libcpp: Ignore CPP_PADDING tokens in _cpp_parse_expr [PR105732]

The first part of the following testcase (m1-m3 macros and its use)
regressed with my PR89971 fix, but as the m1,m4-m5 and its use part shows,
the problem isn't new, we can emit a CPP_PADDING token to avoid it from
being adjacent to whatever comes after the __VA_OPT__ (in this case there
is nothing afterwards, true).

In most cases these CPP_PADDING tokens don't matter, all other
callers of cpp_get_token_with_location either ignore CPP_PADDING tokens
completely (e.g. c_lex_with_flags) or they just remember them and
take them into account when printing stuff whether there should be
added whitespace or not (scan_translation_unit + token_streamer::stream).
So, I think we should just ignore CPP_PADDING tokens the same way in
_cpp_parse_expr.

2022-05-27  Jakub Jelinek  <jakub@redhat.com>

	PR preprocessor/105732
	* expr.cc (_cpp_parse_expr): Handle CPP_PADDING by just another
	token.

	* c-c++-common/cpp/va-opt-10.c: New test.

(cherry picked from commit 58a40e76eb)

gcc/testsuite/c-c++-common/cpp/va-opt-10.c

@@ -0,0 +1,18 @@
+/* PR preprocessor/105732 */
+/* { dg-do compile } */
+/* { dg-options "-std=gnu99" { target c } } */
+/* { dg-options "-std=c++20" { target c++ } } */
+
+#define m1(p1, p2, p3) p3
+#define m2(p1, ...) 1##__VA_OPT__(foo)
+#define m3(...) m1(1, 2, m2)
+#define m4(p1, ...) 1 __VA_OPT__()
+#define m5(...) m1(1, 2, m4)
+#if m3(,)(,)
+#else
+#error
+#endif
+#if m5(,)(,)
+#else
+#error
+#endif

libcpp/expr.cc

@@ -1366,6 +1366,10 @@ _cpp_parse_expr (cpp_reader *pfile, bool is_if)
 	    op.op = CPP_UMINUS;
 	  break;
 
+	case CPP_PADDING:
+	  lex_count--;
+	  continue;
+
 	default:
 	  if ((int) op.op <= (int) CPP_EQ || (int) op.op >= (int) CPP_PLUS_EQ)
 	    SYNTAX_ERROR2_AT (op.loc,