[clang] 21.1.0 C++20 regression "no matching conversion for functional-style cast" for template specialization #157933
Description
The following code worked in previous versions of clang, and works in MSVC, and GCC, but fails in Clang 21.1.0, this code attempts to specialize the function "set_data" via SFINAE with requires for scalars, std::span using a special "ns_is_specialization_of" macro with links to where this was taken from, and explicit specialization for std::string_view and std::string
#include <type_traits>
#include <string_view>
#include <span>
#include <iostream>
#include <cstdint>
namespace ns{
//From https://stackoverflow.com/a/55398444/, only source where I could find that handles non type templates
namespace detail {
template<auto f>
struct is_specialization_of_impl {
private:
template<class T>
static auto value_impl(int) -> std::is_same<T, decltype(f.template operator()<T>())>;
template<class T>
static auto value_impl(...) -> std::false_type;
public:
template<class T>
static constexpr bool value = decltype(value_impl<T>(0))::value;
};
// To replace std::declval which yields T&&
template<class T>
T declrval();
}
}
#define ns_is_specialization_of(...) \
ns::detail::is_specialization_of_impl<[]<class NsImplTemplate_T>() -> decltype(__VA_ARGS__(ns::detail::declrval<NsImplTemplate_T>())) { }>::template value
template <typename T>
void set_data(std::string_view key, T values) = delete;
void set_data(std::string_view key, const void* value, std::size_t value_count){
std::cout << key << (uintptr_t)(value) << value_count;
}
//attempting to specialize set_data for scalars, std::span, string_view, and string all separatley
template <typename T>
requires(ns_is_specialization_of(std::span)<T>)
void set_data(std::string_view key, T values) {
set_data(key, values.data(), values.size());
}
template <typename T>
requires std::integral<T>
|| std::floating_point<T>
|| std::same_as<std::remove_cvref<T>(), bool>
void set_data(std::string_view key, T values) {
set_data(key, &values, 1);
}
template <>
inline void set_data(std::string_view key, std::string_view value) {
set_data(key, value.data(), value.size());
}
template <>
inline void set_data(std::string_view key, std::string value) {
// verify_data_type<std::string>(key, value.size());
set_data(key, value.data(), value.size());
}
int test_function(int num) {
std::span<char> test;
set_data("hello", test);
return num * num;
}
This results in the rather confusing error:
<source>:40:38: error: no matching conversion for functional-style cast from 'std::basic_string<char>' to 'std::span<remove_reference_t<ranges::range_reference_t<std::basic_string<char, std::char_traits<char>, std::allocator<char>> &>>>' (aka 'span<char>')
40 | requires(ns_is_specialization_of(std::span)<T>)
| ~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~
<source>:30:85: note: expanded from macro 'ns_is_specialization_of'
30 | ns::detail::is_specialization_of_impl<[]<class NsImplTemplate_T>() -> decltype(__VA_ARGS__(ns::detail::declrval<NsImplTemplate_T>())) { }>::template value
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<source>:40:14: note: in instantiation of member class '(unnamed class at <source>:40:14)' requested here
40 | requires(ns_is_specialization_of(std::span)<T>)
| ^
<source>:30:44: note: expanded from macro 'ns_is_specialization_of'
30 | ns::detail::is_specialization_of_impl<[]<class NsImplTemplate_T>() -> decltype(__VA_ARGS__(ns::detail::declrval<NsImplTemplate_T>())) { }>::template value
| ^
<source>:21:52: note: while substituting explicitly-specified template arguments into function template 'value_impl'
21 | static constexpr bool value = decltype(value_impl<T>(0))::value;
| ^
<source>:40:14: note: in instantiation of static data member 'ns::detail::is_specialization_of_impl<(lambda at <source>:40:14){}>::value<std::basic_string<char>>' requested here
