c++: Add tests for P2621, no UB in lexer [PR110340]

C++26 P2621 removes UB in the lexer and either makes the construct valid
or ill-formed.  We're already handling this correctly so this patch only
adds tests.

	PR c++/110340

gcc/testsuite/ChangeLog:

	* g++.dg/cpp/string-4.C: New test.
	* g++.dg/cpp/ucn-2.C: New test.

2 files changed, 21 insertions, 0 deletions

diff --git a/gcc/testsuite/g++.dg/cpp/string-4.C b/gcc/testsuite/g++.dg/cpp/string-4.C
new file mode 100644
index 0000000..37d0388
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp/string-4.C
@@ -0,0 +1,6 @@
+// P2621R2 - UB? In My Lexer?
+// { dg-do compile }
+
+// [lex.phases] If a U+0027 APOSTROPHE or a U+0022 QUOTATION
+// MARK matches the last category, the program is ill-formed.
+const char * foo=" // { dg-error "terminating|expected" }
diff --git a/gcc/testsuite/g++.dg/cpp/ucn-2.C b/gcc/testsuite/g++.dg/cpp/ucn-2.C
new file mode 100644
index 0000000..c5583e0
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp/ucn-2.C
@@ -0,0 +1,15 @@
+// P2621R2 - UB? In My Lexer?
+// { dg-do compile }
+
+// Line splicing can form a universal-character-name [lex.charset].
+int \\
+u\
+0\
+3\
+9\
+1 = 0;
+
+// [cpp.concat] Concatenation can form a universal-character-name.
+#define CONCAT(x,y) x##y
+
+int CONCAT(\,u0393)=0;