[PATCH] c++: Instantiate less when evaluating __is_convertible

[PATCH] c++: Instantiate less when evaluating __is_convertible

[Marek Polacek]

Jon reported that evaluating __is_convertible in this test leads to
instantiating char_traits<char>::eq, which is invalid (because we
are trying to call a member function on a char) and so we fail to
compile the test.  __is_convertible doesn't and shouldn't need to
instantiate so much, so let's limit it with a cp_unevaluated guard.

Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

	PR c++/106784

gcc/cp/ChangeLog:

	* method.cc (is_convertible): Use cp_unevaluated.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/is_convertible3.C: New test.
---
 gcc/cp/method.cc                           |  1 +
 gcc/testsuite/g++.dg/ext/is_convertible3.C | 66 ++++++++++++++++++++++
 2 files changed, 67 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/ext/is_convertible3.C

diff --git a/gcc/cp/method.cc b/gcc/cp/method.cc
index c35a59fe56c..45f70f5d3f3 100644
--- a/gcc/cp/method.cc
+++ b/gcc/cp/method.cc
@@ -2246,6 +2246,7 @@ is_convertible (tree from, tree to)
 {
   if (VOID_TYPE_P (from) && VOID_TYPE_P (to))
     return true;
+  cp_unevaluated u;
   tree expr = build_stub_object (from);
   expr = perform_implicit_conversion (to, expr, tf_none);
   if (expr == error_mark_node)
diff --git a/gcc/testsuite/g++.dg/ext/is_convertible3.C b/gcc/testsuite/g++.dg/ext/is_convertible3.C
new file mode 100644
index 00000000000..c817dc6f146
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/is_convertible3.C
@@ -0,0 +1,66 @@
+// PR c++/106784
+// { dg-do compile { target c++11 } }
+// Make sure we don't reject this at runtime by trying to instantiate
+// char_traits<CharT>::eq(CharT, CharT) while evaluating __is_convertible.
+
+template<bool B>
+struct bool_constant { static constexpr bool value = B; };
+using true_type = bool_constant<true>;
+using false_type = bool_constant<false>;
+
+template<typename T> struct is_void : false_type { };
+template<> struct is_void<void> : true_type { };
+
+template<typename T> T&& declval();
+
+template<bool> struct enable_if { };
+template<> struct enable_if<true> { using type = void; };
+template<bool B> using enable_if_t = typename enable_if<B>::type;
+
+template<typename _From, typename _To>
+  struct is_convertible
+  : public bool_constant<__is_convertible(_From, _To)>
+  { };
+
+template<class CharT>
+struct char_traits
+{
+  static unsigned long length(const char* s) { eq(*s, *s); return 0; }
+
+  static void eq(CharT l, CharT r) noexcept { l.f(r); }
+};
+
+template<class CharT>
+struct basic_string_view
+{
+  using traits_type = char_traits<CharT>;
+
+  constexpr basic_string_view() = default;
+  constexpr basic_string_view(const basic_string_view&) = default;
+
+  constexpr
+  basic_string_view(const CharT* __str) noexcept
+  : _M_len{traits_type::length(__str)}
+  { }
+
+  unsigned long _M_len = 0;
+};
+
+template<class CharT>
+struct basic_string
+{
+  template<class T>
+    enable_if_t<is_convertible<const T&, basic_string_view<CharT>>::value
+                && !is_convertible<const T&, const char*>::value>
+    replace(int, T) { }
+
+  void replace(unsigned long, const char*) { }
+
+  void replace(const char* s) { replace(1, s); }
+};
+
+int main()
+{
+  basic_string<char> s;
+  s.replace("");
+}

base-commit: 2460f7cdef7ef9c971de79271afc0db73687a272
-- 
2.37.3

Re: [PATCH] c++: Instantiate less when evaluating __is_convertible

[Patrick Palka]

On Mon, 26 Sep 2022, Marek Polacek via Gcc-patches wrote:

> Jon reported that evaluating __is_convertible in this test leads to
> instantiating char_traits<char>::eq, which is invalid (because we
> are trying to call a member function on a char) and so we fail to
> compile the test.  __is_convertible doesn't and shouldn't need to
> instantiate so much, so let's limit it with a cp_unevaluated guard.
> 
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> 
> 	PR c++/106784
> 
> gcc/cp/ChangeLog:
> 
> 	* method.cc (is_convertible): Use cp_unevaluated.

I think is_nothrow_convertible would need cp_unevaluated too (or maybe we
should define is_nothrow_convertible in terms of is_convertible).

