if constexpr inside lambda, differing compiler behavior

Question

Consider the following code. If my understanding of if constexpr is correct, the else branch should not be compiled so the z() should not be considered an error.

#include <type_traits>

struct Z{};

template<typename T>
void f(T z) {
  auto lam = [z]() {
    if constexpr(std::is_same<T, Z>::value) {
    } else {
      z();
    }
  };
}

int main() {
  f(Z{});
}

In clang and gcc this compiles; but with latest MSVC it does not. Unfortunately goldbolt's MSVC is too old, but on my machine with fully updated VS 2017, cl /std:c++17 yields:

Microsoft (R) C/C++ Optimizing Compiler Version 19.14.26428.1 for x86
Copyright (C) Microsoft Corporation.  All rights reserved.

if_constexpr.cpp
if_constexpr.cpp(10): error C2064: term does not evaluate to a function taking 0 arguments
if_constexpr.cpp(16): note: see reference to function template instantiation 'void f<Z>(T)' being compiled
        with
        [
            T=Z
        ]

If the enclosing lambda is removed, the code compiles on all three compilers.

Am I doing something wrong or unsupported, or is just a MSVC bug?

Answer 1

This is an MSVC bug. Please file a bug report.

The rule, from [stmt.if]/2, is:

During the instantiation of an enclosing templated entity, if the condition is not value-dependent after its instantiation, the discarded substatement (if any) is not instantiated.

During the instantiation of f<Z>, when we instantiate the condition we get true. That is not value-dependent, so the discard substatement (the one where we do z()) is not instantiated. It's only the instantiation of z() that leads to the error - and it should not be happening.