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 (file)
index 0000000..0dce23b
--- /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 be2571034718638751929c830e2c1d76fc167e0e..d869b0239350708f7f5b8d1d2786bfeb0132c964 100644 (file)
--- 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--;
     }