[cfe-dev] clang c++0x template function declaration error

Seth Cantrell seth.cantrell at gmail.com
Sat Jul 16 09:31:36 PDT 2011

Could the diagnostics be improved over

> test.cpp:4:6: note: candidate template ignored: substitution failure
>       [with T = Y]
> auto type_constructor( void (T::*f)() ) -> X< T, f >;
>      ^


Maybe an added note that points at the f in X<T,f> to say that that's why the substitution failed?

Here's an example of a similar mistake:

> #include <functional>
> template<typename T>
> void get(T const &t,size_t N) {
>     std::get<N>(t);
> }
> int main () {
>     get(std::make_tuple(1,'a',1.2),2);
> }

When compiling this the diagnostics include several notes like the following

> /usr/include/c++/v1/__tuple:104:1: note: candidate template ignored: invalid explicitly-specified
>       argument for template parameter '_Ip'
> get(array<_Tp, _Size>&) _NOEXCEPT;

"invalid explicitly-specified argument for template parameter" is a little better but it'd be nice to see something pointing directly at the 'N' in std::get<N>(t) saying it's not legal to use arguments as template parameters. Would this be hard to add? Maybe I can give it a try.

On Jul 4, 2011, at 2:31 PM, John Freeman wrote:

> On 7/4/2011 1:03 PM, Eli Friedman wrote:
>> The declaration of the template type_constructor is broken.  Think
>> about it this way: type_constructor has one template parameter, so
>> type_constructor<Y>  should refer to a single function; however, you're
>> trying to specify it as returning multiple types.
> Or put another way, the second template argument for X, "foo", should be 
> a constant function pointer, but the parameter to type_constructor, "f", 
> is not.
> - John
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