Missing error when constructing deleted classes

[nshahan]

Classes should not be usable after a hot reload that deletes them.

Generation 0:

class A {}

var retained;

void helper() {
  retained = () {
    var a = A();
    print(a);
  };
retained();
}

Generation 1:

var retained;

void helper() {
  retained();  // <-- Should throw an error
}

Unclear what type of error this should be.

[lrhn]

Should the function value itself be callable after the function expression that created it has been deleted? The function body is no longer in the program, so it's a smaller problem that the class that code referred to also doesn't exist.

If the function can be called, then some references can survive being deleted. In that case I see no problem also referencing a class that has been deleted. That's not inconsistent with the current program code — there is no code in the current program that it behaves differently from.

[nshahan]

In practice the VM allows for you run code that has been retained after a hot reload where it was deleted from the source files. At the same time it produces runtime errors if you try to reference a class or interface type that was deleted from the source:

Lookup failed: A in file:///example.dart

This provides a runtime error for code that would be a compile time error if it were all compiled in a single action.

We have a similar issue when implementing hot reload in DDC. We are pruning away code when it gets deleted in a hot reload but references can still remain so we want to implement some helpful error messages if that retained code is ever executed.