diff --git a/lib/Sema/TypeCheckDecl.cpp b/lib/Sema/TypeCheckDecl.cpp
index 514087f2d45..d54410a3d7f 100644
--- a/lib/Sema/TypeCheckDecl.cpp
+++ b/lib/Sema/TypeCheckDecl.cpp
@@ -1202,6 +1202,11 @@ public:
         TAD->setInvalid();
         TAD->overwriteType(ErrorType::get(TC.Context));
         TAD->getUnderlyingTypeLoc().setType(ErrorType::get(TC.Context));
+      } else if (TAD->getDeclContext()->isGenericContext() &&
+                 !TAD->getInterfaceType()) {
+        TAD->setInterfaceType(
+          TC.getInterfaceTypeFromInternalType(TAD->getDeclContext(),
+                                              TAD->getType()));
       }
 
       if (!isa<ProtocolDecl>(TAD->getDeclContext()))
diff --git a/lib/Serialization/Deserialization.cpp b/lib/Serialization/Deserialization.cpp
index 431263d58b5..0675b01c781 100644
--- a/lib/Serialization/Deserialization.cpp
+++ b/lib/Serialization/Deserialization.cpp
@@ -789,11 +789,12 @@ Decl *ModuleFile::getDecl(DeclID DID, Optional<DeclContext *> ForcedContext,
   case decls_block::TYPE_ALIAS_DECL: {
     IdentifierID nameID;
     DeclID contextID;
-    TypeID underlyingTypeID;
+    TypeID underlyingTypeID, interfaceTypeID;
     bool isImplicit;
 
     decls_block::TypeAliasLayout::readRecord(scratch, nameID, contextID,
-                                             underlyingTypeID, isImplicit);
+                                             underlyingTypeID, interfaceTypeID,
+                                             isImplicit);
 
     auto DC = ForcedContext ? *ForcedContext : getDeclContext(contextID);
     auto underlyingType = TypeLoc::withoutLoc(getType(underlyingTypeID));
@@ -806,6 +807,9 @@ Decl *ModuleFile::getDecl(DeclID DID, Optional<DeclContext *> ForcedContext,
                                          DC, { });
     declOrOffset = alias;
 
+    if (auto interfaceType = getType(interfaceTypeID))
+      alias->setInterfaceType(interfaceType);
+
     if (isImplicit)
       alias->setImplicit();
 
diff --git a/lib/Serialization/ModuleFormat.h b/lib/Serialization/ModuleFormat.h
index 92a25eeadbf..51800e62e58 100644
--- a/lib/Serialization/ModuleFormat.h
+++ b/lib/Serialization/ModuleFormat.h
@@ -550,6 +550,7 @@ namespace decls_block {
     IdentifierIDField, // name
     DeclIDField, // context decl
     TypeIDField, // underlying type
+    TypeIDField, // interface type
     BCFixed<1>  // implicit flag
     // Trailed by the conformance info (if any).
   >;
diff --git a/lib/Serialization/Serialization.cpp b/lib/Serialization/Serialization.cpp
index 3a2a8270518..e0ff52aa3ae 100644
--- a/lib/Serialization/Serialization.cpp
+++ b/lib/Serialization/Serialization.cpp
@@ -1094,6 +1094,7 @@ void Serializer::writeDecl(const Decl *D) {
                                 addIdentifierRef(typeAlias->getName()),
                                 addDeclRef(DC),
                                 addTypeRef(underlying),
+                                addTypeRef(typeAlias->getInterfaceType()),
                                 typeAlias->isImplicit());
 
     writeConformances(typeAlias->getProtocols(), typeAlias->getConformances(),