Runtime: Don't crash when overriding a weak-imported method that's not available

Author: slavapestov

include/swift/ABI/Metadata.h

@@ -845,13 +845,15 @@ struct TargetVTableDescriptorHeader {
 template <typename Runtime>
 struct TargetMethodOverrideDescriptor {
   /// The class containing the base method.
-  TargetRelativeIndirectablePointer<Runtime, TargetClassDescriptor<Runtime>> Class;
+  TargetRelativeIndirectablePointer<Runtime, TargetClassDescriptor<Runtime>,
+                                    /*nullable*/ true> Class;
 
   /// The base method.
-  TargetRelativeIndirectablePointer<Runtime, TargetMethodDescriptor<Runtime>> Method;
+  TargetRelativeIndirectablePointer<Runtime, TargetMethodDescriptor<Runtime>,
+                                    /*nullable*/ true> Method;
 
   /// The implementation of the override.
-  TargetRelativeDirectPointer<Runtime, void, /*Nullable=*/true> Impl;
+  TargetRelativeDirectPointer<Runtime, void, /*nullable*/ true> Impl;
 };
 
 /// Header for a class vtable override descriptor. This is a variable-sized

stdlib/public/runtime/Metadata.cpp

@@ -2319,6 +2319,11 @@ static void initClassVTable(ClassMetadata *self) {
       auto *baseClass = descriptor.Class.get();
       auto *baseMethod = descriptor.Method.get();
 
+      // If the base method is null, it's an unavailable weak-linked
+      // symbol.
+      if (baseClass == nullptr || baseMethod == nullptr)
+        continue;
+
       // Calculate the base method's vtable offset from the
       // base method descriptor. The offset will be relative
       // to the base class's vtable start offset.