Why C++ Template function `auto` return type deduction fails on GCC but works on Clang and MSVC

I am trying to build the following code:

#include <functional>#include <iostream>#include <limits>#include <locale>#include <sstream>#include <string>#include <type_traits>const std::locale defaultLocale = std::locale("");template <typename T, const std::locale& locale = defaultLocale>auto convert(const std::string& str) {  auto instream = std::istringstream{str};  instream.imbue(locale);  T result;  instream >> result;  return result;}template <>auto convert<std::string>(const std::string& val) {  return val;}template <typename T>T env(const std::string& var, T deflt,      std::function<T(const std::string&)> transform = convert<T>) {  auto envVar = std::getenv(var.c_str());  if (envVar == nullptr) return (deflt);  try {    return transform(var);  } catch (std::exception& ex) {    return (deflt);  }}int main(int argc, char** argv) {  int n = 0;  auto stupido2 = env<int>("PATH", n);  return 0;}

Here's the compiler explorer link: https://godbolt.org/z/HAAr2-

I tried to go back even to clang 5.0, and it works.

However, If I change the template functions definitions to the following (replacing auto with T and std::string:

template <typename T, const std::locale& locale = defaultLocale>T convert(const std::string& str) {  auto instream = std::istringstream{str};  instream.imbue(locale);  T result;  instream >> result;  return result;}template <>std::string convert<std::string>(const std::string& val) {  return val;}

It works. Is it a bug in GCC or in MSVC/Clang? Some compiler extension that I should turn on for GCC?