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
>
>
