MSVC++: template's static_assert is not triggered inside a lambda

Question

Update 2:

This is fixed now in VS 2019 Preview 16.1 Preview 1.

Update:

I have filed a bug report at visualstudio.com.

---

So I'm starting to get into C++'s templates and I ran into this issue when trying to prevent a template class from being compiled using static_assert.

Basically, the static_assert error is not triggered when it is inside a lambda on VS2017 using C++ Language Standard: ISO C++17 Standard (/std:c++17).

I also tried this on gcc-7 using -std=c++17 and the error is triggered. Is this a bug on VS2017 or is there something I'm missing?

Code sample:

#include <iostream>
#include <string>
#include <type_traits>


template<typename T, typename Enable = void>
class IntegralContainer
{
    static_assert(std::is_integral<T>::value, "Type must be an integral!");
};

template<typename T>
class IntegralContainer<T, typename std::enable_if< std::is_integral<T>::value >::type >
{
private:
    T _value;

public:
    IntegralContainer(T value)
        : _value(value)
    {
    }
};

int main()
{
    IntegralContainer<int> int_container(1);
    // static_assert message is shown here.
    // > error C2338: Type must be an integral!
    // IntegralContainer<std::string> str_container;

    []() {
        // static_assert is not triggered here.
        IntegralContainer<std::string> str_container;
    }();

    std::cout << "Hello World!\n";

    return 0;
}

Answer 1

It's optimising away code which can never be executed. Definitely a bug. The lambda is undergoing some (probably semantic substitution) basic optimisation.

The resulting assembly shows no code being generated for the line IntegralContainer str_container

  []() {
00F72280  push        ebp  
00F72281  mov         ebp,esp  
00F72283  sub         esp,0D8h  
00F72289  push        ebx  
00F7228A  push        esi  
00F7228B  push        edi  
00F7228C  push        ecx  
00F7228D  lea         edi,[ebp-0D8h]  
00F72293  mov         ecx,36h  
00F72298  mov         eax,0CCCCCCCCh  
00F7229D  rep stos    dword ptr es:[edi]  
00F7229F  pop         ecx  
00F722A0  mov         dword ptr [this],ecx  
    // static_assert is not triggered here.
    IntegralContainer<std::string> str_container;
  }();
00F722A3  push        edx  
00F722A4  mov         ecx,ebp  
00F722A6  push        eax  
00F722A7  lea         edx,ds:[0F722BCh]  
00F722AD  call        @_RTC_CheckStackVars@8 (0F712B2h)  
00F722B2  pop         eax  
00F722B3  pop         edx  
00F722B4  pop         edi  
00F722B5  pop         esi  
00F722B6  pop         ebx  
00F722B7  mov         esp,ebp  
00F722B9  pop         ebp  
00F722BA  ret  

If the static assert is placed inside a public ctor though.

#include <iostream>
#include <string>
#include <type_traits>


template<typename T, typename Enable = void>
class IntegralContainer
{
public:
  IntegralContainer(T const& value) {
    static_assert(std::is_integral<T>::value, "Type must be an integral!");
  }
};

template<typename T>
class IntegralContainer<T, typename std::enable_if< std::is_integral<T>::value >::type >
{
private:
  T _value;

public:
  IntegralContainer(T value)
    : _value(value)
  {
  }
};

int main()
{
  IntegralContainer<int> int_container(1);
  // static_assert message is shown here.
  // > error C2338: Type must be an integral!
  //IntegralContainer<std::string> str_container;

  []() {
    // static_assert is not triggered here.
    IntegralContainer<std::string> str_container(std::string(""));
  }();

  std::cout << "Hello World!\n";

  return 0;
}

The static_assert is triggered.