e6ad74697
--- /dev/null
+++ b/src/fmt/errors_test.go
@@ -0,0 +1,534 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fmt_test
+
+import (
+	"errors"
+	"fmt"
+	"io"
+	"os"
+	"path"
+	"reflect"
+	"regexp"
+	"strconv"
+	"strings"
+	"testing"
+)
+
+func TestErrorf(t *testing.T) {
+	chained := &wrapped{"chained", nil}
+	chain := func(s ...string) (a []string) {
+		for _, s := range s {
+			a = append(a, cleanPath(s))
+		}
+		return a
+	}
+	noArgsWrap := "no args: %w" // avoid vet check
+	testCases := []struct {
+		got  error
+		want []string
+	}{{
+		fmt.Errorf("no args"),
+		chain("no args/path.TestErrorf/path.go:xxx"),
+	}, {
+		fmt.Errorf(noArgsWrap),
+		chain("no args: %!w(MISSING)/path.TestErrorf/path.go:xxx"),
+	}, {
+		fmt.Errorf("nounwrap: %s", "simple"),
+		chain(`nounwrap: simple/path.TestErrorf/path.go:xxx`),
+	}, {
+		fmt.Errorf("nounwrap: %v", "simple"),
+		chain(`nounwrap: simple/path.TestErrorf/path.go:xxx`),
+	}, {
+		fmt.Errorf("%s failed: %v", "foo", chained),
+		chain("foo failed/path.TestErrorf/path.go:xxx",
+			"chained/somefile.go:xxx"),
+	}, {
+		fmt.Errorf("no wrap: %s", chained),
+		chain("no wrap/path.TestErrorf/path.go:xxx",
+			"chained/somefile.go:xxx"),
+	}, {
+		fmt.Errorf("%s failed: %w", "foo", chained),
+		chain("wraps:foo failed/path.TestErrorf/path.go:xxx",
+			"chained/somefile.go:xxx"),
+	}, {
+		fmt.Errorf("nowrapv: %v", chained),
+		chain("nowrapv/path.TestErrorf/path.go:xxx",
+			"chained/somefile.go:xxx"),
+	}, {
+		fmt.Errorf("wrapw: %w", chained),
+		chain("wraps:wrapw/path.TestErrorf/path.go:xxx",
+			"chained/somefile.go:xxx"),
+	}, {
+		fmt.Errorf("not wrapped: %+v", chained),
+		chain("not wrapped: chained: somefile.go:123/path.TestErrorf/path.go:xxx"),
+	}}
+	for i, tc := range testCases {
+		t.Run(strconv.Itoa(i)+"/"+path.Join(tc.want...), func(t *testing.T) {
+			got := errToParts(tc.got)
+			if !reflect.DeepEqual(got, tc.want) {
+				t.Errorf("Format:\n got: %+q\nwant: %+q", got, tc.want)
+			}
+
+			gotStr := tc.got.Error()
+			wantStr := fmt.Sprint(tc.got)
+			if gotStr != wantStr {
+				t.Errorf("Error:\n got: %+q\nwant: %+q", gotStr, wantStr)
+			}
+		})
+	}
+}
+
+func TestErrorFormatter(t *testing.T) {
+	testCases := []struct {
+		err    error
+		fmt    string
+		want   string
+		regexp bool
+	}{{
+		err: errors.New("foo"),
+		fmt: "%+v",
+		want: "foo:" +
+			"\n    fmt_test.TestErrorFormatter" +
+			"\n        .+/fmt/errors_test.go:\\d\\d",
+		regexp: true,
+	}, {
+		err:  &wrapped{"simple", nil},
+		fmt:  "%s",
+		want: "simple",
+	}, {
+		err:  &wrapped{"can't adumbrate elephant", outOfPeanuts{}},
+		fmt:  "%s",
+		want: "can't adumbrate elephant: out of peanuts",
+	}, {
+		err:  &wrapped{"a", &wrapped{"b", &wrapped{"c", nil}}},
+		fmt:  "%s",
+		want: "a: b: c",
+	}, {
+		err: &wrapped{"simple", nil},
+		fmt: "%+v",
+		want: "simple:" +
+			"\n    somefile.go:123",
+	}, {
+		err: &wrapped{"can't adumbrate elephant", outOfPeanuts{}},
+		fmt: "%+v",
+		want: "can't adumbrate elephant:" +
+			"\n    somefile.go:123" +
+			"\n  - out of peanuts:" +
+			"\n    the elephant is on strike" +
+			"\n    and the 12 monkeys" +
+			"\n    are laughing",
+	}, {
+		err:  &wrapped{"simple", nil},
+		fmt:  "%#v",
+		want: "&fmt_test.wrapped{msg:\"simple\", err:error(nil)}",
