[interop] do not import function templates with template parameter not present in signature whose default value is dependent

rdar://107561753

Author: hyp

lib/ClangImporter/ImportDecl.cpp

@@ -3301,8 +3301,16 @@ namespace {
           // then still add a generic so that it can be overrieded.
           // TODO(https://github.com/apple/swift/issues/57184): In the future we might want to import two overloads in this case so that the default type could still be used.
           if (templateTypeParam->hasDefaultArgument() &&
-              !templateParamTypeUsedInSignature(templateTypeParam))
+              !templateParamTypeUsedInSignature(templateTypeParam)) {
+            // We do not yet support instantiation of default values of template
+            // parameters when the function template is instantiated, so do not
+            // import the function template if the template parameter has
+            // dependent default value.
+            auto defaultArgumentType = templateTypeParam->getDefaultArgument();
+            if (defaultArgumentType->isDependentType())
+              return nullptr;
             continue;
+          }
 
           auto *typeParam = Impl.createDeclWithClangNode<GenericTypeParamDecl>(
               param, AccessLevel::Public, dc,

test/Interop/Cxx/templates/Inputs/enable-if.h

@@ -26,4 +26,14 @@ struct HasMethodWithEnableIf {
   }
 };
 
+struct HasConstructorWithEnableIf {
+  template<class T, class _ = typename enable_if<is_bool<T>::value, bool>::type>
+  HasConstructorWithEnableIf(const T &);
+};
+
+struct HasConstructorWithEnableIfUsed {
+  template<class T, class U = typename enable_if<is_bool<T>::value, bool>::type>
+  HasConstructorWithEnableIfUsed(const T &, const U &);
+};
+
 #endif // TEST_INTEROP_CXX_TEMPLATES_INPUTS_ENABLE_IF_H

test/Interop/Cxx/templates/enable-if-module-interface.swift

@@ -0,0 +1,11 @@
+// RUN: %target-swift-ide-test -print-module -module-to-print=EnableIf -I %S/Inputs -source-filename=x -enable-experimental-cxx-interop | %FileCheck %s
+
+// The `init<T>` constructor template is not yet supported in Swift.
+
+// CHECK: struct HasConstructorWithEnableIf {
+// CHECK-NEXT:  @available(*, deprecated, message: "This zero-initializes the backing memory of the struct, which is unsafe for some C++ structs. Consider adding an explicit default initializer for this C++ struct.")
+// CHECK-NEXT:  init()
+// CHECK-NEXT:}
+
+// CHECK: struct HasConstructorWithEnableIfUsed {
+// CHECK-NEXT:  init<T, U>(_: T, _: U)