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

[Jason Merrill <jason@redhat.com>]

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
>>
>>
>