[cmake-developers] [PATCH] Let CMake know that clang uses the GCC toolchain.

[Brad King]

On 08/03/2016 03:22 PM, Chaoren Lin wrote:
>> It will take some investigation to decide how/whether to set SIMULATE_ID
>> to GNU. 
> 
> This seems like the right thing to do, and is more future proof than the alternative.

This patch may do it.

```patch
diff --git a/Modules/Compiler/Clang-DetermineCompilerInternal.cmake b/Modules/Compiler/Clang-DetermineCompilerInternal.cmake
index 08c1230..a8ef61b 100644
--- a/Modules/Compiler/Clang-DetermineCompilerInternal.cmake
+++ b/Modules/Compiler/Clang-DetermineCompilerInternal.cmake
@@ -12,4 +12,6 @@ set(_compiler_id_version_compute "
 set(_compiler_id_simulate "
 # if defined(_MSC_VER)
 #  define @PREFIX at SIMULATE_ID \"MSVC\"
+# elif defined(__GNUC__)
+#  define @PREFIX at SIMULATE_ID \"GNU\"
 # endif")
```

However, I remember now why we didn't do this before.  For MSVC, Clang
actually tries to look a lot like `cl`.  It defines _MSC_VER to a
version matching the compiler's standard library.  OTOH for GNU, Clang
is not trying to look like a specific version.  It always defines
__GNUC__ to 4 and __GNUC_MINOR__ to 2, regardless of what standard
library is used.  Essentially Clang is acting as a standalone compiler
that happens to use a GNU standard library as an implementation detail.

Therefore I've decided to go with the direct approach.  Please
try out this change:

 Ninja: Fix response file format for GNU-like Clang on Windows
 https://cmake.org/gitweb?p=cmake.git;a=commitdiff;h=e555480c

Thanks,
-Brad