https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66290
--- Comment #4 from Lewis Hyatt <lhyatt at gcc dot gnu.org> ---
OK, I understand now why done_lexing is necessary, plenty of places call back
into libcpp after lexing, e.g. to interpret strings, and this may generate
warnings.
I think that one line patch is the way to go then.
diff --git a/gcc/c-family/c-opts.cc b/gcc/c-family/c-opts.cc
index b9f01a65ed7..25a3c50de8e 100644
--- a/gcc/c-family/c-opts.cc
+++ b/gcc/c-family/c-opts.cc
@@ -1283,6 +1283,7 @@ c_common_finish (void)
/* For performance, avoid tearing down cpplib's internal structures
with cpp_destroy (). */
+ done_lexing = false;
cpp_finish (parse_in, deps_stream);
if (deps_stream && deps_stream != out_stream && deps_stream != stdout
