[llvm-bugs] [Bug 31311] New: undef doesn't work well with pch files

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https://llvm.org/bugs/show_bug.cgi?id=31311

            Bug ID: 31311
           Summary: undef doesn't work well with pch files
           Product: clang
           Version: unspecified
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Frontend
          Assignee: unassignedclangbugs at nondot.org
          Reporter: nicolasweber at gmx.de
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

$ cat foo2.h
#undef ARG
$ cat foo2.c
const int i = ARG;
$ ./bin/clang -cc1 -DARG -include foo2.h foo2.c -emit-obj -o a.o
foo2.c:1:15: error: use of undeclared identifier 'ARG'
const int i = ARG;
              ^
1 error generated.
$ ./bin/clang -cc1 -emit-pch -o foo2.h.pch foo2.h
$ ./bin/clang -cc1 -DARG -include-pch foo2.h.pch foo2.c -emit-obj -o a.o


This should err when going through a pch as well.


Similar but slightly more difficult to fix: The same with a builtin macro:

$ cat foo2.h
#undef __FILE__
$ cat foo2.c
const char s[] = __FILE__;
s$ bin/clang -cc1 -include foo2.h foo2.c -emit-obj -o a.o
In file included from <built-in>:1:
./foo2.h:1:8: warning: undefining builtin macro
#undef __FILE__
       ^
foo2.c:1:18: error: use of undeclared identifier '__FILE__'
const char s[] = __FILE__;
                 ^
1 warning and 1 error generated.

$ ./bin/clang -cc1 -emit-pch -o foo2.h.pch foo2.h
foo2.h:1:8: warning: undefining builtin macro
#undef __FILE__
       ^
1 warning generated.
$ ./bin/clang -cc1 -include-pch foo2.h.pch foo2.c -emit-obj -o a.o


This too should error out but doesn't.


Problems, partial list:

1. shouldIgnoreMacro() in ASTWriter.cpp stops at isBuiltinMacro(), but that's
true for undefs of builtin macros as well. It probably should have `if
(MD->getKind() != MacroDirective::MD_Define) return false;` as its first line.

2. ASTWriter only writes out identifiers with a ref (see AddIdentifierRef).
ASTWriter::WritePreprocessor() only calls this for macros with a MacroInfo, and
macros that are only #undef'd but never defined (at pch creation time) get
dropped.

3. It seems that pch files get loaded before commandline -D flags are
processed, and Preprocessor::HandleUndefDirective() ignores undefs for macros
without a MacroInfo (which makes sense) (not really sure if the ordering here
is like I say, but if it weren't then `#define FOO` in the .h and -DFOO on the
command passing -include-pch would behave differently)

(Spin-off from bug 29119)

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