c++: fix explicit/copy problem [PR109247]
In the testcase, the user wants the assignment to use the operator= declared
in the class, but because [over.match.list] says that explicit constructors
are also considered for list-initialization, as affirmed in CWG1228, we end
up choosing the implicitly-declared copy assignment operator, using the
explicit constructor template for the argument, which is ill-formed. Other
implementations haven't implemented CWG1228, so we keep getting bug reports.
Discussion in CWG led to the idea for this targeted relaxation: if we use an
explicit constructor for the conversion to the argument of a copy or move
special member function, that makes the candidate worse than another.
DR 2735
PR c++/109247
gcc/cp/ChangeLog:
* call.cc (sfk_copy_or_move): New.
(joust): Add tiebreaker for explicit conv and copy ctor.
gcc/testsuite/ChangeLog:
* g++.dg/cpp0x/initlist-explicit3.C: New test.