On 2/4/19 3:48 PM, Marek Polacek wrote:
In this test we have a user-defined conversion converting const int & to T, and we're binding a const int to const int & -- the parameter of the converting ctor. We call Func with "VIEW_CONVERT_EXPR<const int>(Val)" as an argument. I like to use a diagram for the conversion like this:ck_rvalue <- ck_user <- ck_identity T T const int V_C_E<const int>(Val) when processing the ck_user part we call __ct_comp, whose ICS for its argument is ck_ref_bind <- ck_identity const int & <- const int V_C_E<const int>(Val) this ICS comes from reference_binding, and at that time we still had "Val" so need_temporary_p is 0. But when processing the ck_user conversion before calling build_over_call for the __ct_comp, we call 7009 expr = mark_rvalue_use (expr); for the argument which turns the V_C_E to NON_LVALUE_EXPR<42>. Binding "42" to const int & is possible, but requires a temporary, which we don't create precisely because need_temporary_p is 0. I.e. the original expr of the conversion changed behind reference_binding's back. Now, the call to mark_rvalue_use was added in r159096 to handle -Wunused-but-set-variable. Since r261121, we're actually able to turn a V_C_E into an rvalue. But it seems we shouldn't be trying to turn the argument into an rvalue in the ck_user case; mark_lvalue_use should do the job (= mark is as used) without changing an lvalue into an rvalue, breaking the ICS as above. I'm not sure if I'm describing the issue clearly enough, but I hope this will do.
Yes, thanks, mark_rvalue_use is definitely wrong here. But mark_lvalue_use might be wrong as well; we don't know here how the expression is used by the inner conversions for the user-defined conversion. Can we remove the call entirely? It doesn't seem to break any Wunused* tests.
Jason
