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
