[BUG] Passthrough fails for #if with unbalanced brace on both branches

[simonbuchan]

Describe the bug
When the pre-processor is being used to switch between code that e.g. starts a block with different code, the braces are being counted in both branches, causing it to expect two closing braces.

To Reproduce

#if WINDOWS
int WinMain(HINSTANCE hinstance, HINSTANCE, LPSTR cmdline, int showCmd) {
#else
void main(int argc, char** argv) {
#endif
  // do something cool
}

gives

scratch.cpp2(8,0): error: end of file reached with 1 missing } to match earlier { on line 2

Additional context
Hit this porting some real code (that I didn't write), so this is a potential migration stumbling block.

I don't think it's plausible to catch everything a pre-processor can do, of course, but perhaps some form of pass-through for old syntax so long as the braces in both branches are equal would be fine.

[hsutter]

Thanks! That's good practical feedback and a pragmatic solution proposal. I do care about real-world code and want to reduce adoption friction, even if it can't be bulletproof without implementing the preprocessor and doing macro expansion (which I don't want to do that this point).

perhaps some form of pass-through for old syntax so long as the braces in both branches are equal

OK, that seems like a reasonable heuristic... done, see above commit. It should now handle a net-positive number of braces introduced in both branches. I didn't try to deal with branches that introduce the same net-negative number of braces; that would require a bit more work.

Thanks again!

[simonbuchan]

No problem! It's pretty much why I was throwing some real code at it in the first place, afterall.