<string> forward declares a template for the output operator, but this template is nowhere defined. Instead <ostream> provides an inline definition for that template. This leads to the following program not linking:

x.cc
---
#include <iostream>

void foo(std::ostream &);

int main() {
  foo(std::cout);
}

y.cc
---
#include <string>
#include <iosfwd>

void foo(std::ostream &os)
{
  os << std::string("a");
}

The linker will give an undefined symbol for the operator <<. The reason is that <string> forward declares the template which is not defined neither in <string> nor in the librarie's string.cc. Because of the forward declaration y.cc happily compiles with an undefined reference in the obj-file.

When including <ostream> in y.cc, the linker is successful since there is a inline template definition in <ostream>. According to my reading of the standard this is wrong - the defintion should be in <string> or string.cc should have an instantiation of the template for char and wchar_t.

This is probably an upstream issue?