title: "C++ compiler conformance improvements | Microsoft Docs"

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ms.date: "06/05/2017"

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Visual Studio 2017 correctly raises an error when the left-hand operand of a conditionally evaluating operation is not valid in a constexpr context. The following code compiles in Visual Studio 2015 but not in Visual Studio 2017 (C3615 constexpr function 'f' cannot result in a constant expression):

```cpp

template<int N>

constexpr bool f(const array<1> &arr)

{

return arr.size() == 10 || arr.size() == 11; // C3615

}

To disable the warning, add a #pragma:

struct S {};

template <typename T>

void S::f(T t) {} //C2039: 'f': is not a member of 'S'

#include <memory>

class B { };

class D : B { }; // C2243. should be public B { };

void f()

{

To fix the warning, put extern "C" first:

extern "C" __declspec(noinline) HRESULT __stdcall

Visual Studio 2017 RTW release had a regression in which the C++ compiler would not issue a diagnostic if a 'const' variable was not initialized. This regression has been fixed in Visual Studio 2017 Update 1. The following code now produces "warning C4132: 'Value': const object should be initialized":

```cpp

const int Value; //C4132

To fix the error, assign a value to `Value`.

**std::is_convertible<From, To>** is calculated by checking to see if an imaginary function definition is well formed:

```cpp

To test() { return std::declval<From>(); }

```

```

Private destructors cause a type to be non-constructible. **std::is_constructible<T, Args…>** is calculated as if the following declaration were written:

```cpp

T obj(std::declval<Args>()…)

```

To fix the code, remove the using N::f statement if you intended to call ::f().

Local function declarations hide the function declaration in the enclosing scope and disable argument dependent lookup.

However, previous versions of the Visual C++ compiler performed argument dependent lookup in this case, potentially leading to the wrong overload being chosen and unexpected runtime behavior. Typically, the error is due to an incorrect signature of the local function declaration. In the following example, Visual Studio 2017 Update Version 15.3 correctly raises C2660 'f': function does not take 2 arguments:

struct S {};

void f(S, int);

void g()

{

void f(S); // C2660 'f': function does not take 2 arguments:

// or void f(S, int);

S s;

f(s, 0);

}

To fix the problem arrange the intializer list to have the same order as the declarations. A similar warning is raised when one or both initializers refer to base class members.

Note that the warning is off-by-default and only affects code compiled with /Wall or /WX.

[Visual C++ Language Conformance](visual-cpp-language-conformance.md)