40 | requires(ns_is_specialization_of(std::span)<T>)
| ^
<source>:30:155: note: expanded from macro 'ns_is_specialization_of'
30 | ns::detail::is_specialization_of_impl<[]<class NsImplTemplate_T>() -> decltype(__VA_ARGS__(ns::detail::declrval<NsImplTemplate_T>())) { }>::template value
| ^
<source>:40:14: note: while substituting template arguments into constraint expression here
40 | requires(ns_is_specialization_of(std::span)<T>)
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<source>:30:6: note: expanded from macro 'ns_is_specialization_of'
30 | ns::detail::is_specialization_of_impl<[]<class NsImplTemplate_T>() -> decltype(__VA_ARGS__(ns::detail::declrval<NsImplTemplate_T>())) { }>::template value
| ^
<source>:60:13: note: while checking constraint satisfaction for template 'set_data<std::basic_string<char>>' required here
60 | inline void set_data(std::string_view key, std::string value) {
| ^~~~~~~~
<source>:60:13: note: while substituting deduced template arguments into function template 'set_data' [with T = std::string]
/opt/compiler-explorer/gcc-15.2.0/lib/gcc/x86_64-linux-gnu/15.2.0/../../../../include/c++/15.2.0/span:248:7: note: candidate constructor not viable: no known conversion from 'std::basic_string<char>' to 'const span<char>' for 1st argument
248 | span(const span&) noexcept = default;
| ^ ~~~~~~~~~~~
/opt/compiler-explorer/gcc-15.2.0/lib/gcc/x86_64-linux-gnu/15.2.0/../../../../include/c++/15.2.0/span:203:2: note: candidate template ignored: could not match 'type_identity_t<element_type>[_ArrayExtent]' (aka 'char[_ArrayExtent]') against 'std::basic_string<char>'
203 | span(type_identity_t<element_type> (&__arr)[_ArrayExtent]) noexcept
| ^
/opt/compiler-explorer/gcc-15.2.0/lib/gcc/x86_64-linux-gnu/15.2.0/../../../../include/c++/15.2.0/span:210:2: note: candidate template ignored: could not match 'array' against 'std::basic_string'
210 | span(array<_Tp, _ArrayExtent>& __arr) noexcept
| ^
/opt/compiler-explorer/gcc-15.2.0/lib/gcc/x86_64-linux-gnu/15.2.0/../../../../include/c++/15.2.0/span:217:2: note: candidate template ignored: could not match 'array' against 'std::basic_string'
217 | span(const array<_Tp, _ArrayExtent>& __arr) noexcept
| ^
/opt/compiler-explorer/gcc-15.2.0/lib/gcc/x86_64-linux-gnu/15.2.0/../../../../include/c++/15.2.0/span:229:2: note: candidate template ignored: constraints not satisfied [with _Range = std::basic_string<char>]
229 | span(_Range&& __range)
| ^
/opt/compiler-explorer/gcc-15.2.0/lib/gcc/x86_64-linux-gnu/15.2.0/../../../../include/c++/15.2.0/span:226:8: note: because 'std::basic_string<char>' does not satisfy 'borrowed_range'
226 | && (ranges::borrowed_range<_Range> || is_const_v<element_type>)
| ^
/opt/compiler-explorer/gcc-15.2.0/lib/gcc/x86_64-linux-gnu/15.2.0/../../../../include/c++/15.2.0/bits/ranges_base.h:523:23: note: because 'std::basic_string<char>' does not satisfy '__maybe_borrowed_range'
523 | = range<_Tp> && __detail::__maybe_borrowed_range<_Tp>;
| ^
/opt/compiler-explorer/gcc-15.2.0/lib/gcc/x86_64-linux-gnu/15.2.0/../../../../include/c++/15.2.0/bits/ranges_base.h:101:4: note: because 'is_lvalue_reference_v<std::basic_string<char>>' evaluated to false
101 | = is_lvalue_reference_v<_Tp>
| ^
/opt/compiler-explorer/gcc-15.2.0/lib/gcc/x86_64-linux-gnu/15.2.0/../../../../include/c++/15.2.0/bits/ranges_base.h:102:7: note: and 'enable_borrowed_range<remove_cvref_t<std::basic_string<char, std::char_traits<char>, std::allocator<char>>>>' evaluated to false
102 | || enable_borrowed_range<remove_cvref_t<_Tp>>;
| ^
/opt/compiler-explorer/gcc-15.2.0/lib/gcc/x86_64-linux-gnu/15.2.0/../../../../include/c++/15.2.0/span:226:42: note: and 'is_const_v<element_type>' evaluated to false
226 | && (ranges::borrowed_range<_Range> || is_const_v<element_type>)
| ^
/opt/compiler-explorer/gcc-15.2.0/lib/gcc/x86_64-linux-gnu/15.2.0/../../../../include/c++/15.2.0/span:256:2: note: candidate template ignored: could not match 'span' against 'std::basic_string'
256 | span(const span<_OType, _OExtent>& __s) noexcept
| ^
/opt/compiler-explorer/gcc-15.2.0/lib/gcc/x86_64-linux-gnu/15.2.0/../../../../include/c++/15.2.0/span:470:7: note: candidate constructor not viable: constraints not satisfied
470 | span(_SizedPtr __ptr) noexcept
| ^
/opt/compiler-explorer/gcc-15.2.0/lib/gcc/x86_64-linux-gnu/15.2.0/../../../../include/c++/15.2.0/span:471:17: note: because 'extent != dynamic_extent' (18446744073709551615 != 18446744073709551615) evaluated to false
471 | requires (extent != dynamic_extent)
| ^
/opt/compiler-explorer/gcc-15.2.0/lib/gcc/x86_64-linux-gnu/15.2.0/../../../../include/c++/15.2.0/span:177:7: note: candidate constructor not viable: requires 0 arguments, but 1 was provided
177 | span() noexcept
| ^
/opt/compiler-explorer/gcc-15.2.0/lib/gcc/x86_64-linux-gnu/15.2.0/../../../../include/c++/15.2.0/span:185:2: note: candidate constructor template not viable: requires 2 arguments, but 1 was provided
185 | span(_It __first, size_type __count)
| ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/opt/compiler-explorer/gcc-15.2.0/lib/gcc/x86_64-linux-gnu/15.2.0/../../../../include/c++/15.2.0/span:194:2: note: candidate constructor template not viable: requires 2 arguments, but 1 was provided
194 | span(_It __first, _End __last)
| ^ ~~~~~~~~~~~~~~~~~~~~~~~~
<source>:40:38: error: no matching conversion for functional-style cast from 'std::basic_string<char>' to 'std::span<remove_reference_t<ranges::range_reference_t<std::basic_string<char, std::char_traits<char>, std::allocator<char>> &>>>' (aka 'span<char>')
40 | requires(ns_is_specialization_of(std::span)<T>)
| ~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~
<source>:30:85: note: expanded from macro 'ns_is_specialization_of'
30 | ns::detail::is_specialization_of_impl<[]<class NsImplTemplate_T>() -> decltype(__VA_ARGS__(ns::detail::declrval<NsImplTemplate_T>())) { }>::template value
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/opt/compiler-explorer/gcc-15.2.0/lib/gcc/x86_64-linux-gnu/15.2.0/../../../../include/c++/15.2.0/span:248:7: note: candidate constructor not viable: no known conversion from 'std::basic_string<char>' to 'const span<char>' for 1st argument
248 | span(const span&) noexcept = default;
| ^ ~~~~~~~~~~~
/opt/compiler-explorer/gcc-15.2.0/lib/gcc/x86_64-linux-gnu/15.2.0/../../../../include/c++/15.2.0/span:203:2: note: candidate template ignored: could not match 'type_identity_t<element_type>[_ArrayExtent]' (aka 'char[_ArrayExtent]') against 'std::basic_string<char>'
203 | span(type_identity_t<element_type> (&__arr)[_ArrayExtent]) noexcept
| ^
/opt/compiler-explorer/gcc-15.2.0/lib/gcc/x86_64-linux-gnu/15.2.0/../../../../include/c++/15.2.0/span:210:2: note: candidate template ignored: could not match 'array' against 'std::basic_string'
210 | span(array<_Tp, _ArrayExtent>& __arr) noexcept
| ^
/opt/compiler-explorer/gcc-15.2.0/lib/gcc/x86_64-linux-gnu/15.2.0/../../../../include/c++/15.2.0/span:217:2: note: candidate template ignored: could not match 'array' against 'std::basic_string'
217 | span(const array<_Tp, _ArrayExtent>& __arr) noexcept
| ^
/opt/compiler-explorer/gcc-15.2.0/lib/gcc/x86_64-linux-gnu/15.2.0/../../../../include/c++/15.2.0/span:229:2: note: candidate template ignored: constraints not satisfied [with _Range = std::basic_string<char>]
229 | span(_Range&& __range)
| ^
/opt/compiler-explorer/gcc-15.2.0/lib/gcc/x86_64-linux-gnu/15.2.0/../../../../include/c++/15.2.0/span:226:8: note: because 'std::basic_string<char>' does not satisfy 'borrowed_range'
226 | && (ranges::borrowed_range<_Range> || is_const_v<element_type>)