And the testcase can probably be minimized to something like:

  struct A;
  struct B { template<class T> B(const T&) noexcept { T::nonexistent; } };

  static_assert(__is_convertible(const A&, B));
  static_assert(__is_nothrow_convertible(const A&, B));

> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/ext/is_convertible3.C: New test.
> ---
>  gcc/cp/method.cc                           |  1 +
>  gcc/testsuite/g++.dg/ext/is_convertible3.C | 66 ++++++++++++++++++++++
>  2 files changed, 67 insertions(+)
>  create mode 100644 gcc/testsuite/g++.dg/ext/is_convertible3.C
> 
> diff --git a/gcc/cp/method.cc b/gcc/cp/method.cc
> index c35a59fe56c..45f70f5d3f3 100644
> --- a/gcc/cp/method.cc
> +++ b/gcc/cp/method.cc
> @@ -2246,6 +2246,7 @@ is_convertible (tree from, tree to)
>  {
>    if (VOID_TYPE_P (from) && VOID_TYPE_P (to))
>      return true;
> +  cp_unevaluated u;
>    tree expr = build_stub_object (from);
>    expr = perform_implicit_conversion (to, expr, tf_none);
>    if (expr == error_mark_node)
> diff --git a/gcc/testsuite/g++.dg/ext/is_convertible3.C b/gcc/testsuite/g++.dg/ext/is_convertible3.C
> new file mode 100644
> index 00000000000..c817dc6f146
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/ext/is_convertible3.C
> @@ -0,0 +1,66 @@
> +// PR c++/106784
> +// { dg-do compile { target c++11 } }
> +// Make sure we don't reject this at runtime by trying to instantiate
> +// char_traits<CharT>::eq(CharT, CharT) while evaluating __is_convertible.
> +
> +template<bool B>
> +struct bool_constant { static constexpr bool value = B; };
> +using true_type = bool_constant<true>;
> +using false_type = bool_constant<false>;
> +
> +template<typename T> struct is_void : false_type { };
> +template<> struct is_void<void> : true_type { };
> +
> +template<typename T> T&& declval();
> +
> +template<bool> struct enable_if { };
> +template<> struct enable_if<true> { using type = void; };
> +template<bool B> using enable_if_t = typename enable_if<B>::type;
> +
> +template<typename _From, typename _To>
> +  struct is_convertible
> +  : public bool_constant<__is_convertible(_From, _To)>
> +  { };
> +
> +template<class CharT>
> +struct char_traits
> +{
> +  static unsigned long length(const char* s) { eq(*s, *s); return 0; }
> +
> +  static void eq(CharT l, CharT r) noexcept { l.f(r); }
> +};
> +
> +template<class CharT>
> +struct basic_string_view
> +{
> +  using traits_type = char_traits<CharT>;
> +
> +  constexpr basic_string_view() = default;
> +  constexpr basic_string_view(const basic_string_view&) = default;
> +
> +  constexpr
> +  basic_string_view(const CharT* __str) noexcept
> +  : _M_len{traits_type::length(__str)}
> +  { }
> +
> +  unsigned long _M_len = 0;
> +};
> +
> +template<class CharT>
> +struct basic_string
> +{
> +  template<class T>
> +    enable_if_t<is_convertible<const T&, basic_string_view<CharT>>::value
> +                && !is_convertible<const T&, const char*>::value>
> +    replace(int, T) { }
> +
> +  void replace(unsigned long, const char*) { }
> +
> +  void replace(const char* s) { replace(1, s); }
> +};
> +
> +int main()
> +{
> +  basic_string<char> s;
> +  s.replace("");
> +}
> 
> base-commit: 2460f7cdef7ef9c971de79271afc0db73687a272
> -- 
> 2.37.3
> 
> 

Re: [PATCH] c++: Instantiate less when evaluating __is_convertible

[Jonathan Wakely]

On Mon, 26 Sept 2022 at 16:23, Marek Polacek wrote:
>
> Jon reported that evaluating __is_convertible in this test leads to
> instantiating char_traits<char>::eq, which is invalid (because we
> are trying to call a member function on a char)

N.B. in the original code wasn't trying to do something dumb like call
a member function on a char, but it was using basic_string_view<X>
where X is a class type without an operator== and so
char_traits<X>::eq was invalid. I changed it to just use
basic_string_view<char> and changed char_traits::eq to do something
different, that was invalid for char.

