False alarm when using "return null" and type is known to support nulls

[vlsi]

Sample code:

  public static @PolyNull Boolean not(@PolyNull Boolean b) {
    return (b == null) ? null : !b;
  }

Error:

SqlFunctions.java:2571: error: [return.type.incompatible] incompatible types in return.
    return (b == null) ? null : !b;
                       ^
  type of expression: @Initialized @Nullable Boolean
  method return type: @Initialized @PolyNull Boolean

Could checker be a bit smarter, so it could see that {{b == null}} implies {{@polynull}} is in {{@nullable}} mode, so it should be perfectly valