[ADT] add initializer list specialization for is_contained

Adding an initializer list specialization for is_contained allows for
compile-time evaluation when called with a constant or runtime
evaluation for non-constant values.

This patch doesn't add any uses of this template, but that is coming in
a subsequent patch.

Reviewed By: pete

Differential Revision: https://reviews.llvm.org/D122079

llvm/include/llvm/ADT/STLExtras.h

@@ -1657,6 +1657,15 @@ bool is_contained(R &&Range, const E &Element) {
   return std::find(adl_begin(Range), adl_end(Range), Element) != adl_end(Range);
 }
 
+template <typename T>
+constexpr bool is_contained(std::initializer_list<T> Set, T Value) {
+  // TODO: Use std::find when we switch to C++20.
+  for (T V : Set)
+    if (V == Value)
+      return true;
+  return false;
+}
+
 /// Wrapper function around std::is_sorted to check if elements in a range \p R
 /// are sorted with respect to a comparator \p C.
 template <typename R, typename Compare> bool is_sorted(R &&Range, Compare C) {

llvm/unittests/ADT/STLExtrasTest.cpp

@@ -964,4 +964,29 @@ TEST(STLExtrasTest, TypeAtIndex) {
       (std::is_same<double, llvm::TypeAtIndex<2, int, float, double>>::value));
 }
 
+enum Doggos {
+  Floofer,
+  Woofer,
+  SubWoofer,
+  Pupper,
+  Pupperino,
+  Longboi,
+};
+
+TEST(STLExtrasTest, IsContainedInitializerList) {
+  EXPECT_TRUE(is_contained({Woofer, SubWoofer}, Woofer));
+  EXPECT_TRUE(is_contained({Woofer, SubWoofer}, SubWoofer));
+  EXPECT_FALSE(is_contained({Woofer, SubWoofer}, Pupper));
+  EXPECT_FALSE(is_contained({}, Longboi));
+
+  static_assert(is_contained({Woofer, SubWoofer}, SubWoofer), "SubWoofer!");
+  static_assert(!is_contained({Woofer, SubWoofer}, Pupper), "Missing Pupper!");
+
+  EXPECT_TRUE(is_contained({1, 2, 3, 4}, 3));
+  EXPECT_FALSE(is_contained({1, 2, 3, 4}, 5));
+
+  static_assert(is_contained({1, 2, 3, 4}, 3), "It's there!");
+  static_assert(!is_contained({1, 2, 3, 4}, 5), "It's not there :(");
+}
+
 } // namespace