I can provide a less silly test case if you like, but I don't think it
matters for the purposes of the testsuite.

Re: [PATCH] c++: Instantiate less when evaluating __is_convertible

[Jason Merrill]

On 9/26/22 11:51, Patrick Palka wrote:
> On Mon, 26 Sep 2022, Marek Polacek via Gcc-patches wrote:
> 
>> Jon reported that evaluating __is_convertible in this test leads to
>> instantiating char_traits<char>::eq, which is invalid (because we
>> are trying to call a member function on a char) and so we fail to
>> compile the test.  __is_convertible doesn't and shouldn't need to
>> instantiate so much, so let's limit it with a cp_unevaluated guard.
>>
>> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
>>
>> 	PR c++/106784
>>
>> gcc/cp/ChangeLog:
>>
>> 	* method.cc (is_convertible): Use cp_unevaluated.
> 
> I think is_nothrow_convertible would need cp_unevaluated too (or maybe we
> should define is_nothrow_convertible in terms of is_convertible).

Agreed.

> And the testcase can probably be minimized to something like:
> 
>    struct A;
>    struct B { template<class T> B(const T&) noexcept { T::nonexistent; } };
> 
>    static_assert(__is_convertible(const A&, B));
>    static_assert(__is_nothrow_convertible(const A&, B));
> 
>>
>> gcc/testsuite/ChangeLog:
>>
>> 	* g++.dg/ext/is_convertible3.C: New test.
>> ---
>>   gcc/cp/method.cc                           |  1 +
>>   gcc/testsuite/g++.dg/ext/is_convertible3.C | 66 ++++++++++++++++++++++
>>   2 files changed, 67 insertions(+)
>>   create mode 100644 gcc/testsuite/g++.dg/ext/is_convertible3.C
>>
>> diff --git a/gcc/cp/method.cc b/gcc/cp/method.cc
>> index c35a59fe56c..45f70f5d3f3 100644
>> --- a/gcc/cp/method.cc
>> +++ b/gcc/cp/method.cc
>> @@ -2246,6 +2246,7 @@ is_convertible (tree from, tree to)
>>   {
>>     if (VOID_TYPE_P (from) && VOID_TYPE_P (to))
>>       return true;
>> +  cp_unevaluated u;
>>     tree expr = build_stub_object (from);
>>     expr = perform_implicit_conversion (to, expr, tf_none);
>>     if (expr == error_mark_node)
>> diff --git a/gcc/testsuite/g++.dg/ext/is_convertible3.C b/gcc/testsuite/g++.dg/ext/is_convertible3.C
>> new file mode 100644
>> index 00000000000..c817dc6f146
>> --- /dev/null
>> +++ b/gcc/testsuite/g++.dg/ext/is_convertible3.C
>> @@ -0,0 +1,66 @@
>> +// PR c++/106784
>> +// { dg-do compile { target c++11 } }
>> +// Make sure we don't reject this at runtime by trying to instantiate
>> +// char_traits<CharT>::eq(CharT, CharT) while evaluating __is_convertible.
>> +
>> +template<bool B>
>> +struct bool_constant { static constexpr bool value = B; };
>> +using true_type = bool_constant<true>;
>> +using false_type = bool_constant<false>;
>> +
>> +template<typename T> struct is_void : false_type { };
>> +template<> struct is_void<void> : true_type { };
>> +
>> +template<typename T> T&& declval();
>> +
>> +template<bool> struct enable_if { };
>> +template<> struct enable_if<true> { using type = void; };
>> +template<bool B> using enable_if_t = typename enable_if<B>::type;
>> +
>> +template<typename _From, typename _To>
>> +  struct is_convertible
>> +  : public bool_constant<__is_convertible(_From, _To)>
>> +  { };
>> +
>> +template<class CharT>
>> +struct char_traits
>> +{
>> +  static unsigned long length(const char* s) { eq(*s, *s); return 0; }
>> +
>> +  static void eq(CharT l, CharT r) noexcept { l.f(r); }
>> +};
>> +
>> +template<class CharT>
>> +struct basic_string_view
>> +{
>> +  using traits_type = char_traits<CharT>;
>> +
>> +  constexpr basic_string_view() = default;
>> +  constexpr basic_string_view(const basic_string_view&) = default;
>> +
>> +  constexpr
>> +  basic_string_view(const CharT* __str) noexcept
>> +  : _M_len{traits_type::length(__str)}
>> +  { }
>> +
>> +  unsigned long _M_len = 0;
>> +};
>> +
>> +template<class CharT>
>> +struct basic_string
>> +{
>> +  template<class T>
>> +    enable_if_t<is_convertible<const T&, basic_string_view<CharT>>::value
>> +                && !is_convertible<const T&, const char*>::value>
>> +    replace(int, T) { }
>> +
>> +  void replace(unsigned long, const char*) { }
>> +
>> +  void replace(const char* s) { replace(1, s); }
>> +};
>> +
>> +int main()
>> +{
>> +  basic_string<char> s;
>> +  s.replace("");
>> +}
>>
>> base-commit: 2460f7cdef7ef9c971de79271afc0db73687a272
>> -- 
>> 2.37.3
>>
>>
> 

