[clang] [clang] Add __builtin_start_object_lifetime builtin. (PR #82776)

[Sam McCall via cfe-commits]

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@@ -156,6 +156,37 @@ void test_noexcept(int *i) {
 #undef TEST_TYPE
 } // end namespace test_launder
 
+namespace test_start_object_lifetime {
+// The builtin is non-constant.
+constexpr int test_non_constexpr(int i) { // expected-error {{constexpr function never produces a constant expression}}
+  __builtin_start_object_lifetime(&i); // expected-note {{subexpression not valid in a constant expression}}
+#ifdef CXX11
+  // expected-warning at -2 {{use of this statement in a constexpr function is a C++14 extension}}
+#endif
+  return 0;
+}
+
+struct Incomplete; // expected-note {{forward declaration}}
+void test_incomplete(Incomplete *i) {
+   // Requires a complete type
+   __builtin_start_object_lifetime(i); // expected-error {{incomplete type 'Incomplete' where a complete type is required}}
+}
+
+// The builtin is type-generic.
+#define TEST_TYPE(Ptr, Type) \
+  static_assert(__is_same(decltype(__builtin_launder(Ptr)), Type), "expected same type")
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sam-mccall wrote:

should this be `__builtin_start_object_lifetime`?

https://github.com/llvm/llvm-project/pull/82776