Revert "Revert "Merge to upstream r304942.""

This reverts commit 38a0d5af7e.

Test: make checkbuild
Test: ./run_tests.py
Test: ./run_tests.py --bitness 64

test/std/strings/basic.string/string.modifiers/string_erase/iter.pass.cpp

@@ -24,6 +24,7 @@ test(S s, typename S::difference_type pos, S expected)
     typename S::const_iterator p = s.begin() + pos;
     typename S::iterator i = s.erase(p);
     LIBCPP_ASSERT(s.__invariants());
+    assert(s[s.size()] == typename S::value_type());
     assert(s == expected);
     assert(i - s.begin() == pos);
 }

test/std/strings/basic.string/string.modifiers/string_erase/iter_iter.pass.cpp

@@ -25,6 +25,7 @@ test(S s, typename S::difference_type pos, typename S::difference_type n, S expe
     typename S::const_iterator last = s.cbegin() + pos + n;
     typename S::iterator i = s.erase(first, last);
     LIBCPP_ASSERT(s.__invariants());
+    assert(s[s.size()] == typename S::value_type());
     assert(s == expected);
     assert(i - s.begin() == pos);
 }

test/std/strings/basic.string/string.modifiers/string_erase/pop_back.pass.cpp

@@ -23,6 +23,7 @@ test(S s, S expected)
 {
     s.pop_back();
     LIBCPP_ASSERT(s.__invariants());
+    assert(s[s.size()] == typename S::value_type());
     assert(s == expected);
 }
 

test/std/strings/basic.string/string.modifiers/string_erase/size_size.pass.cpp

@@ -29,6 +29,7 @@ test(S s, typename S::size_type pos, typename S::size_type n, S expected)
     {
         s.erase(pos, n);
         LIBCPP_ASSERT(s.__invariants());
+        assert(s[s.size()] == typename S::value_type());
         assert(s == expected);
     }
 #ifndef TEST_HAS_NO_EXCEPTIONS
@@ -58,6 +59,7 @@ test(S s, typename S::size_type pos, S expected)
     {
         s.erase(pos);
         LIBCPP_ASSERT(s.__invariants());
+        assert(s[s.size()] == typename S::value_type());
         assert(s == expected);
     }
 #ifndef TEST_HAS_NO_EXCEPTIONS
@@ -83,6 +85,7 @@ test(S s, S expected)
 {
     s.erase();
     LIBCPP_ASSERT(s.__invariants());
+    assert(s[s.size()] == typename S::value_type());
     assert(s == expected);
 }