Serialization: When deserializing a typealias, build its generic signature

We were forgetting to do this, triggering crashes when using a
generic typealias from another module.

Fixes <https://bugs.swift.org/browse/SR-1889>.

lib/Serialization/Deserialization.cpp

@@ -2163,6 +2163,21 @@ Decl *ModuleFile::getDecl(DeclID DID, Optional<DeclContext *> ForcedContext) {
                                            genericParams, DC);
     declOrOffset = alias;
 
+    if (genericParams) {
+      SmallVector<GenericTypeParamType *, 4> paramTypes;
+      for (auto &genericParam : *genericParams) {
+        paramTypes.push_back(genericParam->getDeclaredType()
+                               ->castTo<GenericTypeParamType>());
+      }
+
+      // Read the generic requirements.
+      SmallVector<Requirement, 4> requirements;
+      readGenericRequirements(requirements);
+
+      auto sig = GenericSignature::get(paramTypes, requirements);
+      alias->setGenericSignature(sig);
+    }
+
     alias->computeType();
 
     if (auto accessLevel = getActualAccessibility(rawAccessLevel)) {

lib/Serialization/Serialization.cpp

@@ -2191,6 +2191,7 @@ void Serializer::writeDecl(const Decl *D) {
                                 typeAlias->isImplicit(),
                                 rawAccessLevel);
     writeGenericParams(typeAlias->getGenericParams(), DeclTypeAbbrCodes);
+    writeRequirements(typeAlias->getGenericRequirements());
     break;
   }
 

test/multifile/typealias/two-modules/library.swift

@@ -0,0 +1,9 @@
+// RUN: true
+
+public enum Result<T, U>
+{
+    case success(T)
+    case failure(U)
+}
+
+public typealias GenericResult<T> = Result<T, ErrorProtocol>

test/multifile/typealias/two-modules/main.swift

@@ -0,0 +1,13 @@
+// RUN: rm -rf %t && mkdir %t
+
+// RUN: mkdir %t/linker
+// RUN: %target-build-swift -emit-module -c %S/library.swift -o %t/linker/library.o
+// RUN: %target-build-swift -emit-library -c %S/library.swift -o %t/linker/library.o
+// RUN: %target-build-swift %S/main.swift %t/linker/library.o -I %t/linker/ -L %t/linker/ -o %t/linker/main
+
+// REQUIRES: executable_test
+
+import library
+
+func testFunction<T>(withCompletion completion: (Result<T, ErrorProtocol>) -> Void) { }
+testFunction { (result: GenericResult<Int>) in }