Re: [PATCH] c++: fixes for derived-to-base reference binding [PR107085]

[Jason Merrill <jason@redhat.com>]

On 10/5/22 17:27, Marek Polacek wrote:
> This PR reports that
> 
>    struct Base {};
>    struct Derived : Base {};
>    static_assert(__reference_constructs_from_temporary(Base const&, Derived));
> 
> doesn't pass, which it should: it's just like
> 
>    const Base& b(Derived{});
> 
> where we bind 'b' to the Base subobject of a temporary object of type
> Derived.  The ck_base conversion didn't have ->need_temporary_p set because
> we didn't need to create a temporary object just for the base, but the whole
> object is a temporary so we're still binding to a temporary.  Fixed by
> the conv_is_prvalue hunk.
> 
> That broke a bunch of tests.  I've distilled the issue into a simple test
> in elision4.C.  Essentially, we have
> 
>    struct B { /* ... */ };
>    struct D : B { };
>    B b = D();
> 
> and we set force_elide in build_over_call, but we're unable to actually
> elide the B::B(B&&) call, and crash on gcc_assert (!force_elide);.
> 
> <https://en.cppreference.com/w/cpp/language/copy_elision> says that copy
> elision "can only apply when the object being initialized is known not to be
> a potentially-overlapping subobject".  So I suppose we shouldn't force_elide
> the B::B(B&&) call.  I don't belive the CWG 2327 code was added to handle
> derived-to-base conversions, at that time conv_binds_ref_to_prvalue wasn't
> checking ck_base at all.
> 
> Does that make sense?  If so...
> 
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> 
> 	PR c++/107085
> 
> gcc/cp/ChangeLog:
> 
> 	* call.cc (conv_is_prvalue): Return true if the base subobject is part
> 	of a temporary object.

No, the base subobject of a prvalue is an xvalue.

I think the problem is that an expression being a prvalue is a subset of 
binding a reference to a temporary, and we shouldn't try to express both 
of those using the same function: you need a separate 
conv_binds_ref_to_temporary.

> 	(build_over_call): Don't force_elide when it's a derived-to-base
> 	conversion.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/ext/reference_constructs_from_temporary1.C: Adjust expected
> 	result.
> 	* g++.dg/ext/reference_converts_from_temporary1.C: Likewise.
> 	* g++.dg/cpp0x/elision4.C: New test.
> ---
>   gcc/cp/call.cc                                  | 17 +++++++++++++++--
>   gcc/testsuite/g++.dg/cpp0x/elision4.C           | 17 +++++++++++++++++
>   .../ext/reference_constructs_from_temporary1.C  |  2 +-
>   .../ext/reference_converts_from_temporary1.C    |  2 +-
>   4 files changed, 34 insertions(+), 4 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/cpp0x/elision4.C
> 
> diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
> index bd04a1d309a..15e969d6429 100644
> --- a/gcc/cp/call.cc
> +++ b/gcc/cp/call.cc
> @@ -9186,7 +9186,11 @@ conv_is_prvalue (conversion *c)
>   {
>     if (c->kind == ck_rvalue)
>       return true;
> -  if (c->kind == ck_base && c->need_temporary_p)
> +  if (c->kind == ck_base
> +      /* We may not need a temporary object for the base itself, but if it
> +	 is the base subobject of a temporary object, we're still dealing
> +	 with a prvalue.  */
> +      && (c->need_temporary_p || conv_is_prvalue (next_conversion (c))))
>       return true;
>     if (c->kind == ck_user && !TYPE_REF_P (c->type))
>       return true;
> @@ -9417,7 +9421,16 @@ build_over_call (struct z_candidate *cand, int flags, tsubst_flags_t complain)
>         && (DECL_COPY_CONSTRUCTOR_P (fn)
>   	  || DECL_MOVE_CONSTRUCTOR_P (fn))
>         && !unsafe_return_slot_p (first_arg)
> -      && conv_binds_ref_to_prvalue (convs[0]))
> +      && conv_binds_ref_to_prvalue (convs[0])
> +      /* Converting a class to another class that then binds to this
> +	 copy/move constructor's argument is OK, but not when it's a
> +	 derived-to-base conversion.  */
> +      && [convs] {
> +	   for (conversion *t = convs[0]; t; t = next_conversion (t))
> +	     if (t->kind == ck_base)
> +	       return false;
> +	   return true;
> +	 }())
>       {
>         force_elide = true;
>         goto not_really_used;
> diff --git a/gcc/testsuite/g++.dg/cpp0x/elision4.C b/gcc/testsuite/g++.dg/cpp0x/elision4.C
> new file mode 100644
> index 00000000000..4833b50d48e
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp0x/elision4.C
> @@ -0,0 +1,17 @@
> +// PR c++/107085
> +// { dg-do compile { target c++11 } }
> +
> +// Must be non-trivial to exhibit the ICE.
> +struct X {
> +  X();
> +  X(X&&);
> +};
> +struct Z : X {};
> +X x1 = Z();
> +X x2 = X(Z());
> +
> +// ...but let's try the trivial path in build_over_call as well.
> +struct B { };
> +struct D : B { };
> +B b1 = D();
> +B b2 = B(D());
> diff --git a/gcc/testsuite/g++.dg/ext/reference_constructs_from_temporary1.C b/gcc/testsuite/g++.dg/ext/reference_constructs_from_temporary1.C
> index 76de905a35d..5354b1dc4e6 100644
> --- a/gcc/testsuite/g++.dg/ext/reference_constructs_from_temporary1.C
> +++ b/gcc/testsuite/g++.dg/ext/reference_constructs_from_temporary1.C
> @@ -201,7 +201,7 @@ SA(!__reference_constructs_from_temporary(const int&, H));
>   SA(!__reference_constructs_from_temporary(int&&, G2));
>   SA(!__reference_constructs_from_temporary(const int&, H2));
>   
> -SA(!__reference_constructs_from_temporary(const Base&, Der));
> +SA(__reference_constructs_from_temporary(const Base&, Der));
>   
>   // This fails because std::is_constructible_v<int&&, id<int[3]>> is false.
>   SA(!__reference_constructs_from_temporary(int&&, id<int[3]>));
> diff --git a/gcc/testsuite/g++.dg/ext/reference_converts_from_temporary1.C b/gcc/testsuite/g++.dg/ext/reference_converts_from_temporary1.C
> index 90196c38742..e6c159e9b00 100644
> --- a/gcc/testsuite/g++.dg/ext/reference_converts_from_temporary1.C
> +++ b/gcc/testsuite/g++.dg/ext/reference_converts_from_temporary1.C
> @@ -201,7 +201,7 @@ SA( __reference_converts_from_temporary(const int&, H));
>   SA(!__reference_converts_from_temporary(int&&, G2));
>   SA(!__reference_converts_from_temporary(const int&, H2));
>   
> -SA(!__reference_converts_from_temporary(const Base&, Der));
> +SA(__reference_converts_from_temporary(const Base&, Der));
>   
>   // This fails because std::is_constructible_v<int&&, id<int[3]>> is false.
>   SA(!__reference_converts_from_temporary(int&&, id<int[3]>));
> 
> base-commit: e99dcbb54e07b798c3353124f38336f96a826d43