Type refinement not respected in enhanced for loop

[mernst]

import org.checkerframework.checker.nullness.qual.Nullable;

public class ForLoopTest {

    public final Class<?> @Nullable [] types = null;

    public boolean isAssignableFrom(Class<?> argType) {
        if (types == null) {
            return true;
        }

        // OK.
        types.toString();

        // Not OK!
        for (Class<? extends Object> c : types) {
            // body is irrelevant
        }
        return false;
    }
}

The output is:

ForLoopTest.java:13: error: [enhancedfor.type.incompatible] incompatible types in enhanced for loop.
        for (Class<? extends Object> c : types) {
                                         ^
  found   : @Initialized @NonNull Class<? extends @Initialized @Nullable Object>
  required: @UnknownInitialization @Nullable Class<? extends @Initialized @NonNull Object>
1 error

[wmdietl]

The problem is the mismatch in the type argument, not the refined type of types.
types uses the unbounded Class<?>, whereas c uses the constrained Class<? extends Object>.
The error goes away when you change c to Class<?> or change types to Class<? extends Object>.

[mernst]

You are right. Thanks for catching my goof.
(This is more motivation for #3623 which proposes to simplify error messages or highlight the relevant parts.)