[gcc r16-7173] libcpp: Fix up comment handling in -fdirectives-only preprocessing [PR123273]

[Jakub Jelinek]

https://gcc.gnu.org/g:1297b7204aca7fbad741ae02086fa5e134083a5f

commit r16-7173-g1297b7204aca7fbad741ae02086fa5e134083a5f
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Fri Jan 30 10:56:10 2026 +0100

    libcpp: Fix up comment handling in -fdirectives-only preprocessing [PR123273]
    
    Back in 2020 Nathan rewrote the -E -fdirectives-only preprocessing.
    In PR103130 a year and half later I've fixed the handling of comments
    so that /* \*/ is considered as full comment even when * is escaped,
    to match the normal preprocessing.
    The following testcases shows further bugs in the comment handling.
    One is that /* *\/ should not be considered as full comment (i.e.
    when the / after * is escaped).  And another one is that the code
    was treating any number of backslashes as escape, which is wrong,
    only a single backslash is an escape, two backslashes preprocess as
    one backslash, three as one backslash and one escape, etc.
    So, while /* *\
    / is a full comment, /* *\\
    / or /* *\\\\\\\\\\\\\
    / is not.
    
    2026-01-30  Jakub Jelinek  <jakub@redhat.com>
    
            PR preprocessor/123273
            * lex.cc (cpp_directive_only_process): Only go to done_comment
            for '/' if star is true and esc is false.  When seeing '\\' with
            esc set to true, clear esc as well as star instead of keeping esc
            set.
    
            * c-c++-common/cpp/dir-only-10.c: New test.
            * c-c++-common/cpp/dir-only-11.c: New test.

Diff:
---
 gcc/testsuite/c-c++-common/cpp/dir-only-10.c |  5 +++++
 gcc/testsuite/c-c++-common/cpp/dir-only-11.c |  6 ++++++
 libcpp/lex.cc                                | 10 ++++++++--
 3 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/gcc/testsuite/c-c++-common/cpp/dir-only-10.c b/gcc/testsuite/c-c++-common/cpp/dir-only-10.c
new file mode 100644
index 000000000000..902d378fd3a2
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/cpp/dir-only-10.c
@@ -0,0 +1,5 @@
+/* PR preprocessor/123273 */
+/* { dg-do preprocess } */
+/* { dg-options -fdirectives-only } */
+
+/* *\/""" */
diff --git a/gcc/testsuite/c-c++-common/cpp/dir-only-11.c b/gcc/testsuite/c-c++-common/cpp/dir-only-11.c
new file mode 100644
index 000000000000..33437bd508cb
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/cpp/dir-only-11.c
@@ -0,0 +1,6 @@
+/* PR preprocessor/123273 */
+/* { dg-do preprocess } */
+/* { dg-options -fdirectives-only } */
+
+/* *\\
+/""" */
diff --git a/libcpp/lex.cc b/libcpp/lex.cc
index df278534eeee..63f7a5fe5b8a 100644
--- a/libcpp/lex.cc
+++ b/libcpp/lex.cc
@@ -5490,7 +5490,13 @@ cpp_directive_only_process (cpp_reader *pfile,
 		    switch (c)
 		      {
 		      case '\\':
-			esc = true;
+			if (esc)
+			  {
+			    star = false;
+			    esc = false;
+			  }
+			else
+			  esc = true;
 			break;
 
 		      case '\r':
@@ -5521,7 +5527,7 @@ cpp_directive_only_process (cpp_reader *pfile,
 			break;
 
 		      case '/':
-			if (star)
+			if (star && !esc)
 			  goto done_comment;
 			/* FALLTHROUGH  */