On Thu, 9 Apr 2020, Jason Merrill wrote:
> On 4/8/20 7:49 PM, Patrick Palka wrote:
> > When evaluating the initializer of 'a' in the following example
> >
> > struct A { A *p = this; };
> > constexpr A foo() { return {}; }
> > constexpr A a = foo();
> >
> > the PLACEHOLDER_EXPR for 'this' in the aggregate initializer returned by foo
> > gets resolved to the RESULT_DECL of foo. But due to guaranteed RVO, the
> > 'this'
> > should really be resolved to '&a'.
>
> > It seems to me that the right approach would be to immediately resolve the
> > PLACEHOLDER_EXPR to the correct target object during evaluation of 'foo()',
> > so
> > that we could use 'this' to access objects adjacent to the current object in
> > the
> > ultimate storage location. (I think #c2 of PR c++/94537 is an example of
> > such
> > usage of 'this', which currently doesn't work. But as #c1 shows we don't
> > seem
> > to handle this case correctly in non-constexpr initialization either.)
>
> As I commented in the PR, the standard doesn't require this to work because A
> is trivially copyable, and our ABI makes it impossible. But there's still a
> constexpr bug when we add
>
> A() = default; A(const A&);
>
> clang doesn't require the constructors to make this work for constant
> initialization, but similarly can't make it work for non-constant
> initialization.
That makes a lot of sense, thanks for the detailed explanation.
>
> > I haven't yet been able to make a solution using the above approach work --
> > making sure we use the ultimate object instead of the RESULT_DECL whenever
> > we
> > access ctx->global->values is proving to be tricky and subtle.
>
> Do we need to go through ctx->global->values? Would it work for the
> RESULT_DECL case in cxx_eval_constant_expression to go to straight to
> ctx->object or ctx->ctor instead?
I attempted that at some point, but IIRC we still end up not resolving
some RESULT_DECLs because not all of them get processed through
cxx_eval_constant_expression before we manipulate ctx->global->values
with them. I'll try this approach more carefully and report back with
more specifics.