+	}, {
+		err:  &notAFormatterError{},
+		fmt:  "%+v",
+		want: "not a formatter",
+	}, {
+		err: &wrapped{"wrap", &notAFormatterError{}},
+		fmt: "%+v",
+		want: "wrap:" +
+			"\n    somefile.go:123" +
+			"\n  - not a formatter",
+	}, {
+		err: &withFrameAndMore{frame: errors.Caller(0)},
+		fmt: "%+v",
+		want: "something:" +
+			"\n    fmt_test.TestErrorFormatter" +
+			"\n        .+/fmt/errors_test.go:\\d\\d\\d" +
+			"\n    something more",
+		regexp: true,
+	}, {
+		err:  fmtTwice("Hello World!"),
+		fmt:  "%#v",
+		want: "2 times Hello World!",
+	}, {
+		err:  &wrapped{"fallback", os.ErrNotExist},
+		fmt:  "%s",
+		want: "fallback: file does not exist",
+	}, {
+		err: &wrapped{"fallback", os.ErrNotExist},
+		fmt: "%+v",
+		// Note: no colon after the last error, as there are no details.
+		want: "fallback:" +
+			"\n    somefile.go:123" +
+			"\n  - file does not exist:" +
+			"\n    os.init.ializers" +
+			"\n        .+/os/error.go:\\d\\d",
+		regexp: true,
+	}, {
+		err: &wrapped{"outer",
+			errors.Opaque(&wrapped{"mid",
+				&wrapped{"inner", nil}})},
+		fmt:  "%s",
+		want: "outer: mid: inner",
+	}, {
+		err: &wrapped{"outer",
+			errors.Opaque(&wrapped{"mid",
+				&wrapped{"inner", nil}})},
+		fmt: "%+v",
+		want: "outer:" +
+			"\n    somefile.go:123" +
+			"\n  - mid:" +
+			"\n    somefile.go:123" +
+			"\n  - inner:" +
+			"\n    somefile.go:123",
+	}, {
+		err:  &wrapped{"new style", formatError("old style")},
+		fmt:  "%v",
+		want: "new style: old style",
+	}, {
+		err:  &wrapped{"new style", formatError("old style")},
+		fmt:  "%q",
+		want: `"new style: old style"`,
+	}, {
+		err: &wrapped{"new style", formatError("old style")},
+		fmt: "%+v",
+		// Note the extra indentation.
+		// Colon for old style error is rendered by the fmt.Formatter
+		// implementation of the old-style error.
+		want: "new style:" +
+			"\n    somefile.go:123" +
+			"\n  - old style:" +
+			"\n    otherfile.go:456",
+	}, {
+		err:  &wrapped{"simple", nil},
+		fmt:  "%-12s",
+		want: "simple      ",
+	}, {
+		// Don't use formatting flags for detailed view.
+		err: &wrapped{"simple", nil},
+		fmt: "%+12v",
+		want: "simple:" +
+			"\n    somefile.go:123",
+	}, {
+		err:  &wrapped{"can't adumbrate elephant", outOfPeanuts{}},
+		fmt:  "%+50s",
+		want: "          can't adumbrate elephant: out of peanuts",
+	}, {
+		err:  &wrapped{"café", nil},
+		fmt:  "%q",
+		want: `"café"`,
+	}, {
+		err:  &wrapped{"café", nil},
+		fmt:  "%+q",
+		want: `"caf\u00e9"`,
+	}, {
+		err:  &wrapped{"simple", nil},
+		fmt:  "% x",
+		want: "73 69 6d 70 6c 65",
+	}, {
+		err: &wrapped{"msg with\nnewline",
+			&wrapped{"and another\none", nil}},
+		fmt: "%s",
+		want: "msg with" +
+			"\nnewline: and another" +
+			"\none",
+	}, {
+		err: &wrapped{"msg with\nnewline",
+			&wrapped{"and another\none", nil}},
+		fmt: "%+v",
+		want: "msg with" +
+			"\n    newline:" +
+			"\n    somefile.go:123" +
+			"\n  - and another" +
+			"\n    one:" +
+			"\n    somefile.go:123",
+	}, {
+		err: wrapped{"", wrapped{"inner message", nil}},
+		fmt: "%+v",
+		want: "somefile.go:123" +
+			"\n  - inner message:" +
+			"\n    somefile.go:123",
+	}, {
+		err:  detail{"empty detail", "", nil},
+		fmt:  "%s",
+		want: "empty detail",
+	}, {
+		err:  detail{"empty detail", "", nil},
