[gjs: 1/2] wrapperutils: Fix compiler warning on GCC 5.x

[Philip Chimento <pchimento src gnome org>]

commit a6bb10a75a4713c996b6d43e99cd963d2298100a
Author: Philip Chimento <philip chimento gmail com>
Date:   Wed May 29 17:33:06 2019 -0700

    wrapperutils: Fix compiler warning on GCC 5.x
    
    We had a cast to void* which suppressed this warning on later versions
    of GCC, but that doesn't work on GCC 5.x. While researching the warning
    I read advice to not mix static template programming with dynamic
    template programming. In C++17 we could solve this statically with "if
    constexpr", but we only require C++14, so the best option is to change
    the whole thing to dynamic, providing a no_type_struct() function for
    the template specializations that don't have a GType struct.
    
    Closes: #253.

 gi/wrapperutils.cpp | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)
---
diff --git a/gi/wrapperutils.cpp b/gi/wrapperutils.cpp
index cdd97df9..82ff4632 100644
--- a/gi/wrapperutils.cpp
+++ b/gi/wrapperutils.cpp
@@ -105,6 +105,8 @@ bool gjs_wrapper_define_gtype_prop(JSContext* cx, JS::HandleObject constructor,
 template <InfoType::Tag TAG>
 struct InfoMethodsPolicy {};
 
+static GIStructInfo* no_type_struct(GIBaseInfo*) { return nullptr; }
+
 #define DECLARE_POLICY(tag, type, type_struct_func)                            \
     template <>                                                                \
     struct InfoMethodsPolicy<InfoType::tag> {                                  \
@@ -115,10 +117,10 @@ struct InfoMethodsPolicy {};
         static constexpr GIStructInfo* (*type_struct)(T*) = type_struct_func;  \
     };
 
-DECLARE_POLICY(Enum, enum, nullptr)
+DECLARE_POLICY(Enum, enum, no_type_struct)
 DECLARE_POLICY(Interface, interface, g_interface_info_get_iface_struct)
 DECLARE_POLICY(Object, object, g_object_info_get_class_struct)
-DECLARE_POLICY(Struct, struct, nullptr)
+DECLARE_POLICY(Struct, struct, no_type_struct)
 
 #undef DECLARE_POLICY
 
@@ -143,16 +145,14 @@ bool gjs_define_static_methods(JSContext* cx, JS::HandleObject constructor,
         }
     }
 
-    // Casting to void* avoids warning that the function pointer will never be
-    // null in template instantiations where it is not null
-    if (!reinterpret_cast<void*>(InfoMethodsPolicy<TAG>::type_struct))
-        return true;
-
     // Also define class/interface methods if there is a gtype struct
 
     GjsAutoStructInfo type_struct = InfoMethodsPolicy<TAG>::type_struct(info);
+    // Not an error for it to be null even in the case of Object and Interface;
+    // documentation says g_object_info_get_class_struct() and
+    // g_interface_info_get_iface_struct() can validly return a null pointer.
     if (!type_struct)
-        return true;  // not an error?
+        return true;
 
     n_methods = g_struct_info_get_n_methods(type_struct);