Why is templated constexpr with std::string allowed in gcc?

Question

Why is the template version allowed to compile in gcc? Is it a compiler bug or is it actually valid when used with templates? Can someone explain this to me, please?

It does not compile on clang or other compilers used on godbolt.org.

The compile errors generates from both string and stringstream being used in a constexpr.

#include <iostream>
#include <string>
#include <sstream>

template<typename T>
constexpr std::string func1(T a, T b) //Compiles and runs
{
  std::stringstream ss;
  ss << a << b << a+b;
  return ss.str();
}

constexpr std::string func2(int a, int b) //Compile error
{
  std::stringstream ss;
  ss << a << b << a+b;
  return ss.str();
}

int main()
{
  int a = 5;
  int b = 7;
  std::cout << func1(a,b) << std::endl;
  return 0;
}

Answer 1

GCC could be in the right here. According to dcl.constexpr paragraph 6:

If the instantiated template specialization of a constexpr function template or member function of a class template would fail to satisfy the requirements for a constexpr function or constexpr constructor, that specialization is still a constexpr function or constexpr constructor, even though a call to such a function cannot appear in a constant expression. If no specialization of the template would satisfy the requirements for a constexpr function or constexpr constructor when considered as a non-template function or constructor, the template is ill-formed, no diagnostic required.

The program is ill-formed (std::string is not a literal type) but it is not required to emit a diagnostic.