+		fmt:  "%+v",
+		want: "empty detail",
+	}, {
+		err:  detail{"newline at start", "\nextra", nil},
+		fmt:  "%s",
+		want: "newline at start",
+	}, {
+		err: detail{"newline at start", "\n extra", nil},
+		fmt: "%+v",
+		want: "newline at start:" +
+			"\n     extra",
+	}, {
+		err: detail{"newline at start", "\nextra",
+			detail{"newline at start", "\nmore", nil}},
+		fmt: "%+v",
+		want: "newline at start:" +
+			"\n    extra" +
+			"\n  - newline at start:" +
+			"\n    more",
+	}, {
+		err: detail{"two newlines at start", "\n\nextra",
+			detail{"two newlines at start", "\n\nmore", nil}},
+		fmt: "%+v",
+		want: "two newlines at start:" +
+			"\n    " + // note the explicit space
+			"\n    extra" +
+			"\n  - two newlines at start:" +
+			"\n    " +
+			"\n    more",
+	}, {
+		err:  &detail{"single newline", "\n", nil},
+		fmt:  "%+v",
+		want: "single newline",
+	}, {
+		err: &detail{"single newline", "\n",
+			&detail{"single newline", "\n", nil}},
+		fmt: "%+v",
+		want: "single newline:" +
+			"\n  - single newline",
+	}, {
+		err: &detail{"newline at end", "detail\n", nil},
+		fmt: "%+v",
+		want: "newline at end:" +
+			"\n    detail",
+	}, {
+		err: &detail{"newline at end", "detail\n",
+			&detail{"newline at end", "detail\n", nil}},
+		fmt: "%+v",
+		want: "newline at end:" +
+			"\n    detail" +
+			"\n  - newline at end:" +
+			"\n    detail",
+	}, {
+		err: &detail{"two newlines at end", "detail\n\n",
+			&detail{"two newlines at end", "detail\n\n", nil}},
+		fmt: "%+v",
+		want: "two newlines at end:" +
+			"\n    detail" +
+			"\n    " +
+			"\n  - two newlines at end:" +
+			"\n    detail" +
+			"\n    ", // note the additional space
+	}, {
+		err:  nil,
+		fmt:  "%+v",
+		want: "<nil>",
+	}, {
+		err:  (*wrapped)(nil),
+		fmt:  "%+v",
+		want: "<nil>",
+	}, {
+		err:  &wrapped{"simple", nil},
+		fmt:  "%T",
+		want: "*fmt_test.wrapped",
+	}, {
+		err:  &wrapped{"simple", nil},
+		fmt:  "%🤪",
+		want: "%!🤪(*fmt_test.wrapped=&{simple <nil>})",
+	}, {
+		err:  formatError("use fmt.Formatter"),
+		fmt:  "%#v",
+		want: "use fmt.Formatter",
+	}, {
+		err: wrapped{"using errors.Formatter",
+			formatError("use fmt.Formatter")},
+		fmt:  "%#v",
+		want: "fmt_test.wrapped{msg:\"using errors.Formatter\", err:\"use fmt.Formatter\"}",
+	}, {
+		err:  fmtTwice("%s %s", "ok", panicValue{}),
+		fmt:  "%s",
+		want: "ok %!s(PANIC=String method: panic)/ok %!s(PANIC=String method: panic)",
+	}, {
+		err:  fmtTwice("%o %s", panicValue{}, "ok"),
+		fmt:  "%s",
+		want: "{} ok/{} ok",
+	}}
+	for i, tc := range testCases {
+		t.Run(fmt.Sprintf("%d/%s", i, tc.fmt), func(t *testing.T) {
+			got := fmt.Sprintf(tc.fmt, tc.err)
+			var ok bool
+			if tc.regexp {
+				var err error
+				ok, err = regexp.MatchString(tc.want+"$", got)
+				if err != nil {
+					t.Fatal(err)
+				}
+			} else {
+				ok = got == tc.want
+			}
+			if !ok {
+				t.Errorf("\n got: %q\nwant: %q", got, tc.want)
+			}
+		})
+	}
+}
+
+var _ errors.Formatter = wrapped{}
+
+type wrapped struct {
+	msg string
+	err error
+}
+
+func (e wrapped) Error() string { return fmt.Sprint(e) }
+
+func (e wrapped) FormatError(p errors.Printer) (next error) {
+	p.Print(e.msg)
+	p.Detail()
+	p.Print("somefile.go:123")
+	return e.err
+}
+
