Because https://reviews.llvm.org/rL336132 introduces a few function templates with default template arguments, gcc cannot compile <string> anymore, when targeting c++03 or earlier:

$ echo "#include <string>" | g++ -std=c++03 -nostdinc++ -isystem /usr/include/c++/v1 -x c++ -c - -o -
In file included from <stdin>:1:
/usr/include/c++/v1/string:812:35: error: default template arguments may not be used in function templates without -std=c++11 or -std=gnu++11
     basic_string(const _CharT* __s) {
                                   ^
/usr/include/c++/v1/string:822:62: error: default template arguments may not be used in function templates without -std=c++11 or -std=gnu++11
         basic_string(const _CharT* __s, const _Allocator& __a);
                                                              ^
/usr/include/c++/v1/string:833:70: error: default template arguments may not be used in function templates without -std=c++11 or -std=gnu++11
         basic_string(size_type __n, _CharT __c, const _Allocator& __a);
                                                                      ^
/usr/include/c++/v1/string:844:75: error: default template arguments may not be used in function templates without -std=c++11 or -std=gnu++11
                               const allocator_type& __a = allocator_type());
                                                                           ^
/usr/include/c++/v1/string:848:45: error: default template arguments may not be used in function templates without -std=c++11 or -std=gnu++11
         explicit basic_string(const _Tp& __t);
                                             ^
/usr/include/c++/v1/string:852:72: error: default template arguments may not be used in function templates without -std=c++11 or -std=gnu++11
         explicit basic_string(const _Tp& __t, const allocator_type& __a);
                                                                        ^
/usr/include/c++/v1/string:875:43: error: default template arguments may not be used in function templates without -std=c++11 or -std=gnu++11
     basic_string& operator=(const _Tp& __t)
                                           ^

Apparently gcc in c++03 mode does accept class templates with default arguments, but not function templates.

https://reviews.llvm.org/D48616 already stated "I may have stepped on a bug fix for old versions of gcc in C++03 mode - Eric? [ was introduced in r292830 ]", and indeed it looks like this has happened.