libcpp: Fix up comma diagnostics in preprocessor for C++ [PR120778]

libcpp: Fix up comma diagnostics in preprocessor for C++ [PR120778]

The P2843R3 Preprocessing is never undefined paper contains comments
that various compilers handle comma operators in preprocessor expressions
incorrectly and I think they are right.

In both C and C++ the grammar uses constant-expression non-terminal
for #if/#elif and in both C and C++ that NT is conditional-expression,
so
  #if 1, 2
is IMHO clearly wrong in both languages.

C89 then says for constant-expression
"Constant expressions shall not contain assignment, increment, decrement,
function-call, or comma operators, except when they are contained within the
operand of a sizeof operator."
Because all the remaining identifiers in the #if/#elif expression are
replaced with 0 I think assignments, increment, decrement and function-call
aren't that big deal because (0 = 1) or ++4 etc. are all invalid, but
for comma expressions I think it matters.  In r0-56429 PR456 Joseph has
added !CPP_OPTION (pfile, c99) to handle that correctly.
Then C99 changed that to:
"Constant expressions shall not contain assignment, increment, decrement, function-call,
or comma operators, except when they are contained within a subexpression that is not
evaluated."
That made for C99+
  #if 1 || (1, 2)
etc. valid but
  #if (1, 2)
is still invalid, ditto
  #if 1 ? 1, 2 : 3

In C++ I can't find anything like that though, and as can be seen on say
int a[(1, 2)];
int b[1 ? 1, 2 : 3];
being accepted by C++ and rejected by C while
int c[1, 2];
int d[1 ? 2 : 3, 4];
being rejected in both C and C++, so I think for C++ it is indeed just the
grammar that prevents #if 1, 2.  When it is the second operand of ?: or
inside of () the grammar just uses expression and that allows comma
operator.

So, the following patch uses different decisions for C++ when to diagnose
comma operator in preprocessor expressions, for C++ tracks if it is inside
of () (obviously () around #embed clauses don't count unless one uses
limit ((1, 2)) etc.) or inside of the second ?: operand and allows comma
operator there and disallows elsewhere.

BTW, I wonder if anything in the standard disallows <=> in the preprocessor
expressions.  Say
  #if (0 <=> 1) < 0
etc.
  #include <compare>
  constexpr int a = (0 <=> 1) < 0;
is valid (but not valid without #include <compare>) and the expressions
don't use any identifiers.

2025-07-30  Jakub Jelinek  <jakub@redhat.com>

	PR c++/120778
	* internal.h (struct lexer_state): Add comma_ok member.
	* expr.cc (_cpp_parse_expr): Initialize it to 0, increment on
	CPP_OPEN_PAREN and CPP_QUERY, decrement on CPP_CLOSE_PAREN
	and CPP_COLON.
	(num_binary_op): For C++ pedwarn on comma operator if
	pfile->state.comma_ok is 0 instead of !c99 or skip_eval.

	* g++.dg/cpp/if-comma-1.C: New test.