[PATCH v2] c++: Instantiate less when evaluating __is_convertible

[Marek Polacek]

On Mon, Sep 26, 2022 at 11:51:30AM -0400, Patrick Palka wrote:
> On Mon, 26 Sep 2022, Marek Polacek via Gcc-patches wrote:
> 
> > Jon reported that evaluating __is_convertible in this test leads to
> > instantiating char_traits<char>::eq, which is invalid (because we
> > are trying to call a member function on a char) and so we fail to
> > compile the test.  __is_convertible doesn't and shouldn't need to
> > instantiate so much, so let's limit it with a cp_unevaluated guard.
> > 
> > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> > 
> > 	PR c++/106784
> > 
> > gcc/cp/ChangeLog:
> > 
> > 	* method.cc (is_convertible): Use cp_unevaluated.
> 
> I think is_nothrow_convertible would need cp_unevaluated too (or maybe we
> should define is_nothrow_convertible in terms of is_convertible).

/facepalm, that's what I get by not using a single implementation for both!
 
> And the testcase can probably be minimized to something like:
> 
>   struct A;
>   struct B { template<class T> B(const T&) noexcept { T::nonexistent; } };
> 
>   static_assert(__is_convertible(const A&, B));
>   static_assert(__is_nothrow_convertible(const A&, B));

Adjusted, thanks.

Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

-- >8 --
Jon reported that evaluating __is_convertible in a test led to
instantiating something ill-formed and so we failed to compile the test.
__is_convertible doesn't and shouldn't need to instantiate so much, so
let's limit it with a cp_unevaluated guard.  Use a helper function to
implement both built-ins.

	PR c++/106784

gcc/cp/ChangeLog:

	* method.cc (is_convertible_helper): New.
	(is_convertible): Use it.
	(is_nothrow_convertible): Likewise.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/is_convertible3.C: New test.
	* g++.dg/ext/is_nothrow_convertible3.C: New test.
---
 gcc/cp/method.cc                              | 23 ++++++++++++-------
 gcc/testsuite/g++.dg/ext/is_convertible3.C    |  9 ++++++++
 .../g++.dg/ext/is_nothrow_convertible3.C      |  9 ++++++++
 3 files changed, 33 insertions(+), 8 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/ext/is_convertible3.C
 create mode 100644 gcc/testsuite/g++.dg/ext/is_nothrow_convertible3.C

diff --git a/gcc/cp/method.cc b/gcc/cp/method.cc
index c35a59fe56c..9f917f13134 100644
--- a/gcc/cp/method.cc
+++ b/gcc/cp/method.cc
@@ -2236,6 +2236,19 @@ ref_xes_from_temporary (tree to, tree from, bool direct_init_p)
   return ref_conv_binds_directly (to, val, direct_init_p).is_false ();
 }
 
