[Serialization] Recover if a typealias's underlying type is broken (#12979)

We could handle a typealias itself disappearing, but not if the
typealias was okay but the underlying type wasn't. This came up in
real Swift 3/4 mix-and-match code.

rdar://problem/34940079

include/swift/Serialization/ModuleFormat.h

@@ -54,7 +54,7 @@ const uint16_t VERSION_MAJOR = 0;
 /// in source control, you should also update the comment to briefly
 /// describe what change you made. The content of this comment isn't important;
 /// it just ensures a conflict if two people change the module format.
-const uint16_t VERSION_MINOR = 386; // Last change: @callee_guaranted closures
+const uint16_t VERSION_MINOR = 387; // Last change: dependencies for typealiases
 
 using DeclIDField = BCFixed<31>;
 
@@ -819,7 +819,8 @@ namespace decls_block {
     TypeIDField, // interface type (no longer used)
     BCFixed<1>,  // implicit flag
     GenericEnvironmentIDField, // generic environment
-    AccessLevelField // access level
+    AccessLevelField, // access level
+    BCArray<TypeIDField> // dependency types
     // Trailed by generic parameters (if any).
   >;
 

lib/Serialization/Deserialization.cpp

@@ -2514,11 +2514,22 @@ ModuleFile::getDeclCheckedImpl(DeclID DID, Optional<DeclContext *> ForcedContext
     bool isImplicit;
     GenericEnvironmentID genericEnvID;
     uint8_t rawAccessLevel;
+    ArrayRef<uint64_t> dependencyIDs;
 
     decls_block::TypeAliasLayout::readRecord(scratch, nameID, contextID,
                                              underlyingTypeID, interfaceTypeID,
                                              isImplicit, genericEnvID,
-                                             rawAccessLevel);
+                                             rawAccessLevel, dependencyIDs);
+
+    Identifier name = getIdentifier(nameID);
+
+    for (TypeID dependencyID : dependencyIDs) {
+      auto dependency = getTypeChecked(dependencyID);
+      if (!dependency) {
+        return llvm::make_error<TypeError>(
+            name, takeErrorInfo(dependency.takeError()));
+      }
+    }
 
     auto DC = ForcedContext ? *ForcedContext : getDeclContext(contextID);
 
@@ -2526,8 +2537,7 @@ ModuleFile::getDeclCheckedImpl(DeclID DID, Optional<DeclContext *> ForcedContext
     if (declOrOffset.isComplete())
       return declOrOffset;
 
-    auto alias = createDecl<TypeAliasDecl>(SourceLoc(), SourceLoc(),
-                                           getIdentifier(nameID),
+    auto alias = createDecl<TypeAliasDecl>(SourceLoc(), SourceLoc(), name,
                                            SourceLoc(), genericParams, DC);
     declOrOffset = alias;
 

lib/Serialization/Serialization.cpp

@@ -2831,6 +2831,18 @@ void Serializer::writeDecl(const Decl *D) {
 
     auto underlying = typeAlias->getUnderlyingTypeLoc().getType();
 
+    SmallVector<TypeID, 2> dependencies;
+    for (Type dep : collectDependenciesFromType(underlying->getCanonicalType()))
+      dependencies.push_back(addTypeRef(dep));
+
+    for (Requirement req : typeAlias->getGenericRequirements()) {
+      for (Type dep : collectDependenciesFromType(req.getFirstType()))
+        dependencies.push_back(addTypeRef(dep));
+      if (req.getKind() != RequirementKind::Layout)
+        for (Type dep : collectDependenciesFromType(req.getSecondType()))
+          dependencies.push_back(addTypeRef(dep));
+    }
+
     uint8_t rawAccessLevel =
       getRawStableAccessLevel(typeAlias->getFormalAccess());
 
@@ -2843,7 +2855,8 @@ void Serializer::writeDecl(const Decl *D) {
                                 typeAlias->isImplicit(),
                                 addGenericEnvironmentRef(
                                              typeAlias->getGenericEnvironment()),
-                                rawAccessLevel);
+                                rawAccessLevel,
+                                dependencies);
     writeGenericParams(typeAlias->getGenericParams());
     break;
   }

test/Serialization/Recovery/typedefs.swift

@@ -52,6 +52,12 @@ let _ = unwrapped // okay
 _ = usesWrapped(nil) // expected-error {{use of unresolved identifier 'usesWrapped'}}
 _ = usesUnwrapped(nil) // expected-error {{nil is not compatible with expected argument type 'Int32'}}
 
+let _: WrappedAlias = nil // expected-error {{use of undeclared type 'WrappedAlias'}}
+let _: UnwrappedAlias = nil // expected-error {{nil cannot initialize specified type 'UnwrappedAlias' (aka 'Int32')}} expected-note {{add '?'}}
+
+let _: ConstrainedWrapped<Int> = nil // expected-error {{use of undeclared type 'ConstrainedWrapped'}}
+let _: ConstrainedUnwrapped<Int> = nil // expected-error {{type 'Int' does not conform to protocol 'HasAssoc'}}
+
 func testExtensions(wrapped: WrappedInt, unwrapped: UnwrappedInt) {
   wrapped.wrappedMethod() // expected-error {{value of type 'WrappedInt' (aka 'Int32') has no member 'wrappedMethod'}}
   unwrapped.unwrappedMethod() // expected-error {{value of type 'UnwrappedInt' has no member 'unwrappedMethod'}}
@@ -344,4 +350,10 @@ public func returnsWrappedGeneric<T>(_: T.Type) -> WrappedInt { fatalError() }
 public protocol WrappedProto {}
 public protocol UnwrappedProto {}
 
+public typealias WrappedAlias = WrappedInt
+public typealias UnwrappedAlias = UnwrappedInt
+
+public typealias ConstrainedWrapped<T: HasAssoc> = T where T.Assoc == WrappedInt
+public typealias ConstrainedUnwrapped<T: HasAssoc> = T where T.Assoc == UnwrappedInt
+
 #endif // TEST