[interop][SwiftToCxx] prohibit empty tuple param type

Author: hyp

lib/IRGen/IRABIDetailsProvider.cpp

@@ -302,7 +302,8 @@ void LoweredFunctionSignature::visitParameterList(
   for (auto param : *FD->getParameters()) {
     // FIXME: tuples map to more than one sil param (but they're not yet
     // representable by the consumer).
-    silParamMapping.push_back(param);
+    if (!param->getInterfaceType()->isVoid())
+      silParamMapping.push_back(param);
   }
   size_t currentSilParam = 0;
   for (const auto &abiParam : abiDetails.parameters) {

lib/PrintAsClang/PrintClangFunction.cpp

@@ -211,6 +211,9 @@ class CFunctionSignatureTypePrinter
     if (TT->getNumElements() > 0)
       // FIXME: Handle non-void type.
       return ClangRepresentation::unsupported;
+    // FIXME: how to support `()` parameters.
+    if (typeUseKind != FunctionSignatureTypeUse::ReturnType)
+      return ClangRepresentation::unsupported;
     os << "void";
     return ClangRepresentation::representable;
   }

test/Interop/SwiftToCxx/unsupported/unsupported-types-in-cxx.swift

@@ -4,9 +4,11 @@
 
 // RUN: %check-interop-cxx-header-in-clang(%t/functions.h)
 
-// CHECK:     takesFloat
-// CHECK-NOT: takesTuple
+// CHECK:     takeFloat
+// CHECK-NOT: takes
 
-public func takesFloat(_ x: Float) {}
+public func takeFloat(_ x: Float) {}
 
 public func takesTuple(_ x: (Float, Float)) {}
+
+public func takesVoid(_ x: ()) {}