When I compile the following code with -std=c++26

#include <string>

class Foo {
public:
    constexpr static auto foo = std::string{"this_is_ok"};
    constexpr static auto bar = std::string{"a_very_long_string_that_probably_dont_fit_in_SSO"};
};

On Compiler Explorer

I got

<source>:6:27: error: constexpr variable 'bar' must be initialized by a constant expression
    6 |     constexpr static auto bar = std::string{"a_very_long_string_that_probably_dont_fit_in_SSO"};
      |                           ^     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<source>:6:27: note: pointer to subobject of heap-allocated object is not a constant expression
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/15.0.0/../../../../include/c++/15.0.0/bits/allocator.h:193:31: note: heap allocation performed here
  193 |             return static_cast<_Tp*>(::operator new(__n));
      |            

Does this behavior of std::string conform to the standard?

I need a workaround to have some long std::string objects in constexpr.