Querying Function Template Restrictions

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On Monday, 9 May 2016 at 11:57:11 UTC, Nordlöw wrote:
> In what ways can I compile-time introspect the template 
> restrictions of a specific function overload set in D?
>
> In other words if I have, for instance,
>
> T f(T)(T x) if (isFloat!T) {}
> T f(T)(T x) if (isInteger!T) {}
>
> T g(T)(T x) if (isFloat!T && isInteger!T) {}
>
> can I somehow at compile-time query that
>
> - f() is a function having *two* overloads where the first 
> takes an integer and the second takes a float?
> - g() is a function having *one* overload taking either an 
> integer or a float?
>
> Or is it only via trial-and-error calls such as
>
> __traits(compiles, { f(1); } ); // ok
> __traits(compiles, { f(1.2); } ); // ok
> __traits(compiles, { f("1"); } ); // not ok
> __traits(compiles, { f('1'); } ); // not ok
>
> __traits(compiles, { g(1); } ); // ok
> __traits(compiles, { g(1.2); } ); // ok
> __traits(compiles, { g("1"); } ); // not ok
> __traits(compiles, { g('1'); } ); // not ok
>
> I can get that information?

Unfortunately, you cannot introspect template constraints. The 
only thing you can do is check if calling it with given arguments 
compiles or not.