https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117942
--- Comment #3 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
--- a/gcc/opts-common.cc
+++ b/gcc/opts-common.cc
@@ -1077,7 +1077,9 @@ decode_cmdline_options_to_array (unsigned int argc, const
char **argv,
/* Expand -fdiagnostics-plain-output to its constituents. This needs
to happen here so that prune_options can handle -fdiagnostics-color
specially. */
- if (!strcmp (opt, "-fdiagnostics-plain-output"))
+ if (opt[0] == '-'
+ && (opt[1] == '-' || opt[1] == 'f')
+ && !strcmp (opt + 2, "diagnostics-plain-output"))
{
/* If you have changed the default diagnostics output, and this new
output is not appropriately "plain" (e.g., the change needs to be
