为什么编译器现在接受从 std::stringstream::operator<<() 返回的 std::ostream& 上调用 str() 成员？

Question

Consider the following line:

std::string s = (std::stringstream() << "foo").str();

This should not compile because std::stringstream::operator<<() is inherited by std::ostream and returns a std::ostream& which does not have an str() member.

It seems the main compilers are now accepting this code where they didn't in the past. I would like to understand what standard change happened to make this to compile?

I made some tests with gcc , clang and msvc and I could find the version where the change happened:

Compiler	Rejects until (version)	Accepts from (version)
GCC	11.1	11.2
Clang	12.0.1	13.0.0
MSVC	v19.14	v19.15

^{You can find the test here}

Answer 1

They all added the rvalue overload (see here ) very late.

The rvalue overload was introduced in C++11 and returns the same type of stream as its left-hand operand.