From: Jason Merrill <jason@redhat.com>
To: Marek Polacek <polacek@redhat.com>
Cc: GCC Patches <gcc-patches@gcc.gnu.org>
Subject: Re: [PATCH v2] c++: wrong std::is_convertible with cv-qual fn [PR109680]
Date: Wed, 10 May 2023 18:08:19 -0400 [thread overview]
Message-ID: <402c3b96-c6f2-bbcb-ddcf-f4e4b0fbb056@redhat.com> (raw)
In-Reply-To: <ZFwMeKCu4OnsPPaE@redhat.com>
On 5/10/23 17:28, Marek Polacek wrote:
> On Wed, May 03, 2023 at 03:37:03PM -0400, Jason Merrill wrote:
>> On 5/2/23 19:10, Marek Polacek wrote:
>>> This PR points out that std::is_convertible has given the wrong answer
>>> in
>>>
>>> static_assert (!std::is_convertible_v <int () const, int (*) ()>, "");
>>>
>>> since r13-2822 implemented __is_{,nothrow_}convertible.
>>>
>>> std::is_convertible uses the imaginary
>>>
>>> To test() { return std::declval<From>(); }
>>>
>>> to do its job. Here, From is 'int () const'. std::declval is defined as:
>>>
>>> template<class T>
>>> typename std::add_rvalue_reference<T>::type declval() noexcept;
>>>
>>> std::add_rvalue_reference is defined as "If T is a function type that
>>> has no cv- or ref- qualifier or an object type, provides a member typedef
>>> type which is T&&, otherwise type is T."
>>>
>>> In our case, T is cv-qualified, so the result is T, so we end up with
>>>
>>> int () const declval() noexcept;
>>>
>>> which is invalid. In other words, this is pretty much like:
>>>
>>> using T = int () const;
>>> T fn1(); // bad, fn returning a fn
>>> T& fn2(); // bad, cannot declare reference to qualified function type
>>> T* fn3(); // bad, cannot declare pointer to qualified function type
>>>
>>> using U = int ();
>>> U fn4(); // bad, fn returning a fn
>>> U& fn5(); // OK
>>> U* fn6(); // OK
>>>
>>> I think is_convertible_helper needs to simulate std::declval better.
>>> I wouldn't be surprised if other type traits needed a similar fix.
>>>
>>> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk/13?
>>>
>>> I've tested the new test with G++12 and clang++ as well (with
>>> std::is_convertible).
>>>
>>> PR c++/109680
>>>
>>> gcc/cp/ChangeLog:
>>>
>>> * method.cc (is_convertible_helper): Correct simulating std::declval.
>>>
>>> gcc/testsuite/ChangeLog:
>>>
>>> * g++.dg/ext/is_convertible6.C: New test.
>>> ---
>>> gcc/cp/method.cc | 20 ++++++++++++++++++++
>>> gcc/testsuite/g++.dg/ext/is_convertible6.C | 16 ++++++++++++++++
>>> 2 files changed, 36 insertions(+)
>>> create mode 100644 gcc/testsuite/g++.dg/ext/is_convertible6.C
>>>
>>> diff --git a/gcc/cp/method.cc b/gcc/cp/method.cc
>>> index 00eae56eb5b..38eb7520312 100644
>>> --- a/gcc/cp/method.cc
>>> +++ b/gcc/cp/method.cc
>>> @@ -2245,6 +2245,26 @@ is_convertible_helper (tree from, tree to)
>>> {
>>> if (VOID_TYPE_P (from) && VOID_TYPE_P (to))
>>> return integer_one_node;
>>> + /* std::is_{,nothrow_}convertible test whether the imaginary function
>>> + definition
>>> +
>>> + To test() { return std::declval<From>(); }
>>> +
>>> + is well-formed. A function can't return a function... */
>>> + if (FUNC_OR_METHOD_TYPE_P (to)
>>> + /* ...neither can From be a function with cv-/ref-qualifiers:
>>> + std::declval is defined as
>>> +
>>> + template<class T>
>>> + typename std::add_rvalue_reference<T>::type declval() noexcept;
>>> +
>>> + and std::add_rvalue_reference yields T when T is a function with
>>> + cv- or ref-qualifiers, making the definition ill-formed.
>>> + ??? Should we check this in other uses of build_stub_object too? */
>>
>> Probably we want a build_trait_object that wraps build_stub_object with
>> these extra checks, or does something that exercises more of the normal
>> code, maybe by tsubsting into T (U&&) with { to, from }?
>
> I did the former. We only have the To type when processing
> __is_convertible, so I think that can't be part of the declval
> function, which takes another type, From. IOW, if we want to factor
> declval out, build_trait_object will take only one type. Eh, that
> still sounds confusing. But the patch is simple.
>
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
OK.
> -- >8 --
> This PR points out that std::is_convertible has given the wrong answer
> in
>
> static_assert (!std::is_convertible_v <int () const, int (*) ()>, "");
>
> since r13-2822 implemented __is_{,nothrow_}convertible.
>
> std::is_convertible uses the imaginary
>
> To test() { return std::declval<From>(); }
>
> to do its job. Here, From is 'int () const'. std::declval is defined as:
>
> template<class T>
> typename std::add_rvalue_reference<T>::type declval() noexcept;
>
> std::add_rvalue_reference is defined as "If T is a function type that
> has no cv- or ref- qualifier or an object type, provides a member typedef
> type which is T&&, otherwise type is T."
>
> In our case, T is cv-qualified, so the result is T, so we end up with
>
> int () const declval() noexcept;
>
> which is invalid. In other words, this is pretty much like:
>
> using T = int () const;
> T fn1(); // bad, fn returning a fn
> T& fn2(); // bad, cannot declare reference to qualified function type
> T* fn3(); // bad, cannot declare pointer to qualified function type
>
> using U = int ();
> U fn4(); // bad, fn returning a fn
> U& fn5(); // OK
> U* fn6(); // OK
>
> I think is_convertible_helper needs to simulate std::declval better.
> To that end, I'm introducing build_trait_object, to be used where
> a declval is needed.
>
> PR c++/109680
>
> gcc/cp/ChangeLog:
>
> * method.cc (build_trait_object): New.
> (assignable_expr): Use it.
> (ref_xes_from_temporary): Likewise.
> (is_convertible_helper): Likewise. Check FUNC_OR_METHOD_TYPE_P.
>
> gcc/testsuite/ChangeLog:
>
> * g++.dg/ext/is_convertible6.C: New test.
> ---
> gcc/cp/method.cc | 39 +++++++++++++++++++---
> gcc/testsuite/g++.dg/ext/is_convertible6.C | 16 +++++++++
> 2 files changed, 51 insertions(+), 4 deletions(-)
> create mode 100644 gcc/testsuite/g++.dg/ext/is_convertible6.C
>
> diff --git a/gcc/cp/method.cc b/gcc/cp/method.cc
> index 00eae56eb5b..7ec7bfe8387 100644
> --- a/gcc/cp/method.cc
> +++ b/gcc/cp/method.cc
> @@ -1902,6 +1902,27 @@ build_stub_object (tree reftype)
> return convert_from_reference (stub);
> }
>
> +/* Build a std::declval<TYPE>() expression and return it. */
> +
> +tree
> +build_trait_object (tree type)
> +{
> + /* TYPE can't be a function with cv-/ref-qualifiers: std::declval is
> + defined as
> +
> + template<class T>
> + typename std::add_rvalue_reference<T>::type declval() noexcept;
> +
> + and std::add_rvalue_reference yields T when T is a function with
> + cv- or ref-qualifiers, making the definition ill-formed. */
> + if (FUNC_OR_METHOD_TYPE_P (type)
> + && (type_memfn_quals (type) != TYPE_UNQUALIFIED
> + || type_memfn_rqual (type) != REF_QUAL_NONE))
> + return error_mark_node;
> +
> + return build_stub_object (type);
> +}
> +
> /* Determine which function will be called when looking up NAME in TYPE,
> called with a single ARGTYPE argument, or no argument if ARGTYPE is
> null. FLAGS and COMPLAIN are as for build_new_method_call.
> @@ -2050,8 +2071,8 @@ static tree
> assignable_expr (tree to, tree from)
> {
> cp_unevaluated cp_uneval_guard;
> - to = build_stub_object (to);
> - from = build_stub_object (from);
> + to = build_trait_object (to);
> + from = build_trait_object (from);
> tree r = cp_build_modify_expr (input_location, to, NOP_EXPR, from, tf_none);
> return r;
> }
> @@ -2231,7 +2252,9 @@ ref_xes_from_temporary (tree to, tree from, bool direct_init_p)
> return false;
> /* We don't check is_constructible<T, U>: if T isn't constructible
> from U, we won't be able to create a conversion. */
> - tree val = build_stub_object (from);
> + tree val = build_trait_object (from);
> + if (val == error_mark_node)
> + return false;
> if (!TYPE_REF_P (from) && TREE_CODE (from) != FUNCTION_TYPE)
> val = CLASS_TYPE_P (from) ? force_rvalue (val, tf_none) : rvalue (val);
> return ref_conv_binds_to_temporary (to, val, direct_init_p).is_true ();
> @@ -2246,7 +2269,15 @@ is_convertible_helper (tree from, tree to)
> if (VOID_TYPE_P (from) && VOID_TYPE_P (to))
> return integer_one_node;
> cp_unevaluated u;
> - tree expr = build_stub_object (from);
> + tree expr = build_trait_object (from);
> + /* std::is_{,nothrow_}convertible test whether the imaginary function
> + definition
> +
> + To test() { return std::declval<From>(); }
> +
> + is well-formed. A function can't return a function. */
> + if (FUNC_OR_METHOD_TYPE_P (to) || expr == error_mark_node)
> + return error_mark_node;
> deferring_access_check_sentinel acs (dk_no_deferred);
> return perform_implicit_conversion (to, expr, tf_none);
> }
> diff --git a/gcc/testsuite/g++.dg/ext/is_convertible6.C b/gcc/testsuite/g++.dg/ext/is_convertible6.C
> new file mode 100644
> index 00000000000..180582663e8
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/ext/is_convertible6.C
> @@ -0,0 +1,16 @@
> +// PR c++/109680
> +// { dg-do compile { target c++11 } }
> +
> +#define SA(X) static_assert((X),#X)
> +
> +SA(!__is_convertible(int () const, int (*)()));
> +SA(!__is_convertible(int (*)(), int () const));
> +
> +SA( __is_convertible(int (), int (*)()));
> +SA(!__is_convertible(int (*)(), int ()));
> +
> +SA( __is_convertible(int (int), int (*) (int)));
> +SA(!__is_convertible(int (*) (int), int (int)));
> +
> +SA(!__is_convertible(int (int) const, int (*) (int)));
> +SA(!__is_convertible(int (*) (int), int (int) const));
>
> base-commit: bdc10c2bfaceb3be567e0a27d8951a22b4be2ed4
prev parent reply other threads:[~2023-05-10 22:08 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-05-02 23:10 [PATCH] " Marek Polacek
2023-05-03 19:37 ` Jason Merrill
2023-05-10 21:28 ` [PATCH v2] " Marek Polacek
2023-05-10 22:08 ` Jason Merrill [this message]
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