Insert placeholder declarations into winrt/base.h.

PiperOrigin-RevId: 495104919

internal/platform/implementation/windows/generated/base-no-coroutines.patch

@@ -0,0 +1,37 @@
+*** third_party/nearby/internal/platform/implementation/windows/generated/winrt/base.h	2022-12-07 15:29:09.000000000 -0500
+--- third_party/nearby/internal/platform/implementation/windows/generated/winrt/base-no-coroutines.h	2022-12-07 15:28:53.000000000 -0500
+***************
+*** 35,40 ****
+--- 35,66 ----
+  #include <format>
+  #endif
+  
++ #if defined(_LIBCPP_VERSION) && __cplusplus < 202002
++ // If the C++ standard is cxx17, coroutines are not available in libc++.
++ // To compile WinRT without coroutines, define some placeholder types.
++ // These placeholders should not be used. If they are used, they should fail.
++ template <typename T = void>
++ struct coroutine_handle {
++   coroutine_handle() { throw std::logic_error("Not implemented."); }
++   coroutine_handle(std::nullptr_t __h) {
++     throw std::logic_error("Not implemented.");
++   }
++   static coroutine_handle from_address(void*) {
++     throw std::logic_error("Not implemented.");
++   }
++   void operator()() const { throw std::logic_error("Not implemented."); }
++   void* address() const { throw std::logic_error("Not implemented."); }
++ };
++ 
++ class suspend_always {};
++ class suspend_never {};
++ 
++ template <typename _Result, typename...>
++ struct coroutine_traits;
++ 
++ }  // namespace std::experimental
++ #endif
++ 
+  #ifdef __cpp_lib_coroutine
+  
+  #include <coroutine>

internal/platform/implementation/windows/generated/patch_winrt_base.sh

@@ -0,0 +1,7 @@
+#!/bin/bash
+
+patch \
+  --forward \
+  -r - \
+  third_party/nearby/internal/platform/implementation/windows/generated/winrt/base.h \
+  third_party/nearby/internal/platform/implementation/windows/generated/base-no-coroutines.patch

internal/platform/implementation/windows/generated/winrt/base.h

@@ -35,6 +35,32 @@
 #include <format>
 #endif
 
+#if defined(_LIBCPP_VERSION) && __cplusplus < 202002
+// If the C++ standard is cxx17, coroutines are not available in libc++.
+// To compile WinRT without coroutines, define some placeholder types.
+// These placeholders should not be used. If they are used, they should fail.
+template <typename T = void>
+struct coroutine_handle {
+  coroutine_handle() { throw std::logic_error("Not implemented."); }
+  coroutine_handle(std::nullptr_t __h) {
+    throw std::logic_error("Not implemented.");
+  }
+  static coroutine_handle from_address(void*) {
+    throw std::logic_error("Not implemented.");
+  }
+  void operator()() const { throw std::logic_error("Not implemented."); }
+  void* address() const { throw std::logic_error("Not implemented."); }
+};
+
+class suspend_always {};
+class suspend_never {};
+
+template <typename _Result, typename...>
+struct coroutine_traits;
+
+}  // namespace std::experimental
+#endif
+
 #ifdef __cpp_lib_coroutine
 
 #include <coroutine>