Add tests for regex_match ambiguity (aka LWG2273). NFC. Reviewed as https://reviews.llvm.org/D63051

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@365080 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/re/re.alg/re.alg.match/basic.pass.cpp

@@ -1367,6 +1367,30 @@ int main(int, char**)
         assert(m.position(0) == 0);
         assert(m.str(0) == s);
     }
+    { // LWG 2273
+        std::regex re("Foo|FooBar");
+        std::cmatch m;
+        {
+            assert(std::regex_match("FooBar", m, re));
+            assert(m.size() == 1);
+            assert(m[0] == "FooBar");
+        }
+        {
+            assert(std::regex_match("Foo", m, re));
+            assert(m.size() == 1);
+            assert(m[0] == "Foo");
+        }
+        {
+            assert(!std::regex_match("FooBarBaz", m, re));
+            assert(m.size() == 0);
+            assert(m.empty());
+        }
+        {
+            assert(!std::regex_match("FooBa", m, re));
+            assert(m.size() == 0);
+            assert(m.empty());
+        }
+    }
 
   return 0;
 }