Reproducer (js):

$()=i

Error:

Segmentation Fault

Note:
The Segfault is caused by us trying to construct following error message, before the the realm/heap is fully up,
crashing on realm.vm() -> Cell::heap() -> HeapBlockBase::from_cell(this) returning a nullptr

auto executable_result = JS::Bytecode::Generator::generate(vm, script, {}); //  Bytecode/Interpreter.cpp:250
    /* -> */ return Bytecode::CodeGenerationError { lhs, "Unimplemented/invalid node used a reference"sv }; // ASTCodegen.cpp:514ff
[...]
result = JS::throw_completion(JS::InternalError::create(realm(), error_string.release_value())); // Bytecode/Interpreter.cpp:258

So this is two issues:

• Us not rejecting the assignment early enough as a ReferenceError: cannot assign to function call in the Parser, or converting to a corresponding throw ...;
• Us not handling codegen errors gracefully-ish, but something here smells really fishy