| ^
/opt/compiler-explorer/gcc-15.2.0/lib/gcc/x86_64-linux-gnu/15.2.0/../../../../include/c++/15.2.0/bits/ranges_base.h:523:23: note: because 'std::basic_string<char>' does not satisfy '__maybe_borrowed_range'
523 | = range<_Tp> && __detail::__maybe_borrowed_range<_Tp>;
| ^
/opt/compiler-explorer/gcc-15.2.0/lib/gcc/x86_64-linux-gnu/15.2.0/../../../../include/c++/15.2.0/bits/ranges_base.h:101:4: note: because 'is_lvalue_reference_v<std::basic_string<char>>' evaluated to false
101 | = is_lvalue_reference_v<_Tp>
| ^
/opt/compiler-explorer/gcc-15.2.0/lib/gcc/x86_64-linux-gnu/15.2.0/../../../../include/c++/15.2.0/bits/ranges_base.h:102:7: note: and 'enable_borrowed_range<remove_cvref_t<std::basic_string<char, std::char_traits<char>, std::allocator<char>>>>' evaluated to false
102 | || enable_borrowed_range<remove_cvref_t<_Tp>>;
| ^
/opt/compiler-explorer/gcc-15.2.0/lib/gcc/x86_64-linux-gnu/15.2.0/../../../../include/c++/15.2.0/span:226:42: note: and 'is_const_v<element_type>' evaluated to false
226 | && (ranges::borrowed_range<_Range> || is_const_v<element_type>)
| ^
/opt/compiler-explorer/gcc-15.2.0/lib/gcc/x86_64-linux-gnu/15.2.0/../../../../include/c++/15.2.0/span:256:2: note: candidate template ignored: could not match 'span' against 'std::basic_string'
256 | span(const span<_OType, _OExtent>& __s) noexcept
| ^
/opt/compiler-explorer/gcc-15.2.0/lib/gcc/x86_64-linux-gnu/15.2.0/../../../../include/c++/15.2.0/span:470:7: note: candidate constructor not viable: constraints not satisfied
470 | span(_SizedPtr __ptr) noexcept
| ^
/opt/compiler-explorer/gcc-15.2.0/lib/gcc/x86_64-linux-gnu/15.2.0/../../../../include/c++/15.2.0/span:471:17: note: because 'extent != dynamic_extent' (18446744073709551615 != 18446744073709551615) evaluated to false
471 | requires (extent != dynamic_extent)
| ^
/opt/compiler-explorer/gcc-15.2.0/lib/gcc/x86_64-linux-gnu/15.2.0/../../../../include/c++/15.2.0/span:177:7: note: candidate constructor not viable: requires 0 arguments, but 1 was provided
177 | span() noexcept
| ^
/opt/compiler-explorer/gcc-15.2.0/lib/gcc/x86_64-linux-gnu/15.2.0/../../../../include/c++/15.2.0/span:185:2: note: candidate constructor template not viable: requires 2 arguments, but 1 was provided
185 | span(_It __first, size_type __count)
| ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/opt/compiler-explorer/gcc-15.2.0/lib/gcc/x86_64-linux-gnu/15.2.0/../../../../include/c++/15.2.0/span:194:2: note: candidate constructor template not viable: requires 2 arguments, but 1 was provided
194 | span(_It __first, _End __last)
| ^ ~~~~~~~~~~~~~~~~~~~~~~~~
<source>:40:38: error: no matching conversion for functional-style cast from 'std::basic_string<char>' to 'std::span<remove_reference_t<ranges::range_reference_t<std::basic_string<char, std::char_traits<char>, std::allocator<char>> &>>>' (aka 'span<char>')
40 | requires(ns_is_specialization_of(std::span)<T>)
| ~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~
<source>:30:85: note: expanded from macro 'ns_is_specialization_of'
30 | ns::detail::is_specialization_of_impl<[]<class NsImplTemplate_T>() -> decltype(__VA_ARGS__(ns::detail::declrval<NsImplTemplate_T>())) { }>::template value
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/opt/compiler-explorer/gcc-15.2.0/lib/gcc/x86_64-linux-gnu/15.2.0/../../../../include/c++/15.2.0/span:248:7: note: candidate constructor not viable: no known conversion from 'std::basic_string<char>' to 'const span<char>' for 1st argument
248 | span(const span&) noexcept = default;
| ^ ~~~~~~~~~~~
/opt/compiler-explorer/gcc-15.2.0/lib/gcc/x86_64-linux-gnu/15.2.0/../../../../include/c++/15.2.0/span:203:2: note: candidate template ignored: could not match 'type_identity_t<element_type>[_ArrayExtent]' (aka 'char[_ArrayExtent]') against 'std::basic_string<char>'
