[llvm-bugs] [Bug 42972] New: Declaration conflict between __clang_cuda_math_forward_declares.h and libstdc++

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=42972

            Bug ID: 42972
           Summary: Declaration conflict between
                    __clang_cuda_math_forward_declares.h and libstdc++
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: CUDA
          Assignee: unassignedclangbugs at nondot.org
          Reporter: jonathanchesterfield at gmail.com
                CC: llvm-bugs at lists.llvm.org

Tagged as cuda, but the problem may only reproduce from openmp.

The abs definition introduced in https://reviews.llvm.org/D62046 breaks
<iostream> and <algorithm> when mixed with libstdc++ 7.4 and openmp, e.g.

test.cpp:
`#include <iostream>`

~/llvm-install/bin/clang++ -std=c++11 -fopenmp
-fopenmp-targets=nvptx64-nvidia-cuda -Xopenmp-target=nvptx64-nvidia-cuda
-march=sm_50 -nocudalib -o test.x test.cpp

abs returning a const value seems a bit weird, I don't follow the reasoning in
the diff that makes this necessary.

```
In file included from
/usr/lib/gcc/x86_64-linux-gnu/7.4.0/../../../../include/c++/7.4.0/algorithm:62:
In file included from
/usr/lib/gcc/x86_64-linux-gnu/7.4.0/../../../../include/c++/7.4.0/bits/stl_algo.h:59:
In file included from
/usr/lib/gcc/x86_64-linux-gnu/7.4.0/../../../../include/c++/7.4.0/cstdlib:77:
/usr/lib/gcc/x86_64-linux-gnu/7.4.0/../../../../include/c++/7.4.0/bits/std_abs.h:70:3:
error: 
      declaration conflicts with target of using declaration already in scope
  abs(double __x)
  ^
/home/jon/llvm-install/lib/clang/10.0.0/include/__clang_cuda_math_forward_declares.h:50:25:
note: 
      target of using declaration
__DEVICE__ const double abs(const double);
                        ^
/usr/lib/gcc/x86_64-linux-gnu/7.4.0/../../../../include/c++/7.4.0/bits/std_abs.h:52:11:
note: using
      declaration
  using ::abs;
          ^
/usr/lib/gcc/x86_64-linux-gnu/7.4.0/../../../../include/c++/7.4.0/bits/std_abs.h:74:3:
error: 
      declaration conflicts with target of using declaration already in scope
  abs(float __x)
  ^
/home/jon/llvm-install/lib/clang/10.0.0/include/__clang_cuda_math_forward_declares.h:51:24:
note: 
      target of using declaration
__DEVICE__ const float abs(const float);
                       ^
/usr/lib/gcc/x86_64-linux-gnu/7.4.0/../../../../include/c++/7.4.0/bits/std_abs.h:52:11:
note: using
      declaration
  using ::abs;
          ^
2 errors generated.
```

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