https://gcc.gnu.org/g:005b1f418350a3ef7c5280b19a82fb28c0856e7c
commit r13-9299-g005b1f418350a3ef7c5280b19a82fb28c0856e7c
Author: Marek Polacek <pola...@redhat.com>
Date: Tue May 2 17:36:00 2023 -0400
c++: wrong std::is_convertible with cv-qual fn [PR109680]
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.
(cherry picked from commit 4c2ffb02fd4104d77c5d907662f04434dc4c3fe8)
Diff:
---
gcc/cp/method.cc | 39 +++++++++++++++++++++++++++---
gcc/testsuite/g++.dg/ext/is_convertible6.C | 16 ++++++++++++
2 files changed, 51 insertions(+), 4 deletions(-)
diff --git a/gcc/cp/method.cc b/gcc/cp/method.cc
index 09ea6d732dfc..c9d9e3516f3b 100644
--- a/gcc/cp/method.cc
+++ b/gcc/cp/method.cc
@@ -1907,6 +1907,27 @@ 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.
@@ -2055,8 +2076,8 @@ 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;
}
@@ -2235,7 +2256,9 @@ 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 ();
@@ -2250,7 +2273,15 @@ 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 000000000000..180582663e8e
--- /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));
