Clang format breaks macro by removing new lines

[llvmbot]

Bugzilla Link	42742
Version	unspecified
OS	Linux
Attachments	Header and dot-Clang format config
Reporter	LLVM Bugzilla Contributor

Extended Description

The following macros become inlined and so cannot be compiled:

#ifndef badmacrosh
#define badmarcosh

#define NS_FIRSTBAD_NAMESPC              namespace firstbad
#define BEGIN_NS_FIRSTBAD_NAMESPC        NS_FIRSTBAD_NAMESPC {
#define END_NS_FIRSTBAD_NAMESPC          }
#define USING_NS_FIRSTBAD_NAMESPC        using NS_FIRSTBAD_NAMESPC

#define NS_SECONDBAD_NAMESPC             namespace secondbad
#define BEGIN_NS_SECONDBAD_NAMESPC       BEGIN_NS_FIRSTBAD_NAMESPC NS_SECONDBAD_NAMESPC {
#define END_NS_SECONDBAD_NAMESPC         } END_NS_FIRSTBAD_NAMESPC
#define USING_NS_SECONDBAD_NAMESPC       USING_NS_FIRSTBAD_NAMESPC ; using NS_SECONDBAD_NAMESPC

#endif

becomes:

#ifndef badmacrosh
#define badmarcosh

#define NS_FIRSTBAD_NAMESPC namespace firstbad
#define BEGIN_NS_FIRSTBAD_NAMESPC                                                                                      \
    NS_FIRSTBAD_NAMESPC { #define END_NS_FIRSTBAD_NAMESPC }
#define USING_NS_FIRSTBAD_NAMESPC using NS_FIRSTBAD_NAMESPC

#define NS_SECONDBAD_NAMESPC namespace secondbad
#define BEGIN_NS_SECONDBAD_NAMESPC                                                                                     \
    BEGIN_NS_FIRSTBAD_NAMESPC NS_SECONDBAD_NAMESPC { #define END_NS_SECONDBAD_NAMESPC }                                \
    END_NS_FIRSTBAD_NAMESPC
#define USING_NS_SECONDBAD_NAMESPC                                                                                     \
    USING_NS_FIRSTBAD_NAMESPC;                                                                                         \
    using NS_SECONDBAD_NAMESPC

#endif

Header and .clang-format attached

[mkurdej]

It is definitely related to brace wrapping, as I can reproduce it on current main (2022-02-02) with config:

BasedOnStyle:  LLVM
BreakBeforeBraces: Allman

or

BasedOnStyle:  LLVM
BreakBeforeBraces: Custom
BraceWrapping:
  AfterFunction: true

and smaller reproducer:

#define BEGIN        NS {
#define END          }

[mkurdej]

FWIW, I have a quick fix that prevents merging together the two macro definitions (and hence the code does not break anymore), but the formatting is strange.
The code:

#define BEGIN        NS {
#define END          }

becomes

#define BEGIN                                                                  \
  NS                                                                           \
  {
#define END }

Config as above:

BasedOnStyle:  LLVM
BreakBeforeBraces: Custom
BraceWrapping:
  AfterFunction: true

This partial fix is to add:

    // Don't merge with a preprocessor directive.
    if (I[1]->Type == LT_PreprocessorDirective)
      return 0;

at the beginning of LineJoiner::tryMergeSimpleBlock.

[mkurdej]

Revision URI: https://reviews.llvm.org/D118879.