3 files changed, 73 insertions, 3 deletions
diff --git a/src/text/template/exec_test.go b/src/text/template/exec_test.go
index b2ed8e7938..139fc5320d 100644
--- a/src/text/template/exec_test.go
+++ b/src/text/template/exec_test.go
@@ -1183,3 +1183,52 @@ func TestExecuteGivesExecError(t *testing.T) {
 		t.Errorf("expected %q; got %q", expect, err)
 	}
 }
+
+func funcNameTestFunc() int {
+	return 0
+}
+
+func TestGoodFuncNames(t *testing.T) {
+	names := []string{
+		"_",
+		"a",
+		"a1",
+		"a1",
+		"Ӵ",
+	}
+	for _, name := range names {
+		tmpl := New("X").Funcs(
+			FuncMap{
+				name: funcNameTestFunc,
+			},
+		)
+		if tmpl == nil {
+			t.Fatalf("nil result for %q", name)
+		}
+	}
+}
+
+func TestBadFuncNames(t *testing.T) {
+	names := []string{
+		"",
+		"2",
+		"a-b",
+	}
+	for _, name := range names {
+		testBadFuncName(name, t)
+	}
+}
+
+func testBadFuncName(name string, t *testing.T) {
+	defer func() {
+		recover()
+	}()
+	New("X").Funcs(
+		FuncMap{
+			name: funcNameTestFunc,
+		},
+	)
+	// If we get here, the name did not cause a panic, which is how Funcs
+	// reports an error.
+	t.Errorf("%q succeeded incorrectly as function name", name)
+}
diff --git a/src/text/template/funcs.go b/src/text/template/funcs.go
index be13ca2a3e..b514551455 100644
--- a/src/text/template/funcs.go
+++ b/src/text/template/funcs.go
@@ -58,6 +58,9 @@ func createValueFuncs(funcMap FuncMap) map[string]reflect.Value {
 // addValueFuncs adds to values the functions in funcs, converting them to reflect.Values.
 func addValueFuncs(out map[string]reflect.Value, in FuncMap) {
 	for name, fn := range in {
+		if !goodName(name) {
+			panic(fmt.Errorf("function name %s is not a valid identifier", name))
+		}
 		v := reflect.ValueOf(fn)
 		if v.Kind() != reflect.Func {
 			panic("value for " + name + " not a function")
@@ -77,7 +80,7 @@ func addFuncs(out, in FuncMap) {
 	}
 }
 
-// goodFunc checks that the function or method has the right result signature.
+// goodFunc reports whether the function or method has the right result signature.
 func goodFunc(typ reflect.Type) bool {
 	// We allow functions with 1 result or 2 results where the second is an error.
 	switch {
@@ -89,6 +92,23 @@ func goodFunc(typ reflect.Type) bool {
 	return false
 }
 
+// goodName reports whether the function name is a valid identifier.
+func goodName(name string) bool {
+	if name == "" {
+		return false
+	}
+	for i, r := range name {
+		switch {
+		case r == '_':
+		case i == 0 && !unicode.IsLetter(r):
+			return false
+		case !unicode.IsLetter(r) && !unicode.IsDigit(r):
+			return false
+		}
+	}
+	return true
+}
+
 // findFunction looks for a function in the template, and global map.
 func findFunction(name string, tmpl *Template) (reflect.Value, bool) {
 	if tmpl != nil && tmpl.common != nil {
diff --git a/src/text/template/template.go b/src/text/template/template.go
index 3e80982123..69300d8867 100644
--- a/src/text/template/template.go
+++ b/src/text/template/template.go
@@ -162,8 +162,9 @@ func (t *Template) Delims(left, right string) *Template {
 
 // Funcs adds the elements of the argument map to the template's function map.
 // It panics if a value in the map is not a function with appropriate return
-// type. However, it is legal to overwrite elements of the map. The return
-// value is the template, so calls can be chained.
+// type or if the name cannot be used syntactically as a function in a template.
+// It is legal to overwrite elements of the map. The return value is the template,
+// so calls can be chained.
 func (t *Template) Funcs(funcMap FuncMap) *Template {
 	t.init()
 	t.muFuncs.Lock()