Hello all, Can I forward declare a function template that checks concepts using `BOOST_CONCEPT_REQUIRES()`? In the following example, MSVC gives me an ambiguous call error when I try to do that... (I have not yet tried other compilers). Example "c00.cpp": #include <boost/concept/requires.hpp> #include <boost/concept_check.hpp> #include <vector> // (1) Forward declaration with concepts. template<class Iter, class T> BOOST_CONCEPT_REQUIRES( ((boost::ForwardIterator<Iter>)) ((boost::EqualityComparable<T>)) , (bool) ) all_equals(Iter first, Iter last, const T& val); // (2) Actual definition. template<class Iter, class T> BOOST_CONCEPT_REQUIRES( ((boost::ForwardIterator<Iter>)) ((boost::EqualityComparable<T>)) , (bool) ) all_equals(Iter first, Iter last, const T& val) { for (Iter i = first; i < last; ++i) { if (*i != val) return false; } return true; } int main() { std::vector<double> v; // MSVC error: Cannot resolved ambiguous call between (1) and (2). all_equals(v.begin(), v.end(), double()); return 0; } MSVC error: Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 14.00.50727.42 for 80x86 Copyright (C) Microsoft Corporation. All rights reserved. c00.cpp c00.cpp(28) : error C2668: 'all_equals' : ambiguous call to overloaded function c00.cpp(18): could be 'bool all_equals<std::_Vector_iterator<_Ty,_Alloc>,double>(Iter,Iter,const T &)' with [ _Ty=double, _Alloc=std::allocator<double>, Iter=std::_Vector_iterator<double,std::allocator<double>>, T=double ] c00.cpp(11): or 'bool all_equals<std::_Vector_iterator<_Ty,_Alloc>,double>(Iter,Iter,const T &)' with [ _Ty=double, _Alloc=std::allocator<double>, Iter=std::_Vector_iterator<double,std::allocator<double>>, T=double ] while trying to match the argument list '(std::_Vector_iterator<_Ty,_Alloc>, std::_Vector_iterator<_Ty,_Alloc>, double)' with [ _Ty=double, _Alloc=std::allocator<double> ] Thank you very much. -- Lorenzo