On Thu, Sep 05, 2019 at 04:15:06PM -0400, Jason Merrill wrote: > On Thu, Sep 5, 2019 at 2:14 PM Marek Polacek <pola...@redhat.com> wrote: > > > > This patch implements C++20 P1286R2, Contra CWG DR1778 as described here > > <http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1286r2.html>. > > Essentially, it removes the restriction that the exception specification of > > a defaulted special member matches the implicit exception specification. > > "Implementing" it means just removing some code (woo!) that was marking > > a function DECL_DELETED_FN if the exception-specifications didn't match... > > > > ...except there's a snag. We no longer reject > > > > struct A { > > A() noexcept(false) = default; > > }; > > > > because A::A() is no longer deleted. But this assert fails: > > > > static_assert(!noexcept(A()), ""); > > > > and it doesn't seem to me it should, but it's sort of Catch 22. The default > > constructor *is* trivial as per [class.default.ctor]/3, so trivial_fn_p is > > true. When build_over_call sees a default_ctor_p that is trivial_fn_p, it > > elides the call and creates a TARGET_EXPR <D, void_cst>. When evaluating > > noexcept(A()) expr_noexcept_p gets TARGET_EXPR <D, {}> and since there is > > no CALL_EXPR/AGGR_INIT_EXPR that throws, it says "ok, this can't throw". > > > > [except.spec]/6: An expression e is potentially-throwing if > > (6.1) e is a function call [...] with a potentially-throwing exception > > specification, or > > (6.2) e implicitly invokes a function [...] that is potentially-throwing, or > > ... > > (6.6) any of the immediate subexpressions of e is potentially-throwing. > > > which of these apply here, 6.1? But such a TARGET_EXPR can't throw, can it? > > Ah, interesting. It seems to me that a trivial function should not be > considered potentially-throwing. I'm not sure whether it's best to > address this by making it non-trivial or overriding the exception > specification. I think the definition of potentially-throwing is not > the right place to fix it. Do you want to email CWG or shall I?
I'll do it. Please chime in :). Marek
