[Clang][P1061] Add codegen test

Author: ricejasonf

clang/test/CodeGenCXX/cxx2c-decomposition.cpp

@@ -0,0 +1,56 @@
+// RUN: %clang_cc1 -std=c++26 -triple x86_64-linux-gnu -emit-llvm -o - %s | FileCheck %s
+
+namespace std {
+  using size_t = decltype(sizeof(0));
+  template<typename> struct tuple_size;
+  template<size_t, typename> struct tuple_element;
+}
+
+struct Y { int n; };
+struct X { X(); X(Y); X(const X&); ~X(); };
+
+struct A { int a : 13; bool b; };
+
+struct B {};
+template<> struct std::tuple_size<B> { enum { value = 2 }; };
+template<> struct std::tuple_element<0,B> { using type = X; };
+template<> struct std::tuple_element<1,B> { using type = const int&; };
+template<int N> auto get(B) {
+  if constexpr (N == 0)
+    return Y();
+  else
+    return 0.0;
+}
+
+using C = int[2];
+
+template<typename T> T &make();
+
+// CHECK-LABEL: define {{.*}} @_Z8big_testIiEiv()
+template <typename T>
+int big_test() {
+  A& a = make<A>();
+A:
+  auto &[...an] = a;
+  an...[0] = 5;
+  // CHECK: %[[a1:.*]].load = load i16, ptr %[[BITFIELD:.*]],
+  // CHECK: %[[a1]].clear = and i16 %[[a1]].load, -8192
+  // CHECK: %[[a1]].set = or i16 %[[a1]].clear, 5
+  // CHECK: store i16 %[[a1]].set, ptr %[[BITFIELD]],
+B:
+  auto [b1, ...bn] = make<B>();
+  // CHECK: @_Z4makeI1BERT_v()
+  //   CHECK: call i32 @_Z3getILi0EEDa1B()
+  //   CHECK: call void @_ZN1XC1E1Y(ptr {{[^,]*}} %[[b1:.*]], i32
+  //
+  //   CHECK: call noundef double @_Z3getILi1EEDa1B()
+  //   CHECK: %[[cvt:.*]] = fptosi double %{{.*}} to i32
+  //   CHECK: store i32 %[[cvt]], ptr %[[b2:.*]],
+  //   CHECK: store ptr %[[b2]], ptr %[[b2ref:.*]],
+  int bn2 = bn...[0];
+  // CHECK load ptr, ptr %[[b2ref]]
+
+  return 0;
+}
+
+int g = big_test<int>();