Bugzilla

I initially misunderstood my actual problem, so here it is. This code results in successful regular builds but failed MinGW builds:

```c++
#if defined(_M_X64)
constexpr bool kInitiallyTargetingX64 = true;
#else
constexpr bool kInitiallyTargetingX64 = false;
#endif

#include <stdint.h>

#if defined(_M_X64)
constexpr bool kFinallyTargetingX64 = true;
#else
constexpr bool kFinallyTargetingX64 = false;
#endif

static_assert(kInitiallyTargetingX64 == kFinallyTargetingX64);
```

In regular builds, `_M_X64` is defined directly by the compiler if targeting x64. When you can compile something `_M_X64` will be available from the start of the C++ file. However in mingwclang builds, `_M_X64` is not directly defined like this. It only appears after including some header files, such as [`windows.h`](https://treeherder.mozilla.org/jobs?revision=05a681a41e8a0eae6f35030e908798d04c7162fa&repo=try) or [`stdint.h`](https://treeherder.mozilla.org/jobs?revision=9445236bacca2c6b17e9571a944c0f154b8ae1b0&repo=try) (the list in unclear to me), likely because they end up including [`_mingw_mac.h`](https://github.com/mingw-w64/mingw-w64/blob/master/mingw-w64-headers/crt/_mingw_mac.h). Including [`stddef.h`](https://treeherder.mozilla.org/jobs?revision=fda40c1f39e775c9ccd20a63d2b4e37dbfc0b79d&repo=try) does not result in defining `_M_X64`, for example.

I initially misunderstood my actual problem, so here it is. This code results in successful regular builds but failed MinGW builds:

```c++
#if defined(_M_X64)
constexpr bool kInitiallyTargetingX64 = true;
#else
constexpr bool kInitiallyTargetingX64 = false;
#endif

#include <stdint.h>

#if defined(_M_X64)
constexpr bool kFinallyTargetingX64 = true;
#else
constexpr bool kFinallyTargetingX64 = false;
#endif

static_assert(kInitiallyTargetingX64 == kFinallyTargetingX64);
```

In regular builds, `_M_X64` is defined directly by the compiler if targeting x64. When you compile something `_M_X64` will be available from the start of the C++ file. However in mingwclang builds, `_M_X64` is not directly defined like this. It only appears after including some header files, such as [`windows.h`](https://treeherder.mozilla.org/jobs?revision=05a681a41e8a0eae6f35030e908798d04c7162fa&repo=try) or [`stdint.h`](https://treeherder.mozilla.org/jobs?revision=9445236bacca2c6b17e9571a944c0f154b8ae1b0&repo=try) (the list in unclear to me), likely because they end up including [`_mingw_mac.h`](https://github.com/mingw-w64/mingw-w64/blob/master/mingw-w64-headers/crt/_mingw_mac.h). Including [`stddef.h`](https://treeherder.mozilla.org/jobs?revision=fda40c1f39e775c9ccd20a63d2b4e37dbfc0b79d&repo=try) does not result in defining `_M_X64`, for example.

I initially misunderstood my actual problem, so here it is. This code results in successful regular builds but failed MinGW builds:

```c++
#if defined(_M_X64)
constexpr bool kInitiallyTargetingX64 = true;
#else
constexpr bool kInitiallyTargetingX64 = false;
#endif

#include <stdint.h>

#if defined(_M_X64)
constexpr bool kFinallyTargetingX64 = true;
#else
constexpr bool kFinallyTargetingX64 = false;
#endif

static_assert(kInitiallyTargetingX64 == kFinallyTargetingX64);
```

In regular builds, `_M_X64` is defined directly by the compiler if targeting x64. When you compile something `_M_X64` will be available from the start of the C++ file. However in mingwclang builds, `_M_X64` is not directly defined like this. It only appears after including some header files, such as [`windows.h`](https://treeherder.mozilla.org/jobs?revision=05a681a41e8a0eae6f35030e908798d04c7162fa&repo=try) or [`stdint.h`](https://treeherder.mozilla.org/jobs?revision=9445236bacca2c6b17e9571a944c0f154b8ae1b0&repo=try) (the list in unclear to me), likely because they end up including [`_mingw_mac.h`](https://github.com/mingw-w64/mingw-w64/blob/master/mingw-w64-headers/crt/_mingw_mac.h). (Including [`stddef.h`](https://treeherder.mozilla.org/jobs?revision=fda40c1f39e775c9ccd20a63d2b4e37dbfc0b79d&repo=try) does not result in defining `_M_X64`, for example.)