+var _ errors.Formatter = outOfPeanuts{}
+
+type outOfPeanuts struct{}
+
+func (e outOfPeanuts) Error() string { return fmt.Sprint(e) }
+
+func (e outOfPeanuts) Format(fmt.State, rune) {
+	panic("should never be called by one of the tests")
+}
+
+func (outOfPeanuts) FormatError(p errors.Printer) (next error) {
+	p.Printf("out of %s", "peanuts")
+	p.Detail()
+	p.Print("the elephant is on strike\n")
+	p.Printf("and the %d monkeys\nare laughing", 12)
+	return nil
+}
+
+type withFrameAndMore struct {
+	frame errors.Frame
+}
+
+func (e *withFrameAndMore) Error() string { return fmt.Sprint(e) }
+
+func (e *withFrameAndMore) FormatError(p errors.Printer) (next error) {
+	p.Print("something")
+	if p.Detail() {
+		e.frame.Format(p)
+		p.Print("something more")
+	}
+	return nil
+}
+
+type notAFormatterError struct{}
+
+func (e notAFormatterError) Error() string { return "not a formatter" }
+
+type detail struct {
+	msg    string
+	detail string
+	next   error
+}
+
+func (e detail) Error() string { return fmt.Sprint(e) }
+
+func (e detail) FormatError(p errors.Printer) (next error) {
+	p.Print(e.msg)
+	p.Detail()
+	p.Print(e.detail)
+	return e.next
+}
+
+// formatError is an error implementing Format instead of errors.Formatter.
+// The implementation mimics the implementation of github.com/pkg/errors.
+type formatError string
+
+func (e formatError) Error() string { return string(e) }
+
+func (e formatError) Format(s fmt.State, verb rune) {
+	// Body based on pkg/errors/errors.go
+	switch verb {
+	case 'v':
+		if s.Flag('+') {
+			io.WriteString(s, string(e))
+			fmt.Fprintf(s, ":\n%s", "otherfile.go:456")
+			return
+		}
+		fallthrough
+	case 's':
+		io.WriteString(s, string(e))
+	case 'q':
+		fmt.Fprintf(s, "%q", string(e))
+	}
+}
+
+func (e formatError) GoString() string {
+	panic("should never be called")
+}
+
+type fmtTwiceErr struct {
+	format string
+	args   []interface{}
+}
+
+func fmtTwice(format string, a ...interface{}) error {
+	return fmtTwiceErr{format, a}
+}
+
+func (e fmtTwiceErr) Error() string { return fmt.Sprint(e) }
+
+func (e fmtTwiceErr) FormatError(p errors.Printer) (next error) {
+	p.Printf(e.format, e.args...)
+	p.Print("/")
+	p.Printf(e.format, e.args...)
+	return nil
+}
+
+func (e fmtTwiceErr) GoString() string {
+	return "2 times " + fmt.Sprintf(e.format, e.args...)
+}
+
+type panicValue struct{}
+
+func (panicValue) String() string { panic("panic") }
+
+var rePath = regexp.MustCompile(`( [^ ]*)fmt.*test\.`)
+var reLine = regexp.MustCompile(":[0-9]*\n?$")
+
+func cleanPath(s string) string {
+	s = rePath.ReplaceAllString(s, "/path.")
+	s = reLine.ReplaceAllString(s, ":xxx")
+	s = strings.Replace(s, "\n   ", "", -1)
+	s = strings.Replace(s, " /", "/", -1)
+	return s
+}
+
+func errToParts(err error) (a []string) {
+	for err != nil {
+		var p testPrinter
+		if errors.Unwrap(err) != nil {
+			p.str += "wraps:"
+		}
+		f, ok := err.(errors.Formatter)
+		if !ok {
+			a = append(a, err.Error())
+			break
+		}
+		err = f.FormatError(&p)
+		a = append(a, cleanPath(p.str))
+	}
+	return a
+
+}
+
+type testPrinter struct {
+	str string
+}
+
+func (p *testPrinter) Print(a ...interface{}) {
+	p.str += fmt.Sprint(a...)
+}
+
+func (p *testPrinter) Printf(format string, a ...interface{}) {
+	p.str += fmt.Sprintf(format, a...)
+}
+
+func (p *testPrinter) Detail() bool {
+	p.str += " /"
+	return true
+}
diff --git a/src/fmt/format.go b/src/fmt/format.go
index 24e7e9551a..546c456c50 100644
--- a/src/fmt/format.go
+++ b/src/fmt/format.go
@@ -34,6 +34,11 @@ type fmtFlags struct {
 	// different, flagless formats set at the top level.
 	plusV  bool
 	sharpV bool
+
+	// error-related flags.
+	inDetail    bool
+	needNewline bool
+	needColon   bool
 }
 
 // A fmt is the raw formatter used by Printf etc.
diff --git a/src/fmt/format_example_test.go b/src/fmt/format_example_test.go
new file mode 100644
index 0000000000..386f10ef23
--- /dev/null
+++ b/src/fmt/format_example_test.go
@@ -0,0 +1,46 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fmt_test
+
+import (
+	"errors"
+	"fmt"
+	"path/filepath"
+	"regexp"
+)
+
+func baz() error { return errors.New("baz flopped") }
+func bar() error { return fmt.Errorf("bar(nameserver 139): %v", baz()) }
+func foo() error { return fmt.Errorf("foo: %s", bar()) }
+
+func Example_formatting() {
+	err := foo()
+	fmt.Println("Error:")
+	fmt.Printf("%v\n", err)
+	fmt.Println()
+	fmt.Println("Detailed error:")
+	fmt.Println(stripPath(fmt.Sprintf("%+v\n", err)))
+	// Output:
+	// Error:
+	// foo: bar(nameserver 139): baz flopped
+	//
+	// Detailed error:
+	// foo:
+	//     fmt_test.foo
+	//         fmt/format_example_test.go:16
+	//   - bar(nameserver 139):
+	//     fmt_test.bar
+	//         fmt/format_example_test.go:15
+	//   - baz flopped:
+	//     fmt_test.baz
+	//         fmt/format_example_test.go:14
+}
+
+func stripPath(s string) string {
+	rePath := regexp.MustCompile(`( [^ ]*)fmt`)
+	s = rePath.ReplaceAllString(s, " fmt")
+	s = filepath.ToSlash(s)
+	return s
+}
diff --git a/src/fmt/print.go b/src/fmt/print.go
index 121c7c59e4..c4ec73c77a 100644
--- a/src/fmt/print.go
+++ b/src/fmt/print.go
@@ -217,12 +217,6 @@ func Sprintf(format string, a ...interface{}) string {
 	return s
 }
 
-// Errorf formats according to a format specifier and returns the string
-// as a value that satisfies error.
-func Errorf(format string, a ...interface{}) error {
-	return errors.New(Sprintf(format, a...))
-}
-
 // These routines do not take a format string
 
 // Fprint formats using the default formats for its operands and writes to w.
@@ -576,12 +570,22 @@ func (p *pp) handleMethods(verb rune) (handled bool) {
 	if p.erroring {
 		return
 	}
-	// Is it a Formatter?
-	if formatter, ok := p.arg.(Formatter); ok {
+	switch x := p.arg.(type) {
+	case errors.Formatter:
+		handled = true
+		defer p.catchPanic(p.arg, verb, "FormatError")
+		return fmtError(p, verb, x)
+
+	case Formatter:
 		handled = true
 		defer p.catchPanic(p.arg, verb, "Format")
-		formatter.Format(p, verb)
+		x.Format(p, verb)
 		return
+
+	case error:
+		handled = true
+		defer p.catchPanic(p.arg, verb, "Error")
+		return fmtError(p, verb, x)
 	}
 
 	// If we're doing Go syntax and the argument knows how to supply it, take care of it now.
@@ -599,18 +603,7 @@ func (p *pp) handleMethods(verb rune) (handled bool) {
 		// Println etc. set verb to %v, which is "stringable".
 		switch verb {
 		case 'v', 's', 'x', 'X', 'q':
-			// Is it an error or Stringer?
-			// The duplication in the bodies is necessary:
-			// setting handled and deferring catchPanic
-			// must happen before calling the method.
-			switch v := p.arg.(type) {
-			case error:
-				handled = true
-				defer p.catchPanic(p.arg, verb, "Error")
-				p.fmtString(v.Error(), verb)
-				return
-
-			case Stringer:
+			if v, ok := p.arg.(Stringer); ok {
 				handled = true
 				defer p.catchPanic(p.arg, verb, "String")
 				p.fmtString(v.String(), verb)