[PATCH] [libcxx] Fix match_results for alternatives

[Agustín Bergé]

================
Comment at: test/std/re/re.results/re.results.acc/index.pass.cpp:26
@@ -25,3 +25,3 @@
 
     assert(m[0].first == s+2);
     assert(m[0].second == s+9);
----------------
mclow.lists wrote:
> Ok, then I'm really confused.
> 
> The code for `match_results::operator[]` is:
>      return __n < __matches_.size() ? __matches_[__n] : __unmatched_;
> 
> So how does the change in `<regex>` affect this result?
The changes affect unmatched alternative subexpressions. Consider instead the expression `"(z)|cd((e)fg)hi"`, the `(z)` subexpression would then be `m[1]` instead of `m[3]`. In both cases the subscript is within range, so the out-of-range `__unmatched_` result is not referenced.

http://reviews.llvm.org/D7111

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