Re: C++ PATCH to fix ICE in replace_placeholders_r (PR c++/79937)

[Jason Merrill <jason at redhat dot com>]

On Tue, Mar 14, 2017 at 2:33 PM, Jason Merrill <jason@redhat.com> wrote:
> On Tue, Mar 7, 2017 at 12:10 PM, Marek Polacek <polacek@redhat.com> wrote:
>> In this testcase we have
>> C c = bar (X{1});
>> which store_init_value sees as
>> c = TARGET_EXPR <D.2332, bar (TARGET_EXPR <D.2298, {.i=1, .n=(&<PLACEHOLDER_EXPR struct X>)->i}>)>
>> i.e. we're initializing "c" with a TARGET_EXPR.  We call replace_placeholders
>> that walks the whole tree to substitute the placeholders.  Eventually we find
>> the nested <PLACEHOLDER_EXPR struct X> but that's for another object, so we
>> crash.  Seems that we shouldn't have stepped into the second TARGET_EXPR at
>> all; it has nothing to with "c", it's bar's argument.
>>
>> It occurred to me that we shouldn't step into CALL_EXPRs and leave the
>> placeholders in function arguments to cp_gimplify_init_expr which calls
>> replace_placeholders for constructors.  Not sure if it's enough to handle
>> CALL_EXPRs like this, anything else?
>
> Hmm, we might have a DMI containing a call with an argument referring
> to *this, i.e.
>
> struct A
> {
>   int i;
>   int j = frob (this->i);
> };
>
> The TARGET_EXPR seems like a more likely barrier, but even there we
> could have something like
>
> struct A { int i; };
> struct B
> {
>   int i;
>   A a = A{this->i};
> };
>
> I think we need replace_placeholders to keep a stack of objects, so
> that when we see a TARGET_EXPR we add it to the stack and therefore
> can properly replace a PLACEHOLDER_EXPR of its type.

Or actually, avoid replacing such a PLACEHOLDER_EXPR, but rather leave
it for later when we lower the TARGET_EXPR.

Jason