c++: Fix location for -Wunused-macros [PR66290]

In C++, since all tokens are lexed from libcpp up front, diagnostics generated
by libcpp after lexing has completed do not get a valid location from libcpp
(rather, libcpp thinks they all pertain to the end of the file.) This has long
been addressed using the global variable "done_lexing", which the C++ frontend
sets at the appropriate time; when done_lexing is true, then c_cpp_diagnostic(),
which outputs libcpp's diagnostics, uses input_location instead of the wrong
libcpp location. The C++ frontend arranges that input_location will point to the
token it is currently processing, so this generally works fine. However, there
is one exception currently, which is -Wunused-macros. This gets generated at the
end of processing in cpp_finish (), since we need to wait until then to
determine whether a macro was eventually used or not. But the locations it
passes to c_cpp_diagnostic () were remembered from the original lexing and hence
they should not be overridden with input_location, which is now the one
incorrectly pointing to the end of the file.

Fixed by setting done_lexing=false again just prior to calling cpp_finish (). I
also renamed the variable from done_lexing to "override_libcpp_locations", since
it's now not strictly about lexing anymore.

There is no new testcase with this patch, since we already had an xfailed
testcase which is now fixed.

gcc/c-family/ChangeLog:

	PR c++/66290
	* c-common.h: Rename global done_lexing to
	override_libcpp_locations.
	* c-common.cc (c_cpp_diagnostic): Likewise.
	* c-opts.cc (c_common_finish): Set override_libcpp_locations
	(formerly done_lexing) immediately prior to calling cpp_finish ().

gcc/cp/ChangeLog:

	PR c++/66290
	* parser.cc (cp_lexer_new_main): Rename global done_lexing to
	override_libcpp_locations.

gcc/testsuite/ChangeLog:

	PR c++/66290
	* c-c++-common/pragma-diag-15.c: Remove xfail for C++.

gcc/c-family/c-common.cc

@@ -284,9 +284,11 @@ int c_inhibit_evaluation_warnings;
    be generated.  */
 bool in_late_binary_op;
 
-/* Whether lexing has been completed, so subsequent preprocessor
-   errors should use the compiler's input_location.  */
-bool done_lexing = false;
+/* Depending on which phase of processing we are in, we may need
+   to prefer input_location to libcpp's locations.  (Specifically,
+   after the C++ lexer is done lexing tokens, but prior to calling
+   cpp_finish (), we need to do so.  */
+bool override_libcpp_locations;
 
 /* Information about how a function name is generated.  */
 struct fname_var_t
@@ -6681,7 +6683,7 @@ c_cpp_diagnostic (cpp_reader *pfile ATTRIBUTE_UNUSED,
     default:
       gcc_unreachable ();
     }
-  if (done_lexing)
+  if (override_libcpp_locations)
     richloc->set_range (0, input_location, SHOW_RANGE_WITH_CARET);
   diagnostic_set_info_translated (&diagnostic, msg, ap,
 				  richloc, dlevel);

gcc/c-family/c-common.h

@@ -767,10 +767,12 @@ extern int max_tinst_depth;
 
 extern int c_inhibit_evaluation_warnings;
 
-/* Whether lexing has been completed, so subsequent preprocessor
-   errors should use the compiler's input_location.  */
+/* Depending on which phase of processing we are in, we may need
+   to prefer input_location to libcpp's locations.  (Specifically,
+   after the C++ lexer is done lexing tokens, but prior to calling
+   cpp_finish (), we need to do so.  */
 
-extern bool done_lexing;
+extern bool override_libcpp_locations;
 
 /* C types are partitioned into three subsets: object, function, and
    incomplete types.  */

gcc/c-family/c-opts.cc

@@ -1281,6 +1281,12 @@ c_common_finish (void)
 	}
     }
 
+  /* When we call cpp_finish (), it may generate some diagnostics using
+     locations it remembered from the preprocessing phase, e.g. for
+     -Wunused-macros.  So inform c_cpp_diagnostic () not to override those
+     locations with input_location, which would be incorrect now.  */
+  override_libcpp_locations = false;
+
   /* For performance, avoid tearing down cpplib's internal structures
      with cpp_destroy ().  */
   cpp_finish (parse_in, deps_stream);

gcc/cp/parser.cc

@@ -755,7 +755,7 @@ cp_lexer_new_main (void)
 
   /* Subsequent preprocessor diagnostics should use compiler
      diagnostic functions to get the compiler source location.  */
-  done_lexing = true;
+  override_libcpp_locations = true;
 
   maybe_check_all_macros (parse_in);
 

gcc/testsuite/c-c++-common/pragma-diag-15.c

@@ -9,5 +9,5 @@
    because the location of the macro definition is incorrectly set.  This is a
    separate issue, will resolve it in a later patch.  */
 
-#define X /* { dg-warning "-:-Wunused-macros" {} { xfail c++ } } */
+#define X /* { dg-warning "-:-Wunused-macros" } */
 #pragma GCC diagnostic ignored "-Wunused-macros"