[cxx-interop] Don't expose unsupported top-level async or @_transparent functions

Author: WANGJIEKE

lib/PrintAsClang/DeclAndTypePrinter.cpp

@@ -1110,6 +1110,12 @@ class DeclAndTypePrinter::Implementation
 
   void visitFuncDecl(FuncDecl *FD) {
     if (outputLang == OutputLanguageMode::Cxx) {
+      // Don't expose async functions or transparent functions
+      // because they're currently unsupported
+      if (FD->hasAsync() || FD->isTransparent()) {
+        return;
+      }
+
       // Emit the underlying C signature that matches the Swift ABI
       // in the generated C++ implementation prologue for the module.
       auto funcABI = getModuleProloguePrinter()

test/Interop/SwiftToCxx/functions/swift-no-expose-unsupported-func.swift

@@ -0,0 +1,30 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend %s -typecheck -module-name Functions -clang-header-expose-public-decls -emit-clang-header-path %t/functions.h
+// RUN: %FileCheck %s < %t/functions.h
+
+// RUN: %check-interop-cxx-header-in-clang(%t/functions.h)
+
+// CHECK-NOT: SWIFT_EXTERN double $s9Functions10async_funcyS2dYaF(double x) SWIFT_NOEXCEPT SWIFT_CALL; // async_func(_:)
+// CHECK-NOT: SWIFT_EXTERN bool $s9Functions16transparent_funcyS2bF(bool x) SWIFT_NOEXCEPT SWIFT_CALL; // transparent_func(_:)
+
+// CHECK:       namespace Functions {
+// CHECK-EMPTY:
+// CHECK-EMPTY:
+// CHECK-EMPTY:
+// CHECK-EMPTY:
+// CHECK-EMPTY:
+// CHECK-EMPTY:
+// CHECK-NEXT:  } // namespace Functions
+
+// CHECK-NOT:  inline double async_func(double x) noexcept SWIFT_WARN_UNUSED_RESULT {
+// CHECK-NOT:   return _impl::$s9Functions10async_funcyS2dYaF(x);
+// CHECK-NOT: }
+
+// CHECK-NOT:  inline bool transparent_func(bool x) noexcept SWIFT_WARN_UNUSED_RESULT {
+// CHECK-NOT:   return _impl::$s9Functions16transparent_funcyS2bF(x);
+// CHECK-NOT: }
+
+public func async_func(_ x: Double) async -> Double { return 2 * x }
+
+@_transparent
+public func transparent_func(_ x: Bool) -> Bool { return !x }