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

[Patrick Palka <ppalka@redhat.com>]

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
> 
>