Specializing a Template With a Return Type Derived by decltype in Visual Studio

Question

Given the following code:

template<typename ValueType>
decltype(ValueType{} == ValueType{}) Compare(const ValueType& value, const ValueType& expected)
{
    return value == expected;
}

template<>
decltype(float{} == float{}) Compare<float>(const float& value, const float& expected)
{
    return std::abs(value - expected) < 1e-4F;
}

I would expect the call Compare(13.0F, 42.0F) to correctly call the specialization, which it does on gcc. But it fails to on visual-studio-2017. I get the error:

error C2912: explicit specialization bool Compare<float>(const float &,const float &) is not a specialization of a function template

Is there something I can do to help guide visual-studio?

Answer 1

Purely to appease VC++, you may extract the return type as another template argument.

template<typename ValueType, typename RetType = decltype(ValueType{} == ValueType{})>
RetType Compare(const ValueType& value, const ValueType& expected)
{
    return value == expected;
}

template<>
decltype(float{} == float{}) Compare<float>(const float& value, const float& expected)
{
    return std::abs(value - expected) < 1e-4F;
}

That makes VC++ accept the code.