UnicodeString::tempSubString delete && overloads

[philipp-zimmermann]

Changes

The rvalue overloads for icu::UnicodeString::tempSubString and icu::UnicodeString::tempSubStringBetween are deleted.

Justification

Calling tempSubString on a rvalue violates the precondition ("[...] requires that the original string not be modified or deleted during the lifetime of the returned substring object."). Therefore disallowing usage like this prevents bugs.

A example for the buggy behavior is the following code:

#include <iostream>
#include <unicode/unistr.h>

[[nodiscard]] auto get_string() -> icu::UnicodeString
{
  return icu::UnicodeString {"abcdefg"};
}

auto print (icu::UnicodeString const& unicode_string) -> void
{
  std::string s;
  unicode_string.toUTF8String (s);
  std::cout << s << '\n';
}

int main()
{
  {
    auto const substring {get_string().tempSubString(2)};

    print (substring);
    std::string s;
    substring.toUTF8String (s);
    std::cout << s << '\n';
    print (substring);
  }

  std::cout << "\nCorrect behavior:\n";

  {
    auto const full_string {get_string()};
    auto const substring {full_string.tempSubString(2)};

    print (substring);
    std::string s;
    substring.toUTF8String (s);
    std::cout << s << '\n';
    print (substring);
  }
}

This may lead to output like this:

cdefg
efg
晥gg

Correct behavior:
cdefg
cdefg
cdefg

The above output is from a binary compiled with "Release" mode. When using "Debug" the behavior is correct.
This is tested with https://github.com/unicode-org/icu/releases/download/release-78.2/icu4c-78.2-Ubuntu22.04-x64.tgz.

Jira issue

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Checklist

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• [x ] Tests included, if applicable
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