c++: warn about using keywords as identifiers [PR106111]

In C++03, -Wc++11-compat should warn about

  int constexpr;

since 'constexpr' is a keyword in C++11.  Jonathan reports that
we don't emit a similar warning for 'alignas' or 'alignof', and,
as I found out, 'thread_local'.

Similarly, we don't warn for most C++20 keywords.  That happens
because RID_LAST_CXX20 hasn't been updated in a while.

	PR c++/106111

gcc/c-family/ChangeLog:

	* c-common.h (enum rid): Update RID_LAST_CXX20.

gcc/cp/ChangeLog:

	* parser.cc (cp_lexer_get_preprocessor_token): Also warn about
	RID_ALIGNOF, RID_ALIGNAS, RID_THREAD.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/keywords1.C: New test.
	* g++.dg/cpp2a/keywords1.C: New test.

4 files changed, 35 insertions, 4 deletions

diff --git a/gcc/c-family/c-common.h b/gcc/c-family/c-common.h
index 47442c9..a1e6a55 100644
--- a/gcc/c-family/c-common.h
+++ b/gcc/c-family/c-common.h
@@ -271,7 +271,7 @@ enum rid
   RID_FIRST_CXX11 = RID_CONSTEXPR,
   RID_LAST_CXX11 = RID_STATIC_ASSERT,
   RID_FIRST_CXX20 = RID_CONSTINIT,
-  RID_LAST_CXX20 = RID_CONSTINIT,
+  RID_LAST_CXX20 = RID_CO_RETURN,
   RID_FIRST_AT = RID_AT_ENCODE,
   RID_LAST_AT = RID_AT_IMPLEMENTATION,
   RID_FIRST_PQ = RID_IN,
diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index f6bc8db..c5d1f9d 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -890,10 +890,14 @@ cp_lexer_get_preprocessor_token (unsigned flags, cp_token *token)
       else
 	{
           if (warn_cxx11_compat
-              && C_RID_CODE (token->u.value) >= RID_FIRST_CXX11
-              && C_RID_CODE (token->u.value) <= RID_LAST_CXX11)
+	      && ((C_RID_CODE (token->u.value) >= RID_FIRST_CXX11
+		   && C_RID_CODE (token->u.value) <= RID_LAST_CXX11)
+		  /* These are outside the CXX11 range.  */
+		  || C_RID_CODE (token->u.value) == RID_ALIGNOF
+		  || C_RID_CODE (token->u.value) == RID_ALIGNAS
+		  || C_RID_CODE (token->u.value)== RID_THREAD))
             {
-              /* Warn about the C++0x keyword (but still treat it as
+	      /* Warn about the C++11 keyword (but still treat it as
                  an identifier).  */
 	      warning_at (token->location, OPT_Wc__11_compat,
 			  "identifier %qE is a keyword in C++11",
diff --git a/gcc/testsuite/g++.dg/cpp0x/keywords1.C b/gcc/testsuite/g++.dg/cpp0x/keywords1.C
new file mode 100644
index 0000000..2b2ab64
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/keywords1.C
@@ -0,0 +1,15 @@
+// PR c++/106111
+// { dg-do compile { target c++98_only } }
+// { dg-options "-Wc++11-compat" }
+
+int alignof; // { dg-warning "is a keyword in C\\\+\\\+11" }
+int alignas; // { dg-warning "is a keyword in C\\\+\\\+11" }
+int constexpr; // { dg-warning "is a keyword in C\\\+\\\+11" }
+int decltype; // { dg-warning "is a keyword in C\\\+\\\+11" }
+int noexcept; // { dg-warning "is a keyword in C\\\+\\\+11" }
+int nullptr; // { dg-warning "is a keyword in C\\\+\\\+11" }
+int static_assert; // { dg-warning "is a keyword in C\\\+\\\+11" }
+int thread_local; // { dg-warning "is a keyword in C\\\+\\\+11" }
+int _Alignas;
+int _Alignof;
+int _Thread_local;
diff --git a/gcc/testsuite/g++.dg/cpp2a/keywords1.C b/gcc/testsuite/g++.dg/cpp2a/keywords1.C
new file mode 100644
index 0000000..7f4dba2
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/keywords1.C
@@ -0,0 +1,12 @@
+// PR c++/106111
+// { dg-do compile { target c++17_down } }
+// { dg-options "-Wc++20-compat -Wc++11-compat -Wc++14-compat -Wc++17-compat" }
+
+int constinit; // { dg-warning "is a keyword in C\\\+\\\+20" }
+int consteval; // { dg-warning "is a keyword in C\\\+\\\+20" }
+int requires; // { dg-warning "is a keyword in C\\\+\\\+20" }
+int concept; // { dg-warning "is a keyword in C\\\+\\\+20" }
+int co_await; // { dg-warning "is a keyword in C\\\+\\\+20" }
+int co_yield; // { dg-warning "is a keyword in C\\\+\\\+20" }
+int co_return; // { dg-warning "is a keyword in C\\\+\\\+20" }
+int char8_t; // { dg-warning "is a keyword in C\\\+\\\+20" }