text/template: wrap errors returned by template functions instead of stringifying them

Fixes #34201

Change-Id: Ic2e2967e4b01167345cf38bd006cabb206a64377
GitHub-Last-Rev: 5d0c4856550614484a8dbfb68c37aa1abcfcc529
GitHub-Pull-Request: golang/go#42398
Reviewed-on: https://go-review.googlesource.com/c/go/+/267838
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Trust: Daniel Martí <mvdan@mvdan.cc>
Trust: Pontus Leitzler <leitzler@gmail.com>
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Go Bot <gobot@golang.org>

1 files changed, 22 insertions, 0 deletions

diff --git a/src/text/template/exec_test.go b/src/text/template/exec_test.go
index 1a129ed5af..255b111b34 100644
--- a/src/text/template/exec_test.go
+++ b/src/text/template/exec_test.go
@@ -902,6 +902,28 @@ func TestExecError(t *testing.T) {
 	}
 }
 
+type CustomError struct{}
+
+func (*CustomError) Error() string { return "heyo !" }
+
+// Check that a custom error can be returned.
+func TestExecError_CustomError(t *testing.T) {
+	failingFunc := func() (string, error) {
+		return "", &CustomError{}
+	}
+	tmpl := Must(New("top").Funcs(FuncMap{
+		"err": failingFunc,
+	}).Parse("{{ err }}"))
+
+	var b bytes.Buffer
+	err := tmpl.Execute(&b, nil)
+
+	var e *CustomError
+	if !errors.As(err, &e) {
+		t.Fatalf("expected custom error; got %s", err)
+	}
+}
+
 func TestJSEscaping(t *testing.T) {
 	testCases := []struct {
 		in, exp string