libclang: unterminated conditional directive when generating preamble

[Quuxplusone]

Bugzilla Link	PR35029
Status	NEW
Importance	P normal
Reported by	Ben Jackson (puremourning@gmail.com)
Reported on	2017-10-22 11:23:16 -0700
Last modified on	2017-10-26 16:08:45 -0700
Version	5.0
Hardware	All All
CC	contact@micbou.com, llvm-bugs@lists.llvm.org
Fixed by commit(s)
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Since clang 5.0 (and currently in master), libclang reports "unterminated conditional directive" when an "include guarded" file transitively includes itself. Or at least that's how it appears.

There are two simple test cases, both of which compile fine when not in "editing" mode, but raise "unterminated conditional directive" when building a preamble.

More info: https://github.com/Valloric/YouCompleteMe/issues/2795

Tests reproduced with both clang 5.0 prebuilt binaries and a build of master as of yesterday (Sat 21st October).

Test case 1: Header includes itself.

#ifndef TEST_C
#define TEST_C

#include "test.c"

int main() {}

#endif

Normal compilation

BeniMac:basic ben$ /Users/ben/Development/llvm.git-build/bin/c-index-test -test-load-source local circular.h
// CHECK: <invalid loc>:2:9: macro definition=__llvm__
// CHECK: <invalid loc>:3:9: macro definition=__clang__
// CHECK: <invalid loc>:4:9: macro definition=__clang_major__
// CHECK: <invalid loc>:5:9: macro definition=__clang_minor__
// CHECK: <invalid loc>:6:9: macro definition=__clang_patchlevel__
...
works.

In editing mode:

BeniMac:basic ben$ CINDEXTEST_EDITING=1  /Users/ben/Development/llvm.git-build/bin/c-index-test -test-load-source local circular.h
// CHECK: circular.h:6:5: FunctionDecl=main:6:5 (Definition) Extent=[6:1 - 6:14]
// CHECK: circular.h:6:12: CompoundStmt= Extent=[6:12 - 6:14]
./circular.h:1:2: error: unterminated conditional directive
Number FIX-ITs = 0

Test case 2 (perhaps more realistic in editor scenarios, where users use -include b.h to make completion work within a.h):

#include "b.h"
#ifndef A_H
#define A_H
int main() {}
#endif /* A_H */

#ifndef B_H
#define B_H
#include "a.h"
#endif /* B_H */

Again, compilation is fine until using the editor flags:

BeniMac:basic ben$ CINDEXTEST_EDITING=1  /Users/ben/Development/llvm.git-build/bin/c-index-test -test-load-source local a.h
// CHECK: a.h:4:5: FunctionDecl=main:4:5 (Definition) Extent=[4:1 - 4:14]
// CHECK: a.h:4:12: CompoundStmt= Extent=[4:12 - 4:14]
./a.h:2:2: error: unterminated conditional directive
Number FIX-ITs = 0

I suspect (truly a guess), that this commit is the culprit, though I haven't proven it: https://reviews.llvm.org/D15994#95a083c2

[Quuxplusone]

Correction on test case 1, file name was wrong:

```circular.h
#ifndef TEST_C
#define TEST_C

#include "circular.h"

int main() {}

#endif