c: Add C2Y N3370 - Case range expressions support [PR117021]

The following patch adds the C2Y N3370 paper support.
We had the case ranges as a GNU extension for decades, so this patch
simply:
1) adds different diagnostics when it is used in C (depending on flag_isoc2y
   and pedantic and warn_c23_c2y_compat)
2) emits a pedwarn in C if in a range conversion changes the value of
   the low or high bounds and in that case doesn't emit -Woverflow and
   similar warnings anymore if the pedwarn has been diagnosed
3) changes the handling of empty ranges both in C and C++; previously
   we just warned but let the values be still looked up in the splay
   tree/entered into it (and let only gimplification throw away those
   empty cases), so e.g. case -6 ... -8: break; case -6: break;
   complained about duplicate case label.  But that actually isn't
   duplicate case label, case -6 ... -8: stands for nothing at all
   and that is how it is treated later on (thrown away)

2024-10-30  Jakub Jelinek  <jakub@redhat.com>

	PR c/117021
gcc/c-family/
	* c-common.cc (c_add_case_label): Emit different diagnostics for C
	on case ranges.  Diagnose for C using pedwarn conversions of range
	expressions changing value and don't emit further conversion
	diagnostics if the pedwarn has been diagnosed.  For empty ranges
	bail out after emitting warning, don't add anything into splay
	trees nor add a CASE_LABEL_EXPR.
gcc/testsuite/
	* gcc.dg/switch-6.c: Expect different diagnostics.  Add -std=gnu23
	to dg-options.
	* gcc.dg/switch-7.c: Expect different diagnostics.  Add -std=c23
	to dg-options.
	* gcc.dg/gnu23-switch-1.c: New test.
	* gcc.dg/gnu23-switch-2.c: New test.
	* gcc.dg/c23-switch-1.c: New test.
	* gcc.dg/c2y-switch-1.c: New test.
	* gcc.dg/c2y-switch-2.c: New test.
	* gcc.dg/c2y-switch-3.c: New test.

0 files changed, 0 insertions, 0 deletions