text/template: add detailed info for goodFunc check

goodFunc now returns a error describe the exact error it met.
builtin call function can print the name of the callee function
if the goodFunc check failed.

For input {{call .InvalidReturnCountFunc}}

before:
  can't evaluate field InvalidReturnTypeFunc in type *template.T
after:
  invalid function signature for .InvalidReturnTypeFunc: second argument should be error; is bool

Change-Id: I9aa53424ac9a2bffbdbeac889390f41218817575
GitHub-Last-Rev: 7c1e0dbd08884a38d92a42530104884a9ca52b44
GitHub-Pull-Request: golang/go#65509
Reviewed-on: https://go-review.googlesource.com/c/go/+/561115
Reviewed-by: Rob Pike <r@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Commit-Queue: Ian Lance Taylor <iant@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

1 files changed, 11 insertions, 3 deletions

diff --git a/src/text/template/exec.go b/src/text/template/exec.go
index 1a8f2fa0df..5b35b3e5a8 100644
--- a/src/text/template/exec.go
+++ b/src/text/template/exec.go
@@ -734,9 +734,8 @@ func (s *state) evalCall(dot, fun reflect.Value, isBuiltin bool, node parse.Node
 	} else if numIn != typ.NumIn() {
 		s.errorf("wrong number of args for %s: want %d got %d", name, typ.NumIn(), numIn)
 	}
-	if !goodFunc(typ) {
-		// TODO: This could still be a confusing error; maybe goodFunc should provide info.
-		s.errorf("can't call method/function %q with %d results", name, typ.NumOut())
+	if err := goodFunc(name, typ); err != nil {
+		s.errorf("%v", err)
 	}
 
 	unwrap := func(v reflect.Value) reflect.Value {
@@ -800,6 +799,15 @@ func (s *state) evalCall(dot, fun reflect.Value, isBuiltin bool, node parse.Node
 		}
 		argv[i] = s.validateType(final, t)
 	}
+
+	// Special case for the "call" builtin.
+	// Insert the name of the callee function as the first argument.
+	if isBuiltin && name == "call" {
+		calleeName := args[0].String()
+		argv = append([]reflect.Value{reflect.ValueOf(calleeName)}, argv...)
+		fun = reflect.ValueOf(call)
+	}
+
 	v, err := safeCall(fun, argv)
 	// If we have an error that is not nil, stop execution and return that
 	// error to the caller.