[llvm-bugs] [Bug 30240] New: std::string: append(first, last) error when aliasing

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=30240

            Bug ID: 30240
           Summary: std::string: append(first, last) error when aliasing
           Product: libc++
           Version: unspecified
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: All Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: llvm-bugs at daryl.haresign.com
                CC: llvm-bugs at lists.llvm.org, mclow.lists at gmail.com
    Classification: Unclassified

The standard says:

  | template<class InputIterator>
  | basic_string& append(InputIterator first, InputIterator last);
  |
  | Requires: [first, last) is a valid range.
  |
  | Effects: Equivalent to append(basic_string(first, last)).
  |
  | Returns: *this.

http://eel.is/c++draft/string::append#20

Given that, I would expect the following to work:

  | std::string str("hello world//");
  |
  | str.append(str.begin(), str.end());
  | str.append(str.begin(), str.end());

And I should end up with 'str' containing:

  | hello world//hello world//hello world//hello world//

libcxx, however, appears to try to avoid creating a temporary string if it can:

https://github.com/llvm-mirror/libcxx/blob/master/include/string#L2159

If the capacity is not enough for the new string, it will resize the storage:

https://github.com/llvm-mirror/libcxx/blob/master/include/string#L2175

Unfortunately '__grow_by()' destroys the old storage before the string is
copied.

This means the above code doesn't work as expected.  You can see the result
of calling the above code here, compiled against libcxx and libstdc++:

http://coliru.stacked-crooked.com/a/b099fd5dada88798

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