C++ PATCH for c++/90998 - ICE with copy elision in init by ctor and -Wconversion

[Jason Merrill]

On 10/25/19 2:38 PM, Marek Polacek wrote:
> After r269667 which introduced joust_maybe_elide_copy, in C++17 we can elide
> a constructor if it uses a conversion function that returns a prvalue, and
> use the conversion function in its stead.
> 
> This eliding means that if we have a candidate that previously didn't have
> ->second_conv, it can have it after the elision.  This confused the
> -Wconversion warning because it was assuming that if cand1->second_conv is
> non-null, so is cand2->second_conv.  Here cand1->second_conv was non-null
> but cand2->second_conv remained null, so it crashed in compare_ics.
> 
> I checked with clang that both compilers call A::operator B() in C++17 and
> B::B(A const &) otherwise.
> 
> Bootstrapped/regtested on x86_64-linux, ok for trunk and 9?

OK, thanks.

> 2019-10-25  Marek Polacek  <polacek@redhat.com>
> 
> 	PR c++/90998 - ICE with copy elision in init by ctor and -Wconversion.
> 	* call.c (joust): Don't attempt to warn if ->second_conv is null.
> 
> 	* g++.dg/cpp0x/overload-conv-4.C: New test.
> 
> diff --git gcc/cp/call.c gcc/cp/call.c
> index cbd1fe8a0a4..b0c6370107d 100644
> --- gcc/cp/call.c
> +++ gcc/cp/call.c
> @@ -10870,7 +10870,9 @@ joust (struct z_candidate *cand1, struct z_candidate *cand2, bool warn,
>        either between a constructor and a conversion op, or between two
>        conversion ops.  */
>     if ((complain & tf_warning)
> -      && winner && warn_conversion && cand1->second_conv
> +      /* In C++17, the constructor might have been elided, which means that
> +	 an originally null ->second_conv could become non-null.  */
> +      && winner && warn_conversion && cand1->second_conv && cand2->second_conv
>         && (!DECL_CONSTRUCTOR_P (cand1->fn) || !DECL_CONSTRUCTOR_P (cand2->fn))
>         && winner != compare_ics (cand1->second_conv, cand2->second_conv))
>       {
> diff --git gcc/testsuite/g++.dg/cpp0x/overload-conv-4.C gcc/testsuite/g++.dg/cpp0x/overload-conv-4.C
> new file mode 100644
> index 00000000000..6fcdbbaa6a4
> --- /dev/null
> +++ gcc/testsuite/g++.dg/cpp0x/overload-conv-4.C
> @@ -0,0 +1,23 @@
> +// PR c++/90998 - ICE with copy elision in init by ctor and -Wconversion.
> +// { dg-do compile { target c++11 } }
> +// { dg-options "-Wconversion" }
> +
> +struct B;
> +
> +struct A {
> +    operator B();
> +};
> +
> +struct B {
> +    B(A const &rs);
> +    B(B const &rs);
> +};
> +
> +B
> +f (A x)
> +{
> +  // C++14: we call B::B(A const &)
> +  // C++17: we call A::operator B()
> +  return B(x); // { dg-warning "choosing .A::operator B\\(\\). over .B::B\\(const A&\\)" "" { target c++17 } }
> +  // { dg-warning "for conversion from .A. to .B." "" { target c++17 } .-1 }
> +}
>