[Serialization] Recover from a missing conforming type in a protocol conformance

We saw this failure with a Clang module imported @_implementationOnly
with synthesized conformances by the ClangImporter. It caused
issues only in sil-opt as it reads all the witness tables.

rdar://problem/58924131

Author: xymus

lib/Serialization/Deserialization.cpp

@@ -581,7 +581,11 @@ ModuleFile::readConformanceChecked(llvm::BitstreamCursor &Cursor,
     ProtocolConformanceXrefLayout::readRecord(scratch, protoID, nominalID,
                                               moduleID);
 
-    auto nominal = cast<NominalTypeDecl>(getDecl(nominalID));
+    auto maybeNominal = getDeclChecked(nominalID);
+    if (!maybeNominal)
+      return maybeNominal.takeError();
+
+    auto nominal = cast<NominalTypeDecl>(maybeNominal.get());
     PrettyStackTraceDecl trace("cross-referencing conformance for", nominal);
     auto proto = cast<ProtocolDecl>(getDecl(protoID));
     PrettyStackTraceDecl traceTo("... to", proto);

lib/Serialization/DeserializeSIL.cpp

@@ -3104,6 +3104,16 @@ void SILDeserializer::readWitnessTableEntries(
 
 SILWitnessTable *SILDeserializer::readWitnessTable(DeclID WId,
                                                    SILWitnessTable *existingWt) {
+  auto deserialized = readWitnessTableChecked(WId, existingWt);
+  if (!deserialized) {
+    MF->fatal(deserialized.takeError());
+  }
+  return deserialized.get();
+}
+
+llvm::Expected<SILWitnessTable *>
+  SILDeserializer::readWitnessTableChecked(DeclID WId,
+                                           SILWitnessTable *existingWt) {
   if (WId == 0)
     return nullptr;
   assert(WId <= WitnessTables.size() && "invalid WitnessTable ID");
@@ -3150,8 +3160,12 @@ SILWitnessTable *SILDeserializer::readWitnessTable(DeclID WId,
   }
 
   // Deserialize Conformance.
+  auto maybeConformance = MF->readConformanceChecked(SILCursor);
+  if (!maybeConformance)
+    return maybeConformance.takeError();
+
   auto theConformance = cast<RootProtocolConformance>(
-                          MF->readConformance(SILCursor).getConcrete());
+                          maybeConformance.get().getConcrete());
 
   PrettyStackTraceConformance trace(SILMod.getASTContext(),
                                     "deserializing SIL witness table for",
@@ -3228,8 +3242,18 @@ SILWitnessTable *SILDeserializer::readWitnessTable(DeclID WId,
 void SILDeserializer::getAllWitnessTables() {
   if (!WitnessTableList)
     return;
-  for (unsigned I = 0, E = WitnessTables.size(); I < E; I++)
-    readWitnessTable(I + 1, nullptr);
+  for (unsigned I = 0, E = WitnessTables.size(); I < E; I++) {
+    auto maybeTable = readWitnessTableChecked(I + 1, nullptr);
+    if (!maybeTable) {
+      if (maybeTable.errorIsA<XRefNonLoadedModuleError>()) {
+        // This is most likely caused by decls hidden by an implementation-only
+        // import, it is safe to ignore for this function's purpose.
+        consumeError(maybeTable.takeError());
+      } else {
+        MF->fatal(maybeTable.takeError());
+      }
+    }
+  }
 }
 
 SILWitnessTable *

lib/Serialization/DeserializeSIL.h

@@ -126,8 +126,17 @@ namespace swift {
     SILVTable *readVTable(serialization::DeclID);
     SILGlobalVariable *getGlobalForReference(StringRef Name);
     SILGlobalVariable *readGlobalVar(StringRef Name);
-    SILWitnessTable *readWitnessTable(serialization::DeclID,
+
+    /// Read and return the witness table identified with \p WId.
+    SILWitnessTable *readWitnessTable(serialization::DeclID WId,
                                       SILWitnessTable *existingWt);
+
+    /// Read the witness table identified with \p WId, return the table or
+    /// the first error if any.
+    llvm::Expected<SILWitnessTable *>
+      readWitnessTableChecked(serialization::DeclID WId,
+                              SILWitnessTable *existingWt);
+
     void readWitnessTableEntries(
            llvm::BitstreamEntry &entry,
            std::vector<SILWitnessTable::Entry> &witnessEntries,