https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90320
--- Comment #6 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Marek Polacek <mpola...@gcc.gnu.org>: https://gcc.gnu.org/g:feb801f62239528bca2cfb6c3abd70d434b69c0a commit r10-7979-gfeb801f62239528bca2cfb6c3abd70d434b69c0a Author: Marek Polacek <pola...@redhat.com> Date: Wed Apr 22 20:12:47 2020 -0400 c++: Explicit constructor called in copy-initialization [PR90320] This test is rejected with a bogus "use of deleted function" error starting with r225705 whereby convert_like_real/ck_base no longer sets LOOKUP_ONLYCONVERTING for user_conv_p conversions. This does not seem to be always correct. To recap, when we have something like T t = x where T is a class type and the type of x is not T or derived from T, we perform copy-initialization, something like: 1. choose a user-defined conversion to convert x to T, the result is a prvalue, 2. use this prvalue to direct-initialize t. In the second step, explicit constructors should be considered, since we're direct-initializing. This is what r225705 fixed. In this PR we are dealing with the first step, I think, where explicit constructors should be skipped. [over.match.copy] says "The converting constructors of T are candidate functions" which clearly eliminates explicit constructors. But we also have to copy-initialize the argument we are passing to such a converting constructor, and here we should disregard explicit constructors too. In this testcase we have V v = m; and we choose V::V(M) to convert m to V. But we wrongly choose the explicit M::M<M&>(M&) to copy-initialize the argument; it's a better match for a non-const lvalue than the implicit M::M(const M&) but because it's explicit, we shouldn't use it. When convert_like is processing the ck_user conversion -- the convfn is V::V(M) -- it can see that cand->flags contains LOOKUP_ONLYCONVERTING, but then when we're in build_over_call for this convfn, we have no way to pass the flag to convert_like for the argument 'm', because convert_like doesn't take flags. Fixed by creating a new conversion flag, copy_init_p, set in ck_base/ck_rvalue to signal that explicit constructors should be skipped. LOOKUP_COPY_PARM looks relevant, but again, it's a LOOKUP_* flag, so can't pass it to convert_like. DR 899 also seemed related, but that deals with direct-init contexts only. PR c++/90320 * call.c (struct conversion): Add copy_init_p. (standard_conversion): Set copy_init_p in ck_base and ck_rvalue if FLAGS demands LOOKUP_ONLYCONVERTING. (convert_like_real) <case ck_base>: If copy_init_p is set, or LOOKUP_ONLYCONVERTING into FLAGS. * g++.dg/cpp0x/explicit13.C: New test. * g++.dg/cpp0x/explicit14.C: New test.
