https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114393
--- Comment #7 from Patrick Palka <ppalka at gcc dot gnu.org> --- The master branch has been updated by Patrick Palka <ppa...@gcc.gnu.org>: https://gcc.gnu.org/g:d74fe10b13336b9de2e025ced4af00a25ff1d3e7 commit r14-9943-d74fe10b13336b9de2e025ced4af00a25ff1d3e7 Author: Patrick Palka <ppa...@redhat.com> c++: templated substitution into lambda-expr, cont [PR114393] The original PR114393 testcase is unfortunately still not accepted after r14-9938-g081c1e93d56d35 due to return type deduction confusion when a lambda-expr is used as a default template argument. The below reduced testcase demonstrates the bug. Here when forming the dependent specialization b_v<U> we substitute the default argument of F, a lambda-expr, with _Descriptor=U. (In this case in_template_context is true since we're in the context of the template c_v, so we don't defer.) This substitution in turn lowers the level of the lambda's auto return type from 2 to 1 and so later, when instantiating c_v<int, char> we wrongly substitute this auto with the template argument at level=0,index=0, i.e. int, instead of going through do_auto_deduction which would yield char. One way to fix this would be to use a level-less auto to represent a deduced return type of a lambda, but that might be too invasive of a change at this stage, and it might be better to do this across the board for all deduced return types. Another way would be to pass tf_partial from coerce_template_parms during dependent substitution into a default template argument so that the substitution doesn't do any level-lowering, but that wouldn't do the right thing in this case due to the tf_partial early exit in the LAMBDA_EXPR case of tsubst_expr. Yet another way, and the approach that this patch takes, is to just defer all dependent substitution into a lambda-expr, building upon the logic added in r14-9938-g081c1e93d56d35. This also helps ensure LAMBDA_EXPR_REGEN_INFO consists only of the concrete template arguments that were ultimately substituted into the most general lambda. PR c++/114393 gcc/cp/ChangeLog: * pt.cc (tsubst_lambda_expr): Also defer all dependent substitution. gcc/testsuite/ChangeLog: * g++.dg/cpp2a/lambda-targ2a.C: New test. Reviewed-by: Jason Merrill <ja...@redhat.com>
