[cxx-interop] Support conforming to protocols with default assoc types

lib/ClangImporter/ImportDecl.cpp

@@ -10148,6 +10148,15 @@ static void finishTypeWitnesses(
       break;
     }
 
+    if (!satisfied && assocType->hasDefaultDefinitionType()) {
+      auto defaultType = assocType->getDefaultDefinitionType();
+      auto subMap =
+          selfType->getContextSubstitutionMap(assocType->getDeclContext());
+      defaultType = defaultType.subst(subMap);
+      conformance->setTypeWitness(assocType, defaultType, assocType);
+      satisfied = true;
+    }
+
     if (!satisfied) {
       ABORT([&](auto &out) {
         out << "Cannot look up associated type for imported conformance:\n";

test/Interop/Cxx/class/conforms-to-associated-types.swift

@@ -0,0 +1,37 @@
+// RUN: %empty-directory(%t)
+// RUN: %empty-directory(%t/include)
+// RUN: split-file %s %t
+//
+// RUN: %target-swift-frontend -typecheck -module-name a -cxx-interoperability-mode=default -I %t/include %t/a.swift
+
+//--- include/module.modulemap
+module cxx {
+  header "header.h"
+  export *
+}
+
+//--- include/header.h
+struct S {
+  S() {}
+} __attribute__((swift_attr("conforms_to:a.P")));
+
+struct S2 {
+  S2() {}
+  using A = S2;
+} __attribute__((swift_attr("conforms_to:a.P")));
+
+//--- a.swift
+import cxx
+public protocol P {
+  associatedtype A = Int
+  func foo(_: A)
+}
+extension P {
+  func foo(_: A) {}
+}
+func test(s: S) {
+  let _ = s.foo(0)
+}
+func test2(s: S2) {
+  let _ = s.foo(s)
+}