[YAML] Make std::array available (#116059)

`std::array` will be handled like `MutableArrayRef`;

- Extending elements is not acceptable.
- For applying fewer sequence, trailing elements will be initialized by
default.

Not like;

- `std::array` is not the reference but holds values. Supposing to hold
small count of elements.

llvm/include/llvm/Support/YAMLTraits.h

@@ -2006,6 +2006,11 @@ struct SequenceTraits<
     std::enable_if_t<CheckIsBool<SequenceElementTraits<T>::flow>::value>>
     : SequenceTraitsImpl<std::vector<T>, SequenceElementTraits<T>::flow> {};
 template <typename T, unsigned N>
+struct SequenceTraits<
+    std::array<T, N>,
+    std::enable_if_t<CheckIsBool<SequenceElementTraits<T>::flow>::value>>
+    : SequenceTraitsImpl<std::array<T, N>, SequenceElementTraits<T>::flow> {};
+template <typename T, unsigned N>
 struct SequenceTraits<
     SmallVector<T, N>,
     std::enable_if_t<CheckIsBool<SequenceElementTraits<T>::flow>::value>>

llvm/unittests/Support/YAMLIOTest.cpp

@@ -3366,6 +3366,15 @@ struct FixedArray {
   int values[4];
 };
 
+struct StdArray {
+  StdArray() {
+    // Initialize to int max as a sentinel value.
+    for (auto &v : values)
+      v = std::numeric_limits<int>::max();
+  }
+  std::array<int, 4> values;
+};
+
 namespace llvm {
 namespace yaml {
 template <> struct MappingTraits<FixedArray> {
@@ -3374,11 +3383,21 @@ template <> struct MappingTraits<FixedArray> {
     io.mapRequired("Values", array);
   }
 };
+template <> struct MappingTraits<StdArray> {
+  static void mapping(IO &io, StdArray &st) {
+    io.mapRequired("Values", st.values);
+  }
+};
 } // namespace yaml
 } // namespace llvm
 
-TEST(YAMLIO, FixedSizeArray) {
-  FixedArray faval;
+using TestTypes = ::testing::Types<FixedArray, StdArray>;
+
+template <typename T> class YAMLIO : public testing::Test {};
+TYPED_TEST_SUITE(YAMLIO, TestTypes, );
+
+TYPED_TEST(YAMLIO, FixedSizeArray) {
+  TypeParam faval;
   Input yin("---\nValues:  [ 1, 2, 3, 4 ]\n...\n");
   yin >> faval;
 
@@ -3400,9 +3419,9 @@ TEST(YAMLIO, FixedSizeArray) {
   ASSERT_EQ(serialized, expected);
 }
 
-TEST(YAMLIO, FixedSizeArrayMismatch) {
+TYPED_TEST(YAMLIO, FixedSizeArrayMismatch) {
   {
-    FixedArray faval;
+    TypeParam faval;
     Input yin("---\nValues:  [ 1, 2, 3 ]\n...\n");
     yin >> faval;
 
@@ -3415,7 +3434,7 @@ TEST(YAMLIO, FixedSizeArrayMismatch) {
   }
 
   {
-    FixedArray faval;
+    TypeParam faval;
     Input yin("---\nValues:  [ 1, 2, 3, 4, 5 ]\n...\n");
     yin >> faval;