Printing shortest decimal form of floating point number with Mir

[Ola Fosheim Grøstad]

On Wednesday, 23 December 2020 at 20:56:26 UTC, jmh530 wrote:
> concept Bar(T) = requires(U)() {
>     Foo!U; //akin to something like typename T::Foo<U>;
> }
> where we would basically be telling the compiler that T has to 
> be a Foo!U, which would mean you would have to use Bar like 
> Bar!U...at least that's the idea. I don't think anything like 
> this would work currently in C++.

Non-concept version is more verbose, but yeah, works fine in 
C++17:

namespace detail {
     template<template<typename> class F, class U>
     static constexpr void _dummy(const F<U> &a);

     template<class T, template<typename> typename F, class=void>
     struct has_outer_template : std::false_type {};

     template<class T, template<typename> typename F>
     struct 
has_outer_template<T,F,std::void_t<decltype(_dummy<F>(std::declval<T&>()))>>: std::true_type {};
};

template <class T, template<typename> typename F>
inline constexpr bool has_outer_template = 
detail::has_outer_template<T,F>::value;

template<class T>
struct Foo{};

static_assert(has_outer_template<Foo<int>,Foo>);