203 | span(type_identity_t<element_type> (&__arr)[_ArrayExtent]) noexcept
| ^
/opt/compiler-explorer/gcc-15.2.0/lib/gcc/x86_64-linux-gnu/15.2.0/../../../../include/c++/15.2.0/span:210:2: note: candidate template ignored: could not match 'array' against 'std::basic_string'
210 | span(array<_Tp, _ArrayExtent>& __arr) noexcept
| ^
/opt/compiler-explorer/gcc-15.2.0/lib/gcc/x86_64-linux-gnu/15.2.0/../../../../include/c++/15.2.0/span:217:2: note: candidate template ignored: could not match 'array' against 'std::basic_string'
217 | span(const array<_Tp, _ArrayExtent>& __arr) noexcept
| ^
/opt/compiler-explorer/gcc-15.2.0/lib/gcc/x86_64-linux-gnu/15.2.0/../../../../include/c++/15.2.0/span:229:2: note: candidate template ignored: constraints not satisfied [with _Range = std::basic_string<char>]
229 | span(_Range&& __range)
| ^
/opt/compiler-explorer/gcc-15.2.0/lib/gcc/x86_64-linux-gnu/15.2.0/../../../../include/c++/15.2.0/span:226:8: note: because 'std::basic_string<char>' does not satisfy 'borrowed_range'
226 | && (ranges::borrowed_range<_Range> || is_const_v<element_type>)
| ^
/opt/compiler-explorer/gcc-15.2.0/lib/gcc/x86_64-linux-gnu/15.2.0/../../../../include/c++/15.2.0/bits/ranges_base.h:523:23: note: because 'std::basic_string<char>' does not satisfy '__maybe_borrowed_range'
523 | = range<_Tp> && __detail::__maybe_borrowed_range<_Tp>;
| ^
/opt/compiler-explorer/gcc-15.2.0/lib/gcc/x86_64-linux-gnu/15.2.0/../../../../include/c++/15.2.0/bits/ranges_base.h:101:4: note: because 'is_lvalue_reference_v<std::basic_string<char>>' evaluated to false
101 | = is_lvalue_reference_v<_Tp>
| ^
/opt/compiler-explorer/gcc-15.2.0/lib/gcc/x86_64-linux-gnu/15.2.0/../../../../include/c++/15.2.0/bits/ranges_base.h:102:7: note: and 'enable_borrowed_range<remove_cvref_t<std::basic_string<char, std::char_traits<char>, std::allocator<char>>>>' evaluated to false
102 | || enable_borrowed_range<remove_cvref_t<_Tp>>;
| ^
/opt/compiler-explorer/gcc-15.2.0/lib/gcc/x86_64-linux-gnu/15.2.0/../../../../include/c++/15.2.0/span:226:42: note: and 'is_const_v<element_type>' evaluated to false
226 | && (ranges::borrowed_range<_Range> || is_const_v<element_type>)
| ^
/opt/compiler-explorer/gcc-15.2.0/lib/gcc/x86_64-linux-gnu/15.2.0/../../../../include/c++/15.2.0/span:256:2: note: candidate template ignored: could not match 'span' against 'std::basic_string'
256 | span(const span<_OType, _OExtent>& __s) noexcept
| ^
/opt/compiler-explorer/gcc-15.2.0/lib/gcc/x86_64-linux-gnu/15.2.0/../../../../include/c++/15.2.0/span:470:7: note: candidate constructor not viable: constraints not satisfied
470 | span(_SizedPtr __ptr) noexcept
| ^
/opt/compiler-explorer/gcc-15.2.0/lib/gcc/x86_64-linux-gnu/15.2.0/../../../../include/c++/15.2.0/span:471:17: note: because 'extent != dynamic_extent' (18446744073709551615 != 18446744073709551615) evaluated to false
471 | requires (extent != dynamic_extent)
| ^
/opt/compiler-explorer/gcc-15.2.0/lib/gcc/x86_64-linux-gnu/15.2.0/../../../../include/c++/15.2.0/span:177:7: note: candidate constructor not viable: requires 0 arguments, but 1 was provided
177 | span() noexcept
| ^
/opt/compiler-explorer/gcc-15.2.0/lib/gcc/x86_64-linux-gnu/15.2.0/../../../../include/c++/15.2.0/span:185:2: note: candidate constructor template not viable: requires 2 arguments, but 1 was provided
185 | span(_It __first, size_type __count)
| ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/opt/compiler-explorer/gcc-15.2.0/lib/gcc/x86_64-linux-gnu/15.2.0/../../../../include/c++/15.2.0/span:194:2: note: candidate constructor template not viable: requires 2 arguments, but 1 was provided
194 | span(_It __first, _End __last)
| ^ ~~~~~~~~~~~~~~~~~~~~~~~~
3 errors generated.
Compiler returned: 1
Godbolt Links:
Fails Clang21.1.0
Succeeds: Clang20.1.0
Succeeds: msvc v19.43 VS17.13
Succeeds: GCC 15.2