Method type argument inference and @MonotonicNonNull

[wmdietlGC]

Run the Nullness Checker on the following example:

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

abstract class InferenceMonotonicNonNull {
    abstract <T extends @Nullable Object> @NonNull T foo(T p);
    @MonotonicNonNull Object field;

    void foo() {
        this.foo(field);
        this.<@Nullable Object>foo(field);
    }
}

We get:

MNN.java:10: error: [monotonic.type.incompatible] cannot assign org.checkerframework.checker.nullness.qual.MonotonicNonNull to org.checkerframework.checker.nullness.qual.MonotonicNonNull (monotonic type).
                        this.foo(field);
                                     ^
  full type found: @Initialized @MonotonicNonNull Object
1 error

The read of field has type @MonotonicNonNull, which is then the inferred type argument type for foo.
However, when the checker then checks whether the argument type is a subtype of the parameter type, the rule for @MonotonicNonNull prevents that the @MonotonicNonNull is assigned to itself.

I think in general the type of the field read this.field should be @Nullable, not @MonotonicNonNull.
When making the method type argument explicitly @Nullable, the problem goes away.

The alternative would be to change method type argument inference to avoid inferring certain annotations, in particular @MonotonicNonNull.
For @MonotonicNonNull I think I would first try changing the type of the read to @Nullable; if we can already think of other cases where avoiding certain annotations would be required, we should add it as another constraint for #979.

[wmdietlGC]

#306 is probably the same issue. Adding cross-ref.