+/* Worker for is_{,nothrow_}convertible.  Attempt to perform an implicit
+   conversion from FROM to TO and return the result.  */
+
+static tree
+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);
+  return perform_implicit_conversion (to, expr, tf_none);
+}
+
 /* Return true if FROM can be converted to TO using implicit conversions,
    or both FROM and TO are possibly cv-qualified void.  NB: This doesn't
    implement the "Access checks are performed as if from a context unrelated
@@ -2244,10 +2257,7 @@ ref_xes_from_temporary (tree to, tree from, bool direct_init_p)
 bool
 is_convertible (tree from, tree to)
 {
-  if (VOID_TYPE_P (from) && VOID_TYPE_P (to))
-    return true;
-  tree expr = build_stub_object (from);
-  expr = perform_implicit_conversion (to, expr, tf_none);
+  tree expr = is_convertible_helper (from, to);
   if (expr == error_mark_node)
     return false;
   return !!expr;
@@ -2258,10 +2268,7 @@ is_convertible (tree from, tree to)
 bool
 is_nothrow_convertible (tree from, tree to)
 {
-  if (VOID_TYPE_P (from) && VOID_TYPE_P (to))
-    return true;
-  tree expr = build_stub_object (from);
-  expr = perform_implicit_conversion (to, expr, tf_none);
+  tree expr = is_convertible_helper (from, to);
   if (expr == NULL_TREE || expr == error_mark_node)
     return false;
   return expr_noexcept_p (expr, tf_none);
diff --git a/gcc/testsuite/g++.dg/ext/is_convertible3.C b/gcc/testsuite/g++.dg/ext/is_convertible3.C
new file mode 100644
index 00000000000..7a986d075c2
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/is_convertible3.C
@@ -0,0 +1,9 @@
+// PR c++/106784
+// { dg-do compile { target c++11 } }
+// Make sure we don't reject this at runtime by trying to instantiate
+// something that would be ill-formed.
+
+struct A;
+struct B { template<class T> B(const T&) noexcept { T::nonexistent; } };
+
+static_assert(__is_convertible(const A&, B), "");
diff --git a/gcc/testsuite/g++.dg/ext/is_nothrow_convertible3.C b/gcc/testsuite/g++.dg/ext/is_nothrow_convertible3.C
new file mode 100644
index 00000000000..05b1e1d9ad9
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/is_nothrow_convertible3.C
@@ -0,0 +1,9 @@
+// PR c++/106784
+// { dg-do compile { target c++11 } }
+// Make sure we don't reject this at runtime by trying to instantiate
+// something that would be ill-formed.
+
+struct A;
+struct B { template<class T> B(const T&) noexcept { T::nonexistent; } };
+
+static_assert(__is_nothrow_convertible(const A&, B), "");

base-commit: 099a66498bf7a40764002793eba66c881a251b76
-- 
2.37.3

Re: [PATCH] c++: Instantiate less when evaluating __is_convertible

[Marek Polacek]

On Mon, Sep 26, 2022 at 05:02:36PM +0100, Jonathan Wakely wrote:
> On Mon, 26 Sept 2022 at 16:23, Marek Polacek wrote:
> >
> > Jon reported that evaluating __is_convertible in this test leads to
> > instantiating char_traits<char>::eq, which is invalid (because we
> > are trying to call a member function on a char)
> 
> N.B. in the original code wasn't trying to do something dumb like call
> a member function on a char, but it was using basic_string_view<X>
> where X is a class type without an operator== and so
> char_traits<X>::eq was invalid. I changed it to just use
> basic_string_view<char> and changed char_traits::eq to do something
> different, that was invalid for char.

Ack.

> I can provide a less silly test case if you like, but I don't think it
> matters for the purposes of the testsuite.

I think no need to, I'm going to use Patrick's short test.

Thanks,

Marek

Re: [PATCH v2] c++: Instantiate less when evaluating __is_convertible

[Jason Merrill]

On 9/26/22 12:25, Marek Polacek wrote:
> On Mon, Sep 26, 2022 at 11:51:30AM -0400, Patrick Palka wrote:
>> On Mon, 26 Sep 2022, Marek Polacek via Gcc-patches wrote:
>>
>>> Jon reported that evaluating __is_convertible in this test leads to
>>> instantiating char_traits<char>::eq, which is invalid (because we
>>> are trying to call a member function on a char) and so we fail to
>>> compile the test.  __is_convertible doesn't and shouldn't need to
>>> instantiate so much, so let's limit it with a cp_unevaluated guard.
>>>
>>> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
>>>
>>> 	PR c++/106784
>>>
>>> gcc/cp/ChangeLog:
>>>
>>> 	* method.cc (is_convertible): Use cp_unevaluated.
>>
>> I think is_nothrow_convertible would need cp_unevaluated too (or maybe we
>> should define is_nothrow_convertible in terms of is_convertible).
> 
> /facepalm, that's what I get by not using a single implementation for both!
>   
>> And the testcase can probably be minimized to something like:
>>
>>    struct A;
>>    struct B { template<class T> B(const T&) noexcept { T::nonexistent; } };
>>
>>    static_assert(__is_convertible(const A&, B));
>>    static_assert(__is_nothrow_convertible(const A&, B));
> 
> Adjusted, thanks.
> 
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

OK.

