restore missing parentheses (p4lang#4600)

* restore missing parentheses

Restore parentheses that were accidentally dropped from begin call as
part of recent code improvements (p4lang#4513).

* add basic test for ValuesForKeys from map.h

lib/map.h

@@ -207,7 +207,7 @@ class MapForKey {
     M ↦
     typename M::key_type key;
     class iterator {
-        using MapIt = decltype(map.begin);
+        using MapIt = decltype(map.begin());
 
         const MapForKey &self;
         MapIt it;

test/CMakeLists.txt

@@ -37,6 +37,7 @@ set (GTEST_UNITTEST_SOURCES
   gtest/hvec_map.cpp
   gtest/indexed_vector.cpp
   gtest/json_test.cpp
+  gtest/map.cpp
   gtest/midend_def_use.cpp
   gtest/midend_pass.cpp
   gtest/midend_test.cpp

test/gtest/map.cpp

@@ -0,0 +1,53 @@
+/*
+Copyright 2024 Intel Corp.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+#include "lib/map.h"
+
+#include <gtest/gtest.h>
+
+#include <algorithm>
+
+namespace Test {
+
+TEST(ValuesForKey, set_equal) {
+    std::multimap<int, int> a;
+    std::multimap<int, int> b;
+
+    // Populate a
+    a.emplace(1, 1);
+    a.emplace(1, 2);
+    a.emplace(1, 3);
+    a.emplace(1, 4);
+
+    a.emplace(2, 1);
+    a.emplace(2, 2);
+
+    a.emplace(3, 1);
+
+    EXPECT_FALSE(a == b);
+
+    // Walk through the elements in a and populate in b
+    std::set<int> keys({1, 2, 3});
+    for (auto k : keys) {
+        for (auto v : ValuesForKey(a, k)) {
+            b.emplace(k, v);
+        }
+    }
+
+    EXPECT_TRUE(a == b);
+}
+
+}  // namespace Test