errgroup: drop support for Go versions before 1.20

Change-Id: I7de5dfae21c4ffe31d6c16e3df0fed3e2269cb16
Reviewed-on: https://go-review.googlesource.com/c/sync/+/654421
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Commit-Queue: Ian Lance Taylor <iant@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

errgroup/errgroup.go

@@ -46,7 +46,7 @@ func (g *Group) done() {
 // returns a non-nil error or the first time Wait returns, whichever occurs
 // first.
 func WithContext(ctx context.Context) (*Group, context.Context) {
-	ctx, cancel := withCancelCause(ctx)
+	ctx, cancel := context.WithCancelCause(ctx)
 	return &Group{cancel: cancel}, ctx
 }
 

errgroup/errgroup_test.go

@@ -250,6 +250,45 @@ func TestGoLimit(t *testing.T) {
 	}
 }
 
+func TestCancelCause(t *testing.T) {
+	errDoom := errors.New("group_test: doomed")
+
+	cases := []struct {
+		errs []error
+		want error
+	}{
+		{want: nil},
+		{errs: []error{nil}, want: nil},
+		{errs: []error{errDoom}, want: errDoom},
+		{errs: []error{errDoom, nil}, want: errDoom},
+	}
+
+	for _, tc := range cases {
+		g, ctx := errgroup.WithContext(context.Background())
+
+		for _, err := range tc.errs {
+			err := err
+			g.TryGo(func() error { return err })
+		}
+
+		if err := g.Wait(); err != tc.want {
+			t.Errorf("after %T.TryGo(func() error { return err }) for err in %v\n"+
+				"g.Wait() = %v; want %v",
+				g, tc.errs, err, tc.want)
+		}
+
+		if tc.want == nil {
+			tc.want = context.Canceled
+		}
+
+		if err := context.Cause(ctx); err != tc.want {
+			t.Errorf("after %T.TryGo(func() error { return err }) for err in %v\n"+
+				"context.Cause(ctx) = %v; tc.want %v",
+				g, tc.errs, err, tc.want)
+		}
+	}
+}
+
 func BenchmarkGo(b *testing.B) {
 	fn := func() {}
 	g := &errgroup.Group{}