c++: Add testcase for CWG2576 [PR120778]

Another part of the C++26 P2843R3, this time in https://eel.is/c++draft/cpp.include#7
changing former (compile time) undefined behavior to IFNDR.
With IFNDR we wouldn't have to test anything I guess, but this still adds
the cases from the 3.4 section of the paper plus related tests.
The paper wonders about implementation divergence between most compilers and
gcc in the case of glueing tokens together with <> around from multiple
macros, GCC doesn't add CPP_PADDING macros in that case (and even if it
would, it would ignore them through get_token_no_padding).
But I think this behavior is allowed by
https://eel.is/c++draft/cpp.include#7.sentence-3
and changing it now after 25+ years of such behavior might break real-world
stuff, especially making it really hard to construct the <> includes from
some name and adding < and > from some other macro around that.  We handle
  #define A <cstd def>
  #include A
like clang++ and try to open 'cstd def' file.  But not adding spaces for
all the padding means handling even stuff where nothing else can help,
while for cstd def coming from different macros one can use token pasting
to combine them together, e.g. < and following identifier or identifier
followed by . or . followed by identifier or identifier followed by > can't
be pasted together.

2025-08-07  Jakub Jelinek  <jakub@redhat.com>

	PR preprocessor/120778
	* g++.dg/DRs/dr2576.C: New test.

0 files changed, 0 insertions, 0 deletions