[gcc r9-10116] libcpp: Fix up ##__VA_OPT__ handling [PR89971]

[Jakub Jelinek <jakub@gcc.gnu.org>]

https://gcc.gnu.org/g:86b98701bf927ea438354723932e623557c04e42

commit r9-10116-g86b98701bf927ea438354723932e623557c04e42
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Thu Dec 30 22:23:58 2021 +0100

    libcpp: Fix up ##__VA_OPT__ handling [PR89971]
    
    In the following testcase we incorrectly error about pasting / token
    with padding token (which is a result of __VA_OPT__); instead we should
    like e.g. for ##arg where arg is empty macro argument clear PASTE_LEFT
    flag of the previous token if __VA_OPT__ doesn't add any real tokens
    (which can happen either because the macro doesn't have any tokens
    passed to ... (i.e. __VA_ARGS__ expands to empty) or when __VA_OPT__
    doesn't have any tokens in between ()s).
    
    2021-12-30  Jakub Jelinek  <jakub@redhat.com>
    
            PR preprocessor/89971
    libcpp/
            * macro.c (replace_args): For ##__VA_OPT__, if __VA_OPT__ expands
            to no tokens at all, drop PASTE_LEFT flag from the previous token.
    gcc/testsuite/
            * c-c++-common/cpp/va-opt-9.c: New test.
    
    (cherry picked from commit 5545d1edcbdb1701443f94dde7ec97c5ce3e1a6c)

Diff:
---
 gcc/testsuite/c-c++-common/cpp/va-opt-9.c | 20 ++++++++++++++++++++
 libcpp/macro.c                            |  5 +++++
 2 files changed, 25 insertions(+)

diff --git a/gcc/testsuite/c-c++-common/cpp/va-opt-9.c b/gcc/testsuite/c-c++-common/cpp/va-opt-9.c
new file mode 100644
index 00000000000..3f1d42b352f
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/cpp/va-opt-9.c
@@ -0,0 +1,20 @@
+/* PR preprocessor/89971 */
+/* { dg-do compile } */
+/* { dg-options "-std=gnu99" { target c } } */
+/* { dg-options "-std=c++2a" { target c++ } } */
+
+int a, c;
+#define m1(...) a /##__VA_OPT__(b) c
+#define m2(...) a /##__VA_OPT__() c
+#define m3(...) a##__VA_OPT__()##b = 1
+#define m4(...) a##__VA_OPT__(b c d)##e = 2
+
+int
+foo (void)
+{
+  int d = m1();
+  int e = m2(1);
+  int m3(1 2 3);
+  int m4();
+  return d + e + ab + ae;
+}
diff --git a/libcpp/macro.c b/libcpp/macro.c
index 30d3686451c..8dcc21de10a 100644
--- a/libcpp/macro.c
+++ b/libcpp/macro.c
@@ -1886,6 +1886,11 @@ replace_args (cpp_reader *pfile, cpp_hashnode *node, cpp_macro *macro,
 		  paste_flag = tokens_buff_last_token_ptr (buff);
 		}
 
+	      if (start && paste_flag == start && (*start)->flags & PASTE_LEFT)
+		/* If __VA_OPT__ expands to nothing (either because __VA_ARGS__
+		   is empty or because it is __VA_OPT__() ), drop PASTE_LEFT
+		   flag from previous token.  */
+		copy_paste_flag (pfile, start, &pfile->avoid_paste);
 	      if (src->flags & PASTE_LEFT)
 		{
 		  /* With a non-empty __VA_OPT__ on the LHS of ##, the last