preprocessor: Enable __VA_OPT__ for C2x

C2x supports __VA_OPT__, so adjust libcpp not to pedwarn for uses of
it (or of not passing any variable arguments to a variable-arguments
macro) in standard C2x mode.

I didn't try to duplicate existing tests for the details of the
feature, just verified -pedantic-errors handling is as expected.  And
there's a reasonable argument (bug 98859) that __VA_OPT__ shouldn't be
diagnosed in older standard modes at all (as opposed to not passing
any variable arguments to a variable-arguments macro, for which older
versions of the C standard require a diagnostic as a constraint
violation); that argument applies to C as much as to C++, but I
haven't made any changes in that regard.

Bootstrapped with no regressions for x86_64-pc-linux-gnu.

libcpp/
	* init.cc (lang_defaults): Enable va_opt for STDC2X.
	* lex.cc (maybe_va_opt_error): Adjust diagnostic message for C.
	* macro.cc (_cpp_arguments_ok): Update comment.

gcc/testsuite/
	* gcc.dg/cpp/c11-vararg-1.c, gcc.dg/cpp/c2x-va-opt-1.c: New tests.

2 files changed, 20 insertions, 0 deletions

diff --git a/gcc/testsuite/gcc.dg/cpp/c11-vararg-1.c b/gcc/testsuite/gcc.dg/cpp/c11-vararg-1.c
new file mode 100644
index 0000000..6b1bc38
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/cpp/c11-vararg-1.c
@@ -0,0 +1,9 @@
+/* Test error in C11 for no arguments passed for variable arguments to a
+   macro.  */
+/* { dg-do preprocess } */
+/* { dg-options "-std=c11 -pedantic-errors" } */
+
+#define M(X, ...) X
+
+M (x); /* { dg-error "requires at least one argument" } */
+M (x, y);
diff --git a/gcc/testsuite/gcc.dg/cpp/c2x-va-opt-1.c b/gcc/testsuite/gcc.dg/cpp/c2x-va-opt-1.c
new file mode 100644
index 0000000..bd438f7
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/cpp/c2x-va-opt-1.c
@@ -0,0 +1,11 @@
+/* Test __VA_OPT__ and no "..." arguments in a call to a variable-arguments
+   macro accepted for C2X.  */
+/* { dg-do preprocess } */
+/* { dg-options "-std=c2x -pedantic-errors" } */
+
+#define CALL(F, ...) F (7 __VA_OPT__(,) __VA_ARGS__)
+#define M(X, ...) X
+
+CALL (a);
+CALL (b, 1);
+M (x);