Sema: Try not to let protocol type aliases leak into inferred type witnesses

They don't always round-trip through a module interface, because of
request cycles in type resolution.

Fixes rdar://problem/122957148.

Author: slavapestov

lib/Sema/AssociatedTypeInference.cpp

@@ -1851,6 +1851,19 @@ static Type mapErrorTypeToOriginal(Type type) {
   return type;
 }
 
+/// Desugar protocol type aliases, since they can cause request cycles in
+/// type resolution if printed in a module interface and parsed back in.
+static Type getWithoutProtocolTypeAliases(Type type) {
+  return type.transformRec([](TypeBase *t) -> llvm::Optional<Type> {
+    if (auto *aliasTy = dyn_cast<TypeAliasType>(t)) {
+      if (aliasTy->getDecl()->getDeclContext()->getExtendedProtocolDecl())
+        return getWithoutProtocolTypeAliases(aliasTy->getSinglyDesugaredType());
+    }
+
+    return llvm::None;
+  });
+}
+
 /// Produce the type when matching a witness.
 ///
 /// If the witness is a member of the type itself or a superclass, we
@@ -1884,6 +1897,9 @@ static Type getWitnessTypeForMatching(NormalProtocolConformance *conformance,
     conformance->getDeclContext()->mapTypeIntoContext(conformance->getType());
   TypeSubstitutionMap substitutions = model->getMemberSubstitutions(witness);
   Type type = witness->getInterfaceType()->getReferenceStorageReferent();
+
+  type = getWithoutProtocolTypeAliases(type);
+
   LLVM_DEBUG(llvm::dbgs() << "Witness interface type is " << type << "\n";);
 
   if (substitutions.empty())

test/ModuleInterface/protocol_extension_type_witness.swift

@@ -0,0 +1,38 @@
+// RUN: %target-swift-emit-module-interface(%t.swiftinterface) %s -module-name protocol_extension_type_witness -enable-experimental-associated-type-inference
+// RUN: %target-swift-typecheck-module-from-interface(%t.swiftinterface) -module-name protocol_extension_type_witness
+// RUN: %FileCheck %s < %t.swiftinterface
+
+// RUN: %target-swift-emit-module-interface(%t.swiftinterface) %s -module-name protocol_extension_type_witness -disable-experimental-associated-type-inference
+// RUN: %target-swift-typecheck-module-from-interface(%t.swiftinterface) -module-name protocol_extension_type_witness
+// RUN: %FileCheck %s < %t.swiftinterface
+
+public protocol P {
+  associatedtype A
+  associatedtype B
+  associatedtype C
+  associatedtype D
+
+  func b(_: B)
+  func c(_: C)
+  func d(_: D)
+}
+
+extension P {
+  public typealias _Default_A = B
+  public typealias Alias = D
+
+  public func c(_: Alias) {}
+}
+
+public struct S<B>: P {
+  public func b(_: B) {}
+  public func d(_: String) {}
+}
+
+// CHECK-LABEL: public struct S<B> : protocol_extension_type_witness.P {
+// CHECK-NEXT:    public func b(_: B)
+// CHECK-NEXT:    public func d(_: Swift.String)
+// CHECK-NEXT:    public typealias A = B
+// CHECK-NEXT:    public typealias C = Swift.String
+// CHECK-NEXT:    public typealias D = Swift.String
+// CHECK-NEXT:  }