make the try/catch portability macros safer

.clang-format

@@ -72,7 +72,7 @@ EmptyLineBeforeAccessModifier: Leave
 ExperimentalAutoDetectBinPacking: true
 FixNamespaceComments: true
 IfMacros: [
-  STDEXEC_CATCH'
+  'STDEXEC_CATCH'
 ]
 IncludeBlocks: Preserve
 IndentAccessModifiers: false
@@ -88,6 +88,7 @@ KeepEmptyLinesAtTheStartOfBlocks: true
 LambdaBodyIndentation: Signature
 LineEnding: LF
 Macros: [
+  'STDEXEC_CATCH_FALLTHROUGH= ',
   'STDEXEC_MEMFN_DECL(...)=__VA_ARGS__',
   'STDEXEC_ATTRIBUTE(...)=__attribute__((__VA_ARGS__))',
   'STDEXEC_IMMOVABLE_NO_UNIQUE_ADDRESS=[[no_unique_address]]',

.clangd

@@ -53,6 +53,7 @@ CompileFlags:
     - "-ftemplate-backtrace-limit=0"
     - "-std=gnu++20"
     - "-DSTDEXEC_CLANGD_INVOKED"
+    - "-stdlib=libc++"
   Remove:
     - "-stdpar*"
     # strip CUDA fatbin args

examples/hello_coro.cpp

@@ -46,8 +46,11 @@ auto main() -> int {
     auto [i] = stdexec::sync_wait(async_answer2(just(42), just())).value();
     std::cout << "The answer is " << i.value() << '\n';
   }
-  STDEXEC_CATCH(const std::exception& e) {
-    std::cout << e.what() << '\n';
+  STDEXEC_CATCH(std::exception& e) {
+    std::cerr << "error: " << e.what() << '\n';
+  }
+  STDEXEC_CATCH_ALL {
+    std::cerr << "unknown error\n";
   }
 }
 #else

include/exec/__detail/__system_context_default_impl.hpp

@@ -249,7 +249,7 @@ namespace exec::__system_context_default_impl {
           __schedule_operation_t::__construct_maybe_alloc(__storage, &__r, std::move(__sndr));
         __os->start();
       }
-      STDEXEC_CATCH(std::exception & __e) {
+      STDEXEC_CATCH_ALL {
         __r.set_error(std::current_exception());
       }
     }
@@ -278,7 +278,7 @@ namespace exec::__system_context_default_impl {
           __storage, &__r, std::move(__sndr));
         __os->start();
       }
-      STDEXEC_CATCH(std::exception & __e) {
+      STDEXEC_CATCH_ALL {
         __r.set_error(std::current_exception());
       }
     }
@@ -297,7 +297,7 @@ namespace exec::__system_context_default_impl {
           __storage, &__r, std::move(__sndr));
         __os->start();
       }
-      STDEXEC_CATCH(std::exception & __e) {
+      STDEXEC_CATCH_ALL {
         __r.set_error(std::current_exception());
       }
     }

include/stdexec/__detail/__config.hpp

@@ -540,6 +540,8 @@ namespace stdexec {
 #  include <cuda_runtime_api.h>
 #endif
 
+// clang-format off
+
 // The following macros are used to conditionally compile exception handling code. They
 // are used in the same way as `try` and `catch`, but they allow for different behavior
 // based on whether exceptions are enabled or not, and whether the code is being compiled
@@ -559,28 +561,28 @@ namespace stdexec {
 //     printf("unknown error\n");
 //   }
 #if STDEXEC_STD_NO_EXCEPTIONS()
-#  define STDEXEC_TRY if constexpr (true)
-#  define STDEXEC_CATCH(...)                                                                       \
-    else if constexpr ([[maybe_unused]] __VA_ARGS__ = ::stdexec::_catch_any_lvalue{}; false)
-#  define STDEXEC_CATCH_ALL                                                                        \
-    else if constexpr (true) {                                                                     \
-    }                                                                                              \
-    else
-#  define STDEXEC_THROW(...) ::stdexec::__terminate()
+#  define STDEXEC_TRY               if constexpr (true) {
+#  define STDEXEC_CATCH(...)        } else if constexpr (__VA_ARGS__ = ::stdexec::__catch_any_lvalue; false) {
+#  define STDEXEC_CATCH_ALL         } else if constexpr (true) {} else
+#  define STDEXEC_THROW(...)        ::stdexec::__terminate()
+#  define STDEXEC_CATCH_FALLTHROUGH } else {}
 #else
-#  define STDEXEC_TRY        try
-#  define STDEXEC_CATCH      catch
-#  define STDEXEC_CATCH_ALL  STDEXEC_CATCH(...)
-#  define STDEXEC_THROW(...) throw __VA_ARGS__
+#  define STDEXEC_TRY               try
+#  define STDEXEC_CATCH             catch
+#  define STDEXEC_CATCH_ALL         catch(...)
+#  define STDEXEC_THROW(...)        throw __VA_ARGS__
+#  define STDEXEC_CATCH_FALLTHROUGH
 #endif
 
+// clang-format on
+
 namespace stdexec {
   // Used by the STDEXEC_CATCH macro to provide a stub initialization of the exception object.
-  struct _catch_any_lvalue {
+  constexpr struct __catch_any_lvalue_t {
     template <class _Tp>
     STDEXEC_ATTRIBUTE(host, device)
     operator _Tp&() const noexcept;
-  };
+  } __catch_any_lvalue{};
 
   STDEXEC_ATTRIBUTE(noreturn, host, device)
   inline void __terminate() noexcept {

test/exec/async_scope/test_spawn_future.cpp

@@ -165,6 +165,9 @@ namespace {
     STDEXEC_CATCH(const std::logic_error& e) {
       SUCCEED("correct exception caught");
     }
+    STDEXEC_CATCH_ALL {
+      FAIL("invalid exception caught");
+    }
     sync_wait(scope.on_empty());
   }
 #endif // !STDEXEC_STD_NO_EXCEPTIONS()

test/exec/test_libdispatch.cpp

@@ -86,5 +86,8 @@ namespace {
     STDEXEC_CATCH(int e) {
       CHECK(e == 999);
     }
+    STDEXEC_CATCH_ALL {
+      FAIL("invalid exception caught");
+    }
   }
 } // namespace

test/rrd/async_scope.cpp

@@ -75,6 +75,9 @@ struct async_scope_future_set_result : rl::test_suite<async_scope_future_set_res
     STDEXEC_CATCH(const std::logic_error&) {
       threw = true;
     }
+    STDEXEC_CATCH_ALL {
+      RL_ASSERT(false);
+    }
     RL_ASSERT(threw);
     ex::sync_wait(scope.on_empty());
   }

test/stdexec/algos/adaptors/test_let_error.cpp

@@ -95,6 +95,7 @@ namespace {
                             STDEXEC_CATCH(const std::exception& e) {
                               return ex::just(std::string{e.what()});
                             }
+                            STDEXEC_CATCH_FALLTHROUGH
                           });
     wait_for_value(std::move(snd), std::string{"error description"});
     (void) snd;

test/test_common/receivers.hpp

@@ -509,6 +509,9 @@ namespace {
       STDEXEC_CATCH(const std::exception& e) {
         FAIL("Exception thrown: " << e.what());
       }
+      STDEXEC_CATCH_ALL {
+        FAIL("Exception thrown");
+      }
     }
   };