C++Interop: Don't create default argument expressions that are templated

SwiftDeclSynthesizer::makeDefaultArgument does not create substituion map while
creating the implicit FuncDecl which leads to TypeChecker complaining about
the absence when there are type parameters. Avoid importing such C++ functions for now.

Author: meg-gupta

lib/ClangImporter/ImportType.cpp

@@ -2616,12 +2616,11 @@ bool ClangImporter::Implementation::isDefaultArgSafeToImport(
   return true;
 }
 
-static ParamDecl *getParameterInfo(ClangImporter::Implementation *impl,
-                                   const clang::ParmVarDecl *param,
-                                   const Identifier &name,
-                                   const swift::Type &swiftParamTy,
-                                   const bool isInOut,
-                                   const bool isParamTypeImplicitlyUnwrapped) {
+static std::optional<ParamDecl *>
+getParameterInfo(ClangImporter::Implementation *impl,
+                 const clang::ParmVarDecl *param, const Identifier &name,
+                 const swift::Type &swiftParamTy, const bool isInOut,
+                 const bool isParamTypeImplicitlyUnwrapped) {
   // Figure out the name for this parameter.
   Identifier bodyName = impl->importFullName(param, impl->CurrentVersion)
                             .getBaseIdentifier(impl->SwiftContext);
@@ -2657,9 +2656,11 @@ static ParamDecl *getParameterInfo(ClangImporter::Implementation *impl,
   // Swift doesn't support default values of inout parameters.
   // TODO: support default arguments of constructors
   // (https://github.com/apple/swift/issues/70124)
+  // TODO: support params with template parameters
   if (param->hasDefaultArg() && !isInOut &&
       !isa<clang::CXXConstructorDecl>(param->getDeclContext()) &&
-      impl->isDefaultArgSafeToImport(param)) {
+      impl->isDefaultArgSafeToImport(param) &&
+      !isa<clang::FunctionTemplateDecl>(param->getDeclContext())) {
     SwiftDeclSynthesizer synthesizer(*impl);
     if (CallExpr *defaultArgExpr = synthesizer.makeDefaultArgument(
             param, swiftParamTy, paramInfo->getParameterNameLoc())) {
@@ -2724,7 +2725,13 @@ ParameterList *ClangImporter::Implementation::importFunctionParameterList(
 
     auto paramInfo = getParameterInfo(this, param, name, swiftParamTy, isInOut,
                                       isParamTypeImplicitlyUnwrapped);
-    parameters.push_back(paramInfo);
+    if (!paramInfo.has_value()) {
+      addImportDiagnostic(param,
+                          Diagnostic(diag::parameter_type_not_imported, param),
+                          param->getSourceRange().getBegin());
+      return nullptr;
+    }
+    parameters.push_back(*paramInfo);
     ++index;
   }
 
@@ -3349,8 +3356,8 @@ ImportedType ClangImporter::Implementation::importMethodParamsAndReturnType(
     ++nameIndex;
 
     // Set up the parameter info
-    auto paramInfo = getParameterInfo(this, param, name, swiftParamTy, isInOut,
-                                      isParamTypeImplicitlyUnwrapped);
+    auto paramInfo = *getParameterInfo(this, param, name, swiftParamTy, isInOut,
+                                       isParamTypeImplicitlyUnwrapped);
 
     // Determine whether we have a default argument.
     if (kind == SpecialMethodKind::Regular ||