fmt: support %w

When fmt.Errorf is provided with a %w verb with an error operand,
return an error implementing an Unwrap method returning that operand.

It is invalid to use %w with other formatting functions, to use %w
multiple times in a format string, or to use %w with a non-error
operand. When the Errorf format string contains an invalid use of %w,
the returned error does not implement Unwrap.

Change-Id: I534e20d3b163ab22c2b137b1c9095906dc243221
Reviewed-on: https://go-review.googlesource.com/c/go/+/176998
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>

1 files changed, 73 insertions, 0 deletions

diff --git a/src/fmt/errors_test.go b/src/fmt/errors_test.go
new file mode 100644
index 0000000000..0c774bc28b
--- /dev/null
+++ b/src/fmt/errors_test.go
@@ -0,0 +1,73 @@
+// 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"
+	"testing"
+)
+
+func TestErrorf(t *testing.T) {
+	wrapped := errors.New("inner error")
+	for _, test := range []struct {
+		err        error
+		wantText   string
+		wantUnwrap error
+	}{{
+		err:        fmt.Errorf("%w", wrapped),
+		wantText:   "inner error",
+		wantUnwrap: wrapped,
+	}, {
+		err:        fmt.Errorf("added context: %w", wrapped),
+		wantText:   "added context: inner error",
+		wantUnwrap: wrapped,
+	}, {
+		err:        fmt.Errorf("%w with added context", wrapped),
+		wantText:   "inner error with added context",
+		wantUnwrap: wrapped,
+	}, {
+		err:        fmt.Errorf("%s %w %v", "prefix", wrapped, "suffix"),
+		wantText:   "prefix inner error suffix",
+		wantUnwrap: wrapped,
+	}, {
+		err:        fmt.Errorf("%[2]s: %[1]w", wrapped, "positional verb"),
+		wantText:   "positional verb: inner error",
+		wantUnwrap: wrapped,
+	}, {
+		err:      fmt.Errorf("%v", wrapped),
+		wantText: "inner error",
+	}, {
+		err:      fmt.Errorf("added context: %v", wrapped),
+		wantText: "added context: inner error",
+	}, {
+		err:      fmt.Errorf("%v with added context", wrapped),
+		wantText: "inner error with added context",
+	}, {
+		err:      fmt.Errorf("%w is not an error", "not-an-error"),
+		wantText: "%!w(string=not-an-error) is not an error",
+	}, {
+		err:      fmt.Errorf("wrapped two errors: %w %w", errString("1"), errString("2")),
+		wantText: "wrapped two errors: 1 %!w(fmt_test.errString=2)",
+	}, {
+		err:      fmt.Errorf("wrapped three errors: %w %w %w", errString("1"), errString("2"), errString("3")),
+		wantText: "wrapped three errors: 1 %!w(fmt_test.errString=2) %!w(fmt_test.errString=3)",
+	}, {
+		err:        fmt.Errorf("%w", nil),
+		wantText:   "%!w(<nil>)",
+		wantUnwrap: nil, // still nil
+	}} {
+		if got, want := errors.Unwrap(test.err), test.wantUnwrap; got != want {
+			t.Errorf("Formatted error: %v\nerrors.Unwrap() = %v, want %v", test.err, got, want)
+		}
+		if got, want := test.err.Error(), test.wantText; got != want {
+			t.Errorf("err.Error() = %q, want %q", got, want)
+		}
+	}
+}
+
+type errString string
+
+func (e errString) Error() string { return string(e) }