diff --git a/lib/Sema/TypeCheckDecl.cpp b/lib/Sema/TypeCheckDecl.cpp
index 9832d239c83b0..1bf872e638ffb 100644
--- a/lib/Sema/TypeCheckDecl.cpp
+++ b/lib/Sema/TypeCheckDecl.cpp
@@ -3025,6 +3025,34 @@ AllMembersRequest::evaluate(
   return evaluateMembersRequest(idc, MembersRequestKind::All);
 }
 
+static bool isTypeInferredByTypealias(TypeAliasDecl *typealias,
+                                      NominalTypeDecl *nominal) {
+  if (!nominal->isGeneric()){
+    return false;
+  }
+
+  auto nominalGenericArguments = nominal->getDeclaredInterfaceType()
+                                     ->getAs<BoundGenericType>()
+                                     ->getGenericArgs();
+  auto typealiasGenericArguments = typealias->getUnderlyingType()
+                                       ->getAs<BoundGenericType>()
+                                       ->getGenericArgs();
+
+  for (size_t i = 0; i < nominalGenericArguments.size(); i++) {
+    auto nominalBoundGenericType = nominalGenericArguments[i];
+    auto typealiasBoundGenericType = typealiasGenericArguments[i];
+    if (nominalBoundGenericType->isEqual(typealiasBoundGenericType)) {
+      continue;
+    }
+
+    if (typealiasBoundGenericType->hasTypeParameter()) {
+      return false;
+    }
+  }
+
+  return true;
+}
+
 bool TypeChecker::isPassThroughTypealias(TypeAliasDecl *typealias,
                                          NominalTypeDecl *nominal) {
   // Pass-through only makes sense when the typealias refers to a nominal
@@ -3045,17 +3073,39 @@ bool TypeChecker::isPassThroughTypealias(TypeAliasDecl *typealias,
   // If neither is generic, we're done: it's a pass-through alias.
   if (!nominalSig) return true;
 
-  // Check that the type parameters are the same the whole way through.
   auto nominalGenericParams = nominalSig.getGenericParams();
   auto typealiasGenericParams = typealiasSig.getGenericParams();
-  if (nominalGenericParams.size() != typealiasGenericParams.size())
-    return false;
-  if (!std::equal(nominalGenericParams.begin(), nominalGenericParams.end(),
-                  typealiasGenericParams.begin(),
-                  [](GenericTypeParamType *gp1, GenericTypeParamType *gp2) {
-                    return gp1->isEqual(gp2);
-                  }))
-    return false;
+
+  if (nominalGenericParams.size() != typealiasGenericParams.size()) {
+
+    unsigned nominalMaxDepth = nominalGenericParams.back()->getDepth();
+    unsigned typealiasMaxDepth = typealiasGenericParams.back()->getDepth();
+    unsigned maxDepth = std::max(nominalMaxDepth, typealiasMaxDepth);
+
+    while (!nominalGenericParams.empty() &&
+           nominalGenericParams.back()->getDepth() == maxDepth) {
+      nominalGenericParams = nominalGenericParams.drop_back();
+    }
+
+    while (!typealiasGenericParams.empty() &&
+           typealiasGenericParams.back()->getDepth() == maxDepth) {
+      typealiasGenericParams = typealiasGenericParams.drop_back();
+    }
+
+    if (nominalGenericParams.size() != typealiasGenericParams.size()) {
+      return false;
+    }
+
+    if (!std::equal(nominalGenericParams.begin(), nominalGenericParams.end(),
+                    typealiasGenericParams.begin(),
+                    [](GenericTypeParamType *gp1, GenericTypeParamType *gp2) {
+                      return gp1->isEqual(gp2);
+                    })) {
+      return false;
+    }
+
+    return isTypeInferredByTypealias(typealias, nominal);
+  }
 
   // If neither is generic at this level, we have a pass-through typealias.
   if (!typealias->isGeneric()) return true;
diff --git a/test/decl/ext/specialize.swift b/test/decl/ext/specialize.swift
index 84c1faadd7b00..ded791e96c6f8 100644
--- a/test/decl/ext/specialize.swift
+++ b/test/decl/ext/specialize.swift
@@ -32,6 +32,106 @@ extension IntFoo where U == Int {
 
 Foo(x: "test", y: 1).hello()
 
+
+struct Field<Tag,Value> {
+  let tag: Tag
+  let value: Value
+}
+
+typealias IntField<Tag> = Field<Tag,Int>
+
+extension IntField {
+  func adding(_ value: Int) -> Self {
+    Field(tag: tag, value: self.value + value)
+  }
+}
+
+struct S10<X, Y> {}
+typealias InferredSpecializedNestedTypes<X> = S10<X, (Int, [Int]?)>
+extension InferredSpecializedNestedTypes {
+    func returnTuple(value: Y) -> (Int, [Int]?) {
+        return value
+    }
+}
+
+
+struct S2<X,Y,Z> {
+  let x: X
+  let y: Y
+  let z: Z
+}
+
+typealias A2<Y,X> = S2<Int,X,Y>
+
+extension A2 {
+  func test() {
+    let int: Int
+    let _: X = int // expected-error {{cannot convert value of type 'Int' to specified type 'X'}}
+  }
+}
+
+
+struct S4<A, B> {
+  // Generic parameters: <A, B, C>
+  // Depth:               0  0  1
+  struct Nested<C> {
+      let c: C
+  }
+}
+
+struct S5<A> {
+  // Generic parameters: <A, B, C>
+  // Depth:               0  1  1
+  typealias Alias<B, C> = S4<A, B>.Nested<C> where A == Int
+}
+
+extension S5.Alias{
+  func test() {
+    let int: Int
+    let _: A = int // expected-error {{cannot convert value of type 'Int' to specified type 'A'}}
+  }
+}
+
+
+struct S11<T> {
+  struct Inner<U> {}
+}
+
+struct S12<T> {
+  struct Inner<U> {}
+  typealias A1<U> = S11<T>.Inner<U>
+  typealias A2<U> = S12<T>.Inner<U> where T == Int
+}
+
+extension S12<Int>.A1 {
+  func foo1() {
+    let int: Int
+    let _: T = int
+  }
+}
+
+extension S12.A2 {
+  func foo2() {
+    let int: Int
+    let _: T = int
+  }
+}
+
+
+struct S13<T> {
+  struct Inner<U> {}
+}
+struct S14 {
+  typealias A<T> = S13<T>.Inner<Int>
+}
+extension S14.A {
+  func test() {
+    let int: Int
+    let _: U = int // error
+  }
+}
+
+
 struct MyType<TyA, TyB> {
   var a : TyA, b : TyB
 }