[Bug c++/68763] [6 Regression] ICE: in verify_unstripped_args, at cp/pt.c:1132

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68763

--- Comment #18 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Created attachment 37506
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=37506&action=edit
gcc6-pr68763.patch

Partial fix.  With the testcase reduction, I gave up, got only 4.7% reduction
after 6 hours of 16 processes creduction.
With this patch, the testcase compiles fine, but there is still modification in
place - DEFARG_INSTANTIATIONS points to the particular TREE_LIST node
containing the DEFAULT_ARG, while what we'd really need is let it point to the
FUNCTION_DECLs whose TREE_TYPE we should modify similarly.  Thoughts on that?
I'll bootstrap/regtest this partial fix anyway.