[llvm-bugs] [Bug 24387] New: Precompiled headers don't work with `#pragma once`

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https://llvm.org/bugs/show_bug.cgi?id=24387

            Bug ID: 24387
           Summary: Precompiled headers don't work with `#pragma once`
           Product: clang
           Version: 3.6
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: hitchmanr at gmail.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

#pragma once information isn't stored in precompiled headers. This makes PCH
useless for projects that use #pragma once as the only form of header guard.

Test case:

a.h:

    #pragma once
    static int x = 3;

a.c:

    #include "a.h"
    int main(void) { return 0; }

then:

    $ clang a.c -o a
    $ clang a.h -o a.h.pch
    a.h:1:9: warning: #pragma once in main file
    #pragma once
            ^
    1 warning generated.
    $ clang -include-pch a.h.pch a.c -o a
    In file included from a.c:1:
    ./a.h:3:12: error: redefinition of 'x'
    static int x = 3;
               ^
    ./a.h:3:12: note: previous definition is here
    static int x = 3;
               ^
    1 error generated.

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