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List:       gcc-bugs
Subject:    [Bug c++/85580] New: "conflicting C language linkage declaration" warning for variables with identic
From:       "freddie_chopin at op dot pl" <gcc-bugzilla () gcc ! gnu ! org>
Date:       2018-04-30 19:54:58
Message-ID: bug-85580-4 () http ! gcc ! gnu ! org/bugzilla/
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85580

            Bug ID: 85580
           Summary: "conflicting C language linkage declaration" warning
                    for variables with identical names in `extern "C"`
                    functions
           Product: gcc
           Version: 8.0.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: freddie_chopin at op dot pl
  Target Milestone: ---

Here's a minimal test case executed with 8.0.1-RC-20180427 (compiled for
arm-none-eabi target, but I think the problem is not target-specific):

-- >8 -- >8 -- >8 -- >8 -- >8 -- >8 -- >8 -- >8 -- >8 --

$ cat test.cpp

extern "C"
{

void f1()
{
  union some_type{
    char a[2];
    int b;
  } variable;
}

void f2()
{
  union some_type{
    char a[2];
    int b;
  } variable;
}

}

$ arm-none-eabi-gcc test.cpp -c
test.cpp: In function 'void f2()':
test.cpp:17:5: warning: conflicting C language linkage declaration
'f2()::some_type variable'
   } variable;
     ^~~~~~~~
test.cpp:9:5: note: previous declaration 'f1()::some_type variable'
   } variable;
     ^~~~~~~~

$ arm-none-eabi-gcc --version
arm-none-eabi-gcc (bleeding-edge-toolchain) 8.0.1 20180427 (prerelease)
Copyright (C) 2018 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is
NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR
PURPOSE.

-- >8 -- >8 -- >8 -- >8 -- >8 -- >8 -- >8 -- >8 -- >8 --

For the problem to appear:
- functions with the types and variables have to be extern "C"
- the file must be C++
- there has to be both a type and a variable
- the variables must have identical names

The problem was spotted with ARM CMSIS headers, where similar pattern exists:
https://github.com/ARM-software/CMSIS_5/blob/develop/CMSIS/Core/Include/cmsis_gcc.h#L1929

Related discussion on mailing list:
https://gcc.gnu.org/ml/gcc/2018-04/msg00191.html=
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