[Serialization] Recover from deserializing an enum from a missing module

rdar://problem/58022345

Author: xymus

lib/Serialization/Deserialization.cpp

@@ -1889,14 +1889,25 @@ DeclContext *ModuleFile::getLocalDeclContext(LocalDeclContextID DCID) {
 }
 
 DeclContext *ModuleFile::getDeclContext(DeclContextID DCID) {
+  auto deserialized = getDeclContextChecked(DCID);
+  if (!deserialized) {
+    fatal(deserialized.takeError());
+  }
+  return deserialized.get();
+}
+
+Expected<DeclContext *> ModuleFile::getDeclContextChecked(DeclContextID DCID) {
   if (!DCID)
     return FileContext;
 
   if (Optional<LocalDeclContextID> contextID = DCID.getAsLocalDeclContextID())
     return getLocalDeclContext(contextID.getValue());
 
-  auto D = getDecl(DCID.getAsDeclID().getValue());
+  auto deserialized = getDeclChecked(DCID.getAsDeclID().getValue());
+  if (!deserialized)
+    return deserialized.takeError();
 
+  auto D = deserialized.get();
   if (auto GTD = dyn_cast<GenericTypeDecl>(D))
     return GTD;
   if (auto ED = dyn_cast<ExtensionDecl>(D))
@@ -3496,7 +3507,6 @@ class swift::DeclDeserializer {
                                         numConformances, numInherited,
                                         rawInheritedAndDependencyIDs);
 
-    auto DC = MF.getDeclContext(contextID);
     if (declOrOffset.isComplete())
       return declOrOffset;
 
@@ -3510,6 +3520,11 @@ class swift::DeclDeserializer {
       }
     }
 
+    auto DCOrError = MF.getDeclContextChecked(contextID);
+    if (!DCOrError)
+      return DCOrError.takeError();
+    auto DC = DCOrError.get();
+
     auto genericParams = MF.maybeReadGenericParams(DC);
     if (declOrOffset.isComplete())
       return declOrOffset;

lib/Serialization/ModuleFile.h

@@ -913,6 +913,11 @@ class ModuleFile
   /// Returns the decl context with the given ID, deserializing it if needed.
   DeclContext *getDeclContext(serialization::DeclContextID DID);
 
+  /// Returns the decl context with the given ID, deserializing it if needed,
+  /// or the first error.
+  llvm::Expected<DeclContext *>
+  getDeclContextChecked(serialization::DeclContextID DCID);
+
   /// Returns the local decl context with the given ID, deserializing it if needed.
   DeclContext *getLocalDeclContext(serialization::LocalDeclContextID DID);