diff --git a/lib/AST/GenericSignatureBuilder.cpp b/lib/AST/GenericSignatureBuilder.cpp
index 4b08216270f95..136106b9c69c2 100644
--- a/lib/AST/GenericSignatureBuilder.cpp
+++ b/lib/AST/GenericSignatureBuilder.cpp
@@ -48,6 +48,8 @@
 #include "llvm/Support/SaveAndRestore.h"
 #include <algorithm>
 
+#include <iostream>
+
 using namespace swift;
 using llvm::DenseMap;
 
@@ -3625,6 +3627,8 @@ ResolvedType GenericSignatureBuilder::maybeResolveEquivalenceClass(
                                     Type type,
                                     ArchetypeResolutionKind resolutionKind,
                                     bool wantExactPotentialArchetype) {
+  type->dump();
+  
   // An error type is best modeled as an unresolved potential archetype, since
   // there's no way to be sure what it is actually meant to be.
   if (type->is<ErrorType>()) {
@@ -3633,12 +3637,16 @@ ResolvedType GenericSignatureBuilder::maybeResolveEquivalenceClass(
 
   // The equivalence class of a generic type is known directly.
   if (auto genericParam = type->getAs<GenericTypeParamType>()) {
+    std::cout << "0" << std::endl;
     unsigned index = GenericParamKey(genericParam).findIndexIn(
                                                             getGenericParams());
+    std::cout << "1" << std::endl;
     if (index < Impl->GenericParams.size()) {
+      std::cout << "2" << std::endl;
       return ResolvedType(Impl->PotentialArchetypes[index]);
     }
 
+    std::cout << "3" << std::endl;
     return ResolvedType::forUnresolved(nullptr);
   }
 
@@ -3646,6 +3654,7 @@ ResolvedType GenericSignatureBuilder::maybeResolveEquivalenceClass(
   // base equivalence class, if there is one.
   if (auto depMemTy = type->getAs<DependentMemberType>()) {
     // Find the equivalence class of the base.
+    std::cout << "4" << std::endl;
     auto resolvedBase =
       maybeResolveEquivalenceClass(depMemTy->getBase(),
                                    resolutionKind,
@@ -4850,6 +4859,16 @@ ConstraintResult GenericSignatureBuilder::addSameTypeRequirement(
     FloatingRequirementSource source,
     UnresolvedHandlingKind unresolvedHandling,
     llvm::function_ref<void(Type, Type)> diagnoseMismatch) {
+  
+  if (paOrT1.is<Type>())
+    paOrT1.get<Type>()->dump();
+  if (paOrT1.is<PotentialArchetype*>())
+    paOrT1.get<PotentialArchetype*>()->dump();
+  
+  if (paOrT2.is<Type>())
+    paOrT2.get<Type>()->dump();
+  if (paOrT2.is<PotentialArchetype*>())
+    paOrT2.get<PotentialArchetype*>()->dump();
 
   auto resolved1 = resolve(paOrT1, source);
   if (!resolved1) {
diff --git a/lib/Sema/TypeCheckType.cpp b/lib/Sema/TypeCheckType.cpp
index a5ed86edb2b5b..23272fe63c7cf 100644
--- a/lib/Sema/TypeCheckType.cpp
+++ b/lib/Sema/TypeCheckType.cpp
@@ -47,6 +47,7 @@
 #include "llvm/ADT/StringSwitch.h"
 #include "llvm/ADT/Twine.h"
 #include "llvm/Support/SaveAndRestore.h"
+#include <iostream>
 
 using namespace swift;
 
@@ -566,8 +567,7 @@ Type TypeChecker::resolveTypeInContext(TypeDecl *typeDecl, DeclContext *foundDC,
     if (selfType->is<GenericTypeParamType>()) {
       if (typeDecl->getDeclContext()->getSelfProtocolDecl()) {
         if (isa<AssociatedTypeDecl>(typeDecl) ||
-            (isa<TypeAliasDecl>(typeDecl) &&
-             !cast<TypeAliasDecl>(typeDecl)->isGeneric())) {
+            typeDecl->getAsGenericContext()) {
           // FIXME: We should use this lookup method for the Interface
           // stage too, but right now that causes problems with
           // Sequence.SubSequence vs Collection.SubSequence; the former
@@ -576,10 +576,16 @@ Type TypeChecker::resolveTypeInContext(TypeDecl *typeDecl, DeclContext *foundDC,
           // because we use the Sequence.SubSequence default instead of
           // the Collection.SubSequence default, even when the conforming
           // type wants to conform to Collection.
+//          selfType->dump();
+//          typeDecl->dump();
+//          std::cout << typeDecl->getName().get() << std::endl;
+          
           if (resolution.getStage() == TypeResolutionStage::Structural) {
             return resolution.resolveSelfAssociatedType(selfType, foundDC,
                                                         typeDecl->getName());
-          } else if (auto assocType = dyn_cast<AssociatedTypeDecl>(typeDecl)) {
+          }
+          
+          if (auto assocType = dyn_cast<AssociatedTypeDecl>(typeDecl)) {
             typeDecl = assocType->getAssociatedTypeAnchor();
           }
         }
@@ -1295,10 +1301,23 @@ resolveTopLevelIdentTypeComponent(TypeResolution resolution,
     // Otherwise, check for an ambiguity.
     if (!resolution.areSameType(current, type)) {
       isAmbiguous = true;
-      break;
+      // If a typealias collides with an associatedtype in generic requirement
+      // position, prefer the associated type.
+      if (options.getContext() == TypeResolverContext::GenericRequirement) {
+        if (isa<TypeAliasDecl>(currentDecl) && isa<AssociatedTypeDecl>(typeDecl)) {
+          current = type;
+          currentDecl = typeDecl;
+          currentDC = foundDC;
+          isAmbiguous = false;
+        } else if (isa<AssociatedTypeDecl>(currentDecl) &&
+                   isa<TypeAliasDecl>(typeDecl)) {
+          isAmbiguous = false;
+        }
+      }
+      if (isAmbiguous) break;
     }
 
-    // We have a found multiple type aliases that refer to the same thing.
+    // We have found multiple type aliases that refer to the same thing.
     // Ignore the duplicate.
   }
 
diff --git a/validation-test/compiler_crashers_2_fixed/sr11639.swift b/validation-test/compiler_crashers_2_fixed/sr11639.swift
new file mode 100644
index 0000000000000..4d0cca842c837
--- /dev/null
+++ b/validation-test/compiler_crashers_2_fixed/sr11639.swift
@@ -0,0 +1,16 @@
+// RUN: %target-typecheck-verify-swift
+
+protocol ProtocolA {
+    associatedtype T1
+}
+
+struct S<T> : ProtocolA {
+  typealias T1 = T
+}
+
+protocol ProtocolB: ProtocolA {
+    associatedtype T2: ProtocolB where T2.T1 == T3.T1
+    associatedtype X
+    typealias T3 = S<X>
+}
+