[Serialization] Recover from conformances with missing decls

Components of a requirement may be hidden behind an implementation-only
import. Attempts at deserializing them would fail on a 'module not
loaded' error. We only see failures in non-compilation paths, either in
indexing or with tools like ide-test as they try to deserialize
things that are private.

Author: xymus

lib/Serialization/Deserialization.cpp

@@ -571,7 +571,11 @@ ModuleFile::readConformanceChecked(llvm::BitstreamCursor &Cursor,
   case NORMAL_PROTOCOL_CONFORMANCE_ID: {
     NormalConformanceID conformanceID;
     NormalProtocolConformanceIdLayout::readRecord(scratch, conformanceID);
-    return ProtocolConformanceRef(readNormalConformance(conformanceID));
+
+    auto conformance = readNormalConformanceChecked(conformanceID);
+    if (!conformance)
+      return conformance.takeError();
+    return ProtocolConformanceRef(conformance.get());
   }
 
   case PROTOCOL_CONFORMANCE_XREF: {
@@ -614,7 +618,7 @@ ModuleFile::readConformanceChecked(llvm::BitstreamCursor &Cursor,
   }
 }
 
-NormalProtocolConformance *ModuleFile::readNormalConformance(
+Expected<NormalProtocolConformance *> ModuleFile::readNormalConformanceChecked(
                              NormalConformanceID conformanceID) {
   auto &conformanceEntry = NormalConformances[conformanceID-1];
   if (conformanceEntry.isComplete()) {
@@ -653,7 +657,11 @@ NormalProtocolConformance *ModuleFile::readNormalConformance(
   Type conformingType = dc->getDeclaredInterfaceType();
   PrettyStackTraceType trace(ctx, "reading conformance for", conformingType);
 
-  auto proto = cast<ProtocolDecl>(getDecl(protoID));
+  auto protoOrError = getDeclChecked(protoID);
+  if (!protoOrError)
+    return protoOrError.takeError();
+  auto proto = cast<ProtocolDecl>(protoOrError.get());
+
   PrettyStackTraceDecl traceTo("... to", proto);
   ++NumNormalProtocolConformancesLoaded;
 
@@ -5761,9 +5769,22 @@ ModuleFile::loadAllConformances(const Decl *D, uint64_t contextData,
   fatalIfNotSuccess(DeclTypeCursor.JumpToBit(bitPosition));
 
   while (numConformances--) {
-    auto conf = readConformance(DeclTypeCursor);
-    if (conf.isConcrete())
-      conformances.push_back(conf.getConcrete());
+    auto conformance = readConformanceChecked(DeclTypeCursor);
+
+    if (!conformance) {
+      // Missing module errors are most likely caused by an
+      // implementation-only import hiding types and decls.
+      // rdar://problem/60291019
+      if (conformance.errorIsA<XRefNonLoadedModuleError>()) {
+        consumeError(conformance.takeError());
+        return;
+      }
+      else
+        fatal(conformance.takeError());
+    }
+
+    if (conformance.get().isConcrete())
+      conformances.push_back(conformance.get().getConcrete());
   }
 }
 

lib/Serialization/ModuleFile.h

@@ -978,13 +978,14 @@ class ModuleFile
   llvm::Expected<ProtocolConformanceRef>
   readConformanceChecked(llvm::BitstreamCursor &Cursor,
                          GenericEnvironment *genericEnv = nullptr);
-  
+
   /// Read a SILLayout from the given cursor.
   SILLayout *readSILLayout(llvm::BitstreamCursor &Cursor);
 
-  /// Read the given normal conformance from the current module file.
-  NormalProtocolConformance *
-  readNormalConformance(serialization::NormalConformanceID id);
+  /// Read the given normal conformance from the current module file,
+  /// returns the conformance or the first error.
+  llvm::Expected<NormalProtocolConformance *>
+  readNormalConformanceChecked(serialization::NormalConformanceID id);
 
   /// Reads a foreign error conformance from \c DeclTypeCursor, if present.
   Optional<ForeignErrorConvention> maybeReadForeignErrorConvention();

test/Serialization/Recovery/implementation-only-missing.swift

@@ -15,6 +15,10 @@
 // RUN: %target-swift-frontend -typecheck -DCLIENT_APP -primary-file %s -I %t -index-system-modules -index-store-path %t
 // RUN: %target-swift-frontend -emit-sil -DCLIENT_APP -primary-file %s -I %t -module-name client
 
+//// Printing the public module should not crash when checking for overrides of
+//// methods from the private module.
+// RUN: %target-swift-ide-test -print-module -module-to-print=public_lib -source-filename=x -skip-overrides -I %t
+
 #if PRIVATE_LIB
 
 @propertyWrapper
@@ -38,11 +42,15 @@ public protocol HiddenProtocol {
   associatedtype Value
 }
 
+public protocol HiddenProtocolWithOverride {
+  func hiddenOverride()
+}
+
 #elseif PUBLIC_LIB
 
 @_implementationOnly import private_lib
 
-struct LibProtocolContraint { }
+struct LibProtocolConstraint { }
 
 protocol LibProtocolTABound { }
 struct LibProtocolTA: LibProtocolTABound { }
@@ -52,13 +60,13 @@ protocol LibProtocol {
 
   func hiddenRequirement<A>(
       param: HiddenGenStruct<A>
-  ) where A.Value == LibProtocolContraint
+  ) where A.Value == LibProtocolConstraint
 }
 
 extension LibProtocol where TA == LibProtocolTA {
   func hiddenRequirement<A>(
       param: HiddenGenStruct<A>
-  ) where A.Value == LibProtocolContraint { }
+  ) where A.Value == LibProtocolConstraint { }
 }
 
 public struct PublicStruct: LibProtocol {
@@ -70,6 +78,10 @@ public struct PublicStruct: LibProtocol {
   public var wrappedVar: String
 }
 
+struct StructWithOverride: HiddenProtocolWithOverride {
+  func hiddenOverride() {}
+}
+
 #elseif CLIENT_APP
 
 import public_lib