Fix syntax error parsing unnamed template parameters with a default.

Closes #961

CHANGES.current

@@ -7,6 +7,9 @@ the issue number to the end of the URL: https://github.com/swig/swig/issues/
 Version 4.2.0 (in progress)
 ===========================
 
+2022-11-25: wsfulton
+            #961 Fix syntax error parsing unnamed template parameters with a default value.
+
 2022-11-25: olly
 	    #2447 Fix undefined behaviour in swig's parser when handling
 	    default parameter expressions containing method calls.

Examples/test-suite/cpp17_enable_if_t.i

@@ -32,11 +32,12 @@ template <typename A, typename B, std::enable_if_t<(std::is_integral_v<A> and st
 
 void tester() {
   enableif5<int, int>(10, 20);
+  enableif5(10, 20);
 }
 %}
 
 // non-type template parameters working well in SWIG, below is a simple workaround as the 3rd parameter is defaulted for enable_if_t (which is just SFINAE to give a nice C++ compiler error)
-%template(enableif5) enableif5<int, int, true>; // workaround
+%template(enableif5) enableif5<int, int, true>; // workaround (overriding default)
 
 
 %inline %{
@@ -47,3 +48,42 @@ void destId(T el) {}
 template <typename T, std::enable_if_t<sizeof(T) >= 3>>
 void destId(const T& el) {}
 %}
+
+%inline %{
+// #961 no name for defaulted template parameter
+template<typename T, typename = std::enable_if_t<std::is_enum<T>::value>>
+void uuu() {}
+template<typename T, typename E = std::enable_if_t<std::is_enum<T>::value>>
+void uuuE() {}
+
+template<typename T, typename = typename std::enable_if<std::is_floating_point<T>::value>::type>
+void vvv() {}
+template<typename T, typename E = typename std::enable_if<std::is_floating_point<T>::value>::type>
+void vvvE() {}
+
+// More variations of enable_if and enable_if_t
+template<typename T, typename std::enable_if<std::is_floating_point<T>::value>::type* = nullptr>
+void www() {}
+
+template<typename T, typename std::enable_if_t<std::is_enum<T>::value, int> = 0>
+void xxx() {}
+
+enum TestEnum { Enum1 = 1, Enum2 };
+struct TestStruct {};
+
+void tester2() {
+  uuu<TestEnum>();
+//  uuu<TestStruct>(); // compilation error
+  uuuE<TestEnum>();
+//  uuuE<TestStruct>(); // compilation error
+  vvv<double>();
+//  vvv<TestStruct>(); // compilation error
+  vvvE<double>();
+//  vvvE<TestStruct>(); // compilation error
+
+  www<double>();
+//  www<TestStruct>(); // compilation error
+  xxx<TestEnum>();
+//  xxx<TestStruct>(); // compilation error
+}
+%}

Source/CParse/parser.y

@@ -4489,8 +4489,9 @@ templateparameters : templateparameter templateparameterstail {
                    | empty { $$ = 0; }
                    ;
 
-templateparameter : templcpptype {
+templateparameter : templcpptype def_args {
 		    $$ = NewParmWithoutFileLineInfo(NewString($1), 0);
+		    Setattr($$, "value", $2.rawval ? $2.rawval : $2.val);
                   }
                   | parm {
                     $$ = $1;