> -- >8 --
> Jon reported that evaluating __is_convertible in a test led to
> instantiating something ill-formed and so we failed to compile the test.
> __is_convertible doesn't and shouldn't need to instantiate so much, so
> let's limit it with a cp_unevaluated guard.  Use a helper function to
> implement both built-ins.
> 
> 	PR c++/106784
> 
> gcc/cp/ChangeLog:
> 
> 	* method.cc (is_convertible_helper): New.
> 	(is_convertible): Use it.
> 	(is_nothrow_convertible): Likewise.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/ext/is_convertible3.C: New test.
> 	* g++.dg/ext/is_nothrow_convertible3.C: New test.
> ---
>   gcc/cp/method.cc                              | 23 ++++++++++++-------
>   gcc/testsuite/g++.dg/ext/is_convertible3.C    |  9 ++++++++
>   .../g++.dg/ext/is_nothrow_convertible3.C      |  9 ++++++++
>   3 files changed, 33 insertions(+), 8 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/ext/is_convertible3.C
>   create mode 100644 gcc/testsuite/g++.dg/ext/is_nothrow_convertible3.C
> 
> diff --git a/gcc/cp/method.cc b/gcc/cp/method.cc
> index c35a59fe56c..9f917f13134 100644
> --- a/gcc/cp/method.cc
> +++ b/gcc/cp/method.cc
> @@ -2236,6 +2236,19 @@ ref_xes_from_temporary (tree to, tree from, bool direct_init_p)
>     return ref_conv_binds_directly (to, val, direct_init_p).is_false ();
>   }
>   
> +/* Worker for is_{,nothrow_}convertible.  Attempt to perform an implicit
> +   conversion from FROM to TO and return the result.  */
> +
> +static tree
> +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);
> +  return perform_implicit_conversion (to, expr, tf_none);
> +}
> +
>   /* Return true if FROM can be converted to TO using implicit conversions,
>      or both FROM and TO are possibly cv-qualified void.  NB: This doesn't
>      implement the "Access checks are performed as if from a context unrelated
> @@ -2244,10 +2257,7 @@ ref_xes_from_temporary (tree to, tree from, bool direct_init_p)
>   bool
>   is_convertible (tree from, tree to)
>   {
> -  if (VOID_TYPE_P (from) && VOID_TYPE_P (to))
> -    return true;
> -  tree expr = build_stub_object (from);
> -  expr = perform_implicit_conversion (to, expr, tf_none);
> +  tree expr = is_convertible_helper (from, to);
>     if (expr == error_mark_node)
>       return false;
>     return !!expr;
> @@ -2258,10 +2268,7 @@ is_convertible (tree from, tree to)
>   bool
>   is_nothrow_convertible (tree from, tree to)
>   {
> -  if (VOID_TYPE_P (from) && VOID_TYPE_P (to))
> -    return true;
> -  tree expr = build_stub_object (from);
> -  expr = perform_implicit_conversion (to, expr, tf_none);
> +  tree expr = is_convertible_helper (from, to);
>     if (expr == NULL_TREE || expr == error_mark_node)
>       return false;
>     return expr_noexcept_p (expr, tf_none);
> diff --git a/gcc/testsuite/g++.dg/ext/is_convertible3.C b/gcc/testsuite/g++.dg/ext/is_convertible3.C
> new file mode 100644
> index 00000000000..7a986d075c2
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/ext/is_convertible3.C
> @@ -0,0 +1,9 @@
> +// PR c++/106784
> +// { dg-do compile { target c++11 } }
> +// Make sure we don't reject this at runtime by trying to instantiate
> +// something that would be ill-formed.
> +
> +struct A;
> +struct B { template<class T> B(const T&) noexcept { T::nonexistent; } };
> +
> +static_assert(__is_convertible(const A&, B), "");
> diff --git a/gcc/testsuite/g++.dg/ext/is_nothrow_convertible3.C b/gcc/testsuite/g++.dg/ext/is_nothrow_convertible3.C
> new file mode 100644
> index 00000000000..05b1e1d9ad9
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/ext/is_nothrow_convertible3.C
> @@ -0,0 +1,9 @@
> +// PR c++/106784
> +// { dg-do compile { target c++11 } }
> +// Make sure we don't reject this at runtime by trying to instantiate
> +// something that would be ill-formed.
> +
> +struct A;
> +struct B { template<class T> B(const T&) noexcept { T::nonexistent; } };
> +
> +static_assert(__is_nothrow_convertible(const A&, B), "");
> 
> base-commit: 099a66498bf7a40764002793eba66c881a251b76