Re: [PATCH v5] c++: ICE with temporary of class type in DMI [PR100252]

[Marek Polacek <polacek@redhat.com>]

On Tue, May 24, 2022 at 04:01:37PM -0400, Jason Merrill wrote:
> On 5/24/22 09:55, Marek Polacek wrote:
> > On Tue, May 24, 2022 at 08:36:39AM -0400, Jason Merrill wrote:
> > > On 5/16/22 11:36, Marek Polacek wrote:
> > > > +static tree
> > > > +replace_placeholders_for_class_temp_r (tree *tp, int *, void *data)
> > > > +{
> > > > +  tree t = *tp;
> > > > +  tree full_expr = *static_cast<tree *>(data);
> > > > +
> > > > +  /* We're looking for a TARGET_EXPR nested in the whole expression.  */
> > > > +  if (TREE_CODE (t) == TARGET_EXPR
> > > > +      && !potential_prvalue_result_of (t, full_expr))
> > > > +    {
> > > > +      tree init = TARGET_EXPR_INITIAL (t);
> > > > +      while (TREE_CODE (init) == COMPOUND_EXPR)
> > > > +	init = TREE_OPERAND (init, 1);
> > > 
> > > Hmm, how do we get a COMPOUND_EXPR around a CONSTRUCTOR?
> > 
> > Sadly, that's possible for code like (from nsdmi-aggr18.C)
> > 
> > struct D {
> >    int x = 42;
> >    B b = (true, A{x});
> > };
> > 
> > where the TARGET_EXPR_INITIAL is
> > <<< Unknown tree: void_cst >>>, {.x=((struct D *) this)->x, .y=(&<PLACEHOLDER_EXPR struct A>)->x}
> 
> Hmm, perhaps cp_build_compound_expr should build an additional TARGET_EXPR
> around the COMPOUND_EXPR but leave the one inside alone. Feel free to
> investigate that if you'd like, or the patch is OK as is.

Sorry, I was unclear.  The whole expression is:

TARGET_EXPR <D.2439, A::operator B (&TARGET_EXPR <D.2405, <<< Unknown tree: void_cst >>>, {.x=((struct D *) this)->x, .y=(&<PLACEHOLDER_EXPR struct A>)->x}>)>

so there *is* a TARGET_EXPR around the COMPOUND_EXPR.  We'd have to build
a TARGET_EXPR around the COMPOUND_EXPR's RHS = the CONSTRUCTOR.  Frankly,
I'm not sure if it's worth the effort.  The while loop is somewhat unsightly
but not too bad.

Marek