[llvm-bugs] [Bug 31886] New: in -traditional mode, the preprocessor should only process directives whose '#' appears in column 1

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https://llvm.org/bugs/show_bug.cgi?id=31886

            Bug ID: 31886
           Summary: in -traditional mode, the preprocessor should only
                    process directives whose '#' appears in column 1
           Product: clang
           Version: unspecified
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: froydnj at gmail.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

Relaying this bug report on behalf of Zack Weinberg:

In -traditional mode, the preprocessor should only process directives
whose '#' appears in column 1.  For instance, in this fragment ...

    #ifndef foo
        #ifdef bar
            quux
        #endif
    #endif

... no tokens survive preprocessing in the standard mode, but the
expected output of 'cc -E -traditional' (ignoring blank lines and line
number annotations) is

        #ifdef bar
            quux
        #endif

clang, however, has a bug in which the nested #endif is *not* ignored,
causing the subsequent non-nested #endif to trigger an error:

        #ifdef bar
            quux
    test.c:5:2: error: #endif without #if
    #endif
     ^
    1 error generated.

This may not just affect #endif: clang preprocesses *this* fragment ...

    /* this comment is not indented */
        #ifdef bar
            quux
        #else
            greeble
        #endif

... to just 'greeble' and no errors, with or without -traditional.

I observe this behavior with both
'clang version 3.8.1-17 (tags/RELEASE_381/final)' and
'clang version 3.9.1-4 (tags/RELEASE_391/rc2)' as supplied by Debian.
Another person has reported identical behavior with
'Apple LLVM version 8.0.0 (clang-800.0.42.1)'.

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