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  1. #include <iostream>
  2.  
  3. struct A
  4. {
  5.     static A const DEFAULT;
  6.  
  7.     constexpr A(int value = {}) : _value{ value } {}
  8.  
  9.     constexpr operator int() const { return _value; }
  10.  
  11.     int const _value;
  12. };
  13.  
  14. constexpr A const A::DEFAULT{};
  15.  
  16. template<typename = void>
  17. struct B_impl
  18. {
  19.     static B_impl const DEFAULT;
  20.  
  21.     constexpr B_impl(int value = {}) : _value{ value } {}
  22.  
  23.     constexpr operator int() const { return _value; }
  24.  
  25.     int const _value;
  26. };
  27.  
  28. template<typename T>
  29. constexpr B_impl<T> const B_impl<T>::DEFAULT{};
  30.  
  31. using B = B_impl<>;
  32.  
  33. template<int N>
  34. constexpr int foo() { return N; }
  35.  
  36. int main(int, char**) noexcept
  37. {
  38.     std::cout << "A: " << foo<A::DEFAULT>() << "\n"; // IntelliSense hates this
  39.     std::cout << "B: " << foo<B::DEFAULT>() << "\n"; // VC++ and clang die here:
  40.     // MSVC error:  C2975  'N': invalid template argument for 'foo', expected compile-time constant expression
  41.     // clang error:  note: candidate template ignored: invalid explicitly-specified argument for template parameter 'N'
  42.     return 0;
  43. }
  44.  
Success #stdin #stdout 0s 4252KB
comments (?)
stdin
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Standard input is empty
stdout
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A: 0
B: 0
https://ideone.com/VaNoL9
language:
C++14 (gcc 8.3)
created:
6 years ago
visibility:
public

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