[C++20][Coroutines] Lambda-coroutine with operator new in promise_type (#84193)

Fix #84064

According to http://eel.is/c++draft/dcl.fct.def.coroutine#9 the first
parameter for overload resolution of `operator new` is `size_t` followed
by the arguments of the coroutine function.
http://eel.is/c++draft/dcl.fct.def.coroutine#4 states that the first
argument is the lvalue of `*this` if the coroutine is a member function.

Before this patch, Clang handled class types correctly but ignored
lambdas. This patch adds support for lambda coroutines with a
`promise_type` that implements a custom `operator new`.

The patch does consider C++23 `static operator()`, which already worked
as there is no `this` parameter.

2 files changed, 132 insertions, 0 deletions

diff --git a/clang/test/SemaCXX/gh84064-1.cpp b/clang/test/SemaCXX/gh84064-1.cpp
new file mode 100644
index 0000000..d9c2738
--- /dev/null
+++ b/clang/test/SemaCXX/gh84064-1.cpp
@@ -0,0 +1,79 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -I%S/Inputs -std=c++20 %s
+
+// expected-no-diagnostics
+
+#include "std-coroutine.h"
+
+using size_t = decltype(sizeof(0));
+
+struct Generator {
+  struct promise_type {
+    int _val{};
+
+    Generator get_return_object() noexcept
+    {
+      return {};
+    }
+
+    std::suspend_never initial_suspend() noexcept
+    {
+      return {};
+    }
+
+    std::suspend_always final_suspend() noexcept
+    {
+      return {};
+    }
+
+    void return_void() noexcept {}
+    void unhandled_exception() noexcept {}
+
+    template<typename This, typename... TheRest>
+    static void*
+    operator new(size_t size,
+                 This&,
+                 TheRest&&...) noexcept
+    {
+        return nullptr;
+    }
+
+    static void operator delete(void*, size_t)
+    {
+    }
+  };
+};
+
+struct CapturingThisTest
+{
+    int x{};
+
+    void AsPointer()
+    {
+      auto lamb = [=,this]() -> Generator {
+        int y = x;
+        co_return;
+      };
+
+      static_assert(sizeof(decltype(lamb)) == sizeof(void*));
+    }
+
+    void AsStarThis()
+    {
+      auto lamb = [*this]() -> Generator {
+        int y = x;
+        co_return;
+      };
+
+      static_assert(sizeof(decltype(lamb)) == sizeof(int));
+    }
+};
+
+int main()
+{
+  auto lamb = []() -> Generator {
+    co_return;
+  };
+
+  static_assert(sizeof(decltype(lamb)) == 1);
+}
+
diff --git a/clang/test/SemaCXX/gh84064-2.cpp b/clang/test/SemaCXX/gh84064-2.cpp
new file mode 100644
index 0000000..457de43
--- /dev/null
+++ b/clang/test/SemaCXX/gh84064-2.cpp
@@ -0,0 +1,53 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -I%S/Inputs -std=c++23 %s
+
+// expected-no-diagnostics
+
+#include "std-coroutine.h"
+
+using size_t = decltype(sizeof(0));
+
+struct GeneratorStatic {
+  struct promise_type {
+    int _val{};
+
+    GeneratorStatic get_return_object() noexcept
+    {
+      return {};
+    }
+
+    std::suspend_never initial_suspend() noexcept
+    {
+      return {};
+    }
+
+    std::suspend_always final_suspend() noexcept
+    {
+      return {};
+    }
+
+    void return_void() noexcept {}
+    void unhandled_exception() noexcept {}
+
+    template<typename... TheRest>
+    static void*
+    operator new(size_t  size,
+                 TheRest&&...) noexcept
+    {
+        return nullptr;
+    }
+
+    static void operator delete(void*, size_t)
+    {
+    }
+  };
+};
+
+
+int main()
+{
+  auto lambCpp23 = []() static -> GeneratorStatic {
+    co_return;
+  };
+
+  static_assert(sizeof(decltype(lambCpp23)) == 1);
+}