[libcxx] [test] Add string nullptr asserts to erase functions.

[BillyONeal]

Summary

In my perf overhaul for MSVC++'s basic_string I wrote the null at data[oldsize] instead of data[newsize]; adding asserts to catch that bug.

[EricWF]

Sorry for the delay reviewing such a simple change.

[EricWF]

@BillyONeal Changes like this are good for post-commit review (ie just commit it). Adding clearly correct assertions and test cases is never an objectionable thing.

[BillyONeal]

In D33340#764206, @EricWF wrote:

Sorry for the delay reviewing such a simple change.

No problem! Their simplicity also means no likely merge conflicts :)

In D33340#764213, @EricWF wrote:

@BillyONeal Changes like this are good for post-commit review (ie just commit it). Adding clearly correct assertions and test cases is never an objectionable thing.

Sounds good, thanks!

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

 
 template <class S>
 void
 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);
 }
 
 int main()
 {
     {
     typedef std::string S;

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

 template <class S>
 void
 test(S s, typename S::difference_type pos, typename S::difference_type n, S expected)
 {
     typename S::const_iterator first = s.cbegin() + pos;
     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);
 }
 
 int main()
 {
     {
     typedef std::string S;

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

 #include "min_allocator.h"
 
 template <class S>
 void
 test(S s, S expected)
 {
     s.pop_back();
     LIBCPP_ASSERT(s.__invariants());
+    assert(s[s.size()] == typename S::value_type());
     assert(s == expected);
 }
 
 int main()
 {
     {
     typedef std::string S;
     test(S("abcde"), S("abcd"));

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

 test(S s, typename S::size_type pos, typename S::size_type n, S expected)
 {
     const typename S::size_type old_size = s.size();
     S s0 = s;
     if (pos <= old_size)
     {
         s.erase(pos, n);
         LIBCPP_ASSERT(s.__invariants());
+        assert(s[s.size()] == typename S::value_type());
         assert(s == expected);
     }
 #ifndef TEST_HAS_NO_EXCEPTIONS
     else
     {
         try
         {
             s.erase(pos, n);

 test(S s, typename S::size_type pos, S expected)
 {
     const typename S::size_type old_size = s.size();
     S s0 = s;
     if (pos <= old_size)
     {
         s.erase(pos);
         LIBCPP_ASSERT(s.__invariants());
+        assert(s[s.size()] == typename S::value_type());
         assert(s == expected);
     }
 #ifndef TEST_HAS_NO_EXCEPTIONS
     else
     {
         try
         {
             s.erase(pos);

 }
 
 template <class S>
 void
 test(S s, S expected)
 {
     s.erase();
     LIBCPP_ASSERT(s.__invariants());
+    assert(s[s.size()] == typename S::value_type());
     assert(s == expected);
 }
 
 int main()
 {
     {
     typedef std::string S;
     test(S(""), 0, 0, S(""));