From: Jakub Jelinek <jakub@redhat.com>
Date: Mon, 4 Aug 2025 12:27:14 +0000 (+0200)
Subject: libcpp: Use pedwarn instead of warning for CWG2578 diagnostics [PR120778]
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=b9f443b6177881c490fbce9ba65b86a7cd65f11f;p=thirdparty%2Fgcc.git

libcpp: Use pedwarn instead of warning for CWG2578 diagnostics [PR120778]

This is another case which changed from compile time undefined behavior
to ill-formed, diagnostic required.  Now, we warn on this, so pedantically
that is good enough, maybe all we need is a testcase, but the following
patch changes it to a pedwarn for C++26.

2025-08-04  Jakub Jelinek  <jakub@redhat.com>

	PR preprocessor/120778
	* macro.cc (stringify_arg): For C++26 emit a pedarn instead of warning
	for \ at the end of stringification.

	* g++.dg/DRs/dr2578.C: New test.
---

diff --git a/gcc/testsuite/g++.dg/DRs/dr2578.C b/gcc/testsuite/g++.dg/DRs/dr2578.C
new file mode 100644
index 00000000000..0dce23b9464
--- /dev/null
+++ b/gcc/testsuite/g++.dg/DRs/dr2578.C
@@ -0,0 +1,10 @@
+// DR 2578 - Undefined behavior when creating an invalid string literal via stringizing
+// { dg-do preprocess }
+// { dg-options "-pedantic-errors" }
+
+#define A(a) #a
+#define B(a) A(a)
+#define C \\
+
+const char *x = B(C);	// { dg-warning "invalid string literal, ignoring final '\\\\'" "" { target c++23_down } }
+// { dg-error "invalid string literal, ignoring final '\\\\'" "" { target c++26 }  .-1 }
diff --git a/libcpp/macro.cc b/libcpp/macro.cc
index be257103471..d869b023935 100644
--- a/libcpp/macro.cc
+++ b/libcpp/macro.cc
@@ -1003,7 +1003,10 @@ stringify_arg (cpp_reader *pfile, const cpp_token **first, unsigned int count)
   /* Ignore the final \ of invalid string literals.  */
   if (backslash_count & 1)
     {
-      cpp_error (pfile, CPP_DL_WARNING,
+      cpp_error (pfile,
+		 CPP_OPTION (pfile, cplusplus)
+		 && CPP_OPTION (pfile, lang) >= CLK_GNUCXX26
+		 ? CPP_DL_PEDWARN : CPP_DL_WARNING,
 		 "invalid string literal, ignoring final %<\\%>");
       dest--;
     }