[interop][SwiftToCxx] add a restriction for calling only supported type metadata accessor function for generic types

Author: hyp

lib/PrintAsClang/ClangSyntaxPrinter.cpp

@@ -191,6 +191,11 @@ void ClangSyntaxPrinter::printValueWitnessTableAccessSequenceFromTypeMetadata(
 void ClangSyntaxPrinter::printCTypeMetadataTypeFunction(
     const NominalTypeDecl *typeDecl, StringRef typeMetadataFuncName,
     llvm::ArrayRef<GenericRequirement> genericRequirements) {
+  // FIXME: Support generic requirements > 3.
+  if (!genericRequirements.empty())
+    os << "static_assert(" << genericRequirements.size()
+       << " <= " << NumDirectGenericTypeMetadataAccessFunctionArgs
+       << ", \"unsupported generic requirement list for metadata func\");\n";
   os << "// Type metadata accessor for " << typeDecl->getNameStr() << "\n";
   os << "SWIFT_EXTERN ";
   printSwiftImplQualifier();

test/Interop/SwiftToCxx/generics/generic-struct-in-cxx.swift

@@ -29,6 +29,7 @@ public func takeGenericPair<T, T1>(_ x: GenericPair<T, T1>) {
 // CHECK-NEXT: requires swift::isUsableInGenericContext<T_0_0> && swift::isUsableInGenericContext<T_0_1>
 // CHECK-NEXT: class _impl_GenericPair;
 // CHECK-EMPTY:
+// CHECK-NEXT: static_assert(2 <= 3, "unsupported generic requirement list for metadata func"); 
 // CHECK-NEXT: // Type metadata accessor for GenericPair
 // CHECK-NEXT: SWIFT_EXTERN swift::_impl::MetadataResponseTy $s8Generics11GenericPairVMa(swift::_impl::MetadataRequestTy, void * _Nonnull, void * _Nonnull) SWIFT_NOEXCEPT SWIFT_CALL;