[cfe-dev] Assertion generating code

[Eli Friedman]

On Mon, Aug 29, 2011 at 12:37 PM, Sebastian Redl
<sebastian.redl at getdesigned.at> wrote:
> Hi,
>
> While working on Clang, I hit an assertion I don't understand. CodeGen wants to generate this expression:
>
> #20 0x00000001001f9f30 in (anonymous namespace)::AggExprEmitter::VisitCXXConstructExpr (this=0x7fff5fbfd050, E=0x106841540) at clang/lib/CodeGen/CGExprAgg.cpp:552
> 552       CGF.EmitCXXConstructExpr(E, Slot);
>
> (gdb) call E->dump()
> (CXXConstructExpr 0x106841540 'struct B''void (struct B &&)'
>  (MaterializeTemporaryExpr 0x106841410 'struct B' xvalue
>    (ImplicitCastExpr 0x1068413f8 'struct B' <LValueToRValue>
>      (ImplicitCastExpr 0x1068413d8 'struct B' lvalue <UncheckedDerivedToBase (B)>
>        (DeclRefExpr 0x1068413b0 'struct S' lvalue ParmVar 0x1068410b0 '' 'struct S &&')))))
>
> Note the two nested implicit casts. This eventually reaches AggExprEmitter::VisitCastExpr(CastExpr *E) for the derived-to-base cast, which asserts.
>
> Questions:
> 1) Why does it reach the assertion? (I think it's because of the MaterializeTemporaryExpr)
>
> 2) How do I avoid that?

How exactly do you get an LValueToRValue on a struct in C++?
LValueToRValue corresponds to a C-style struct copy, which isn't
generally usable in C++.

-Eli