[libcxx] Fix match_results for alternatives

[K-ballo]

Summary

Initialize submatch results to unmatched. They will not be modified for failed alternatives (they might not even be looked at), and would otherwise leave the pair of iterators value-initialized instead of pointing to the end of the searched sequence. Fixes PR22061.

[mclow.lists]

The fix looks reasonable, it fixes the OP's problem, and the new test fails w/o the fix.

However, the change to <regex> is in two places, and I don't see how this test exercises both code paths.

Other than that (and the nits in the test) this looks good.

test/std/re/re.results/re.results.acc/index.pass.cpp

27

How about assert(m.size() == 6) here? - or >5 since we're checking elements 0..5

50

Should this be m[5] ?

[K-ballo]

In D7111#112810, @mclow.lists wrote:

The fix looks reasonable, it fixes the OP's problem, and the new test fails w/o the fix.

However, the change to <regex> is in two places, and I don't see how this test exercises both code paths.

Good point, I'll parametrize the test and invoke it for both ECMAScript and extended POSIX.

test/std/re/re.results/re.results.acc/index.pass.cpp

27

assert(m.size() == 4) actually, the main match plus three subexpressions. Any out of range access shall return an unmatched result, and the original test was exercising that.

50

Indeed, good catch.

[K-ballo]

Address review comments.

[mclow.lists]

test/std/re/re.results/re.results.acc/index.pass.cpp

28

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?

[K-ballo]

test/std/re/re.results/re.results.acc/index.pass.cpp

28

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.

[K-ballo]

SVN r227384

include/regex

basic_regex<_CharT, _Traits>::__match_at_start_ecma(
         const _CharT* __first, const _CharT* __last,
         match_results<const _CharT*, _Allocator>& __m,
         regex_constants::match_flag_type __flags, bool __at_first) const
 {
     vector<__state> __states;
     __node* __st = __start_.get();
     if (__st)
     {
+        sub_match<const _CharT*> __unmatched;
+        __unmatched.first   = __last;
+        __unmatched.second  = __last;
+        __unmatched.matched = false;
+
         __states.push_back(__state());
         __states.back().__do_ = 0;
         __states.back().__first_ = __first;
         __states.back().__current_ = __first;
         __states.back().__last_ = __last;
-        __states.back().__sub_matches_.resize(mark_count());
+        __states.back().__sub_matches_.resize(mark_count(), __unmatched);
         __states.back().__loop_data_.resize(__loop_count());
         __states.back().__node_ = __st;
         __states.back().__flags_ = __flags;
         __states.back().__at_first_ = __at_first;
         do
         {
             __state& __s = __states.back();
             if (__s.__node_)
basic_regex<_CharT, _Traits>::__match_at_start_posix_subs(
     vector<__state> __states;
     __state __best_state;
     ptrdiff_t __j = 0;
     ptrdiff_t __highest_j = 0;
     ptrdiff_t _Np = _VSTD::distance(__first, __last);
     __node* __st = __start_.get();
     if (__st)
     {
+        sub_match<const _CharT*> __unmatched;
+        __unmatched.first   = __last;
+        __unmatched.second  = __last;
+        __unmatched.matched = false;
+
         __states.push_back(__state());
         __states.back().__do_ = 0;
         __states.back().__first_ = __first;
         __states.back().__current_ = __first;
         __states.back().__last_ = __last;
-        __states.back().__sub_matches_.resize(mark_count());
+        __states.back().__sub_matches_.resize(mark_count(), __unmatched);
         __states.back().__loop_data_.resize(__loop_count());
         __states.back().__node_ = __st;
         __states.back().__flags_ = __flags;
         __states.back().__at_first_ = __at_first;
         const _CharT* __current = __first;
         bool __matched = false;
         do
         {

test/std/re/re.results/re.results.acc/index.pass.cpp

 // class match_results<BidirectionalIterator, Allocator>
 
 // const_reference operator[](size_type n) const;
 
 #include <regex>
 #include <cassert>
 
 void
-test()
+test(std::regex_constants::syntax_option_type syntax)
 {
     std::match_results<const char*> m;
     const char s[] = "abcdefghijk";
-    assert(std::regex_search(s, m, std::regex("cd((e)fg)hi")));
+    assert(std::regex_search(s, m, std::regex("cd((e)fg)hi|(z)", syntax)));
+
+    assert(m.size() == 4);
 
     assert(m[0].first == s+2);
     assert(m[0].second == s+9);
     assert(m[0].matched == true);
 
     assert(m[1].first == s+4);
     assert(m[1].second == s+7);
     assert(m[1].matched == true);
 
     assert(m[2].first == s+4);
     assert(m[2].second == s+5);
     assert(m[2].matched == true);
 
     assert(m[3].first == s+11);
     assert(m[3].second == s+11);
     assert(m[3].matched == false);
 
     assert(m[4].first == s+11);
     assert(m[4].second == s+11);
     assert(m[4].matched == false);
 }
 
 int main()
 {
-    test();
+    test(std::regex_constants::ECMAScript);
+    test(std::regex_constants::extended);
 }