Incorrect warning for nullable type parameter in Map.computeIfAbsent

[amalloy]

Commands

$ ./checker-framework-3.18.0/checker/bin/javac -processor nullness CFRepro.java
$ ./checker-framework-3.22.1/checker/bin/javac -processor nullness CFRepro.java

Inputs

File CFRepro.java:

import java.util.Map;
import java.util.HashMap;
import org.checkerframework.checker.nullness.qual.Nullable;

class CFRepro {
  final Map<Integer, Box<@Nullable String>> m = new HashMap<>();
  Box<@Nullable String> get(Integer k) {
    return m.computeIfAbsent(k, unused -> new Box<>(null));
  }
}

class Box<T> {
  Box(T t) {}
}

Outputs

3.18.0 produces no errors. 3.22.1 produces the following errors:

CFRepro.java:15: error: [return] incompatible types in return.                  
    return m.computeIfAbsent(k, unused -> new Box<>(null));                     
                                          ^
  type of expression: @Initialized @NonNull Box<@Initialized @NonNull String>
  method return type: @Initialized @NonNull Box<@Initialized @Nullable String>
CFRepro.java:15: error: [argument] incompatible argument for parameter t of Box.
    return m.computeIfAbsent(k, unused -> new Box<>(null));
                                                    ^
  found   : null (NullType)
  required: @Initialized @NonNull String 
2 errors

Expectation

3.22.1 should behave like 3.18.0, because this code is not erroneous. We have a Map<K, Box<@Nullable V>>, and new Box(null) is s valid Box<@Nullable V>. I don't see why CF infers that new Box<>(null) has type @Initialized @NonNull Box<@Initialized @NonNull String>.

3.18.0 is an arbitary older version I happen to have handy for testing. I have not tried to identify the specific version or commit between 3.18.0 and 3.22.1 that introduced the regression.

[smillst]

This was fixed with #979.