[libc++] Fix handling of negated character classes in regex

Summary:
This commit fixes a regression introduced in r316095, where we don't match
inverted character classes when there's no negated characrers in the []'s.

rdar://problem/43060054

Reviewers: mclow.lists, timshen, EricWF

Subscribers: christof, dexonsmith, cfe-commits

Differential Revision: https://reviews.llvm.org/D50534

llvm-svn: 340609
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 954d4a2235865af388f3d8ce4dc7793ba4703139
diff --git a/include/regex b/include/regex
index 84aacc0..dcdb14a 100644
--- a/include/regex
+++ b/include/regex
@@ -2414,20 +2414,17 @@
                 goto __exit;
             }
         }
-        // set of "__found" chars =
+        // When there's at least one of __neg_chars_ and __neg_mask_, the set
+        // of "__found" chars is
         //   union(complement(union(__neg_chars_, __neg_mask_)),
         //         other cases...)
         //
-        // __neg_chars_ and __neg_mask_'d better be handled together, as there
-        // are no short circuit opportunities.
-        //
-        // In addition, when __neg_mask_/__neg_chars_ is empty, they should be
-        // treated as all ones/all chars.
+        // It doesn't make sense to check this when there are no __neg_chars_
+        // and no __neg_mask_.
+        if (!(__neg_mask_ == 0 && __neg_chars_.empty()))
         {
-          const bool __in_neg_mask = (__neg_mask_ == 0) ||
-              __traits_.isctype(__ch, __neg_mask_);
+            const bool __in_neg_mask = __traits_.isctype(__ch, __neg_mask_);
           const bool __in_neg_chars =
-              __neg_chars_.empty() ||
               std::find(__neg_chars_.begin(), __neg_chars_.end(), __ch) !=
               __neg_chars_.end();
           if (!(__in_neg_mask || __in_neg_chars))