Home SFINAE works as return type, but not parameter type
 I was trying to use enable_if to avoid duplicating code. It works fine as long as placed in the return type, but not if it's in the parameters. Before this gets closed as a dup of this, the error I'm getting is not a redefinition, but a "no matching function for call." Here's my MCVE (not so "C", or so "M" for that matter) using VS2015 and g++ 7.2.0 (mingw): #include #include #include template class Matrix { public: static const size_t ROWS = M; static const size_t COLS = N; typedef T SCALAR; SCALAR operator[](const size_t index) const { static_assert((COLS == 1 || ROWS == 1), "operator[] is only for vectors (single row or column)."); return m_elements.at(index); } SCALAR& operator[](const size_t index) { static_assert((COLS == 1 || ROWS == 1), "operator[] is only for vectors (single row or column)."); return m_elements.at(index); } std::array m_elements; }; template static inline T Length( const Matrix::type, N, M> & input) { T value = 0; for (size_t i = 0; i < std::max(N, M); ++i) { value += (input[i] * input[i]); } return std::sqrt(value); } template static inline Matrix::type , M, N> CrossProduct(const Matrix & a, const Matrix & b) { Matrix result; result[0] = a[1] * b[2] - a[2] * b[1]; result[1] = a[2] * b[0] - a[0] * b[2]; result[2] = a[0] * b[1] - a[1] * b[0]; return result; } Matrix m11; Matrix m31; Matrix m13; Matrix m33; auto l0 = Length(m11); // Should work, but doesn't: no matching function for call to 'Length(Matrix&)' auto l1 = Length(m31); // Should work, but doesn't: no matching function for call to 'Length(Matrix&)' auto l2 = Length(m13); // Should work, but doesn't: no matching function for call to 'Length(Matrix&)' //auto l3 = Length(m33); // Shouldn't work, and doesn't: no matching function for call to 'Length(Matrix&)' auto v1 = CrossProduct(m13, m13); //Works, as expected //auto v2 = CrossProduct(m11, m11); // As expected: enable_if.cpp:71:32: error: no matching function for // call to 'CrossProduct(Matrix&, Matrix&)'  If I change the signature of Length to static inline typename std::enable_if<(M == 1 || N == 1), T>::type \ Length(const math::Matrix & input)  it works fine. But the error it gives me seems to indicate that it was able to determine the correct signature (e.g. Length(Matrix&)). Why is the compiler unable to find a matching function if the enable_if is in the parameter list, but is able to if it's in the return type?