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? -- >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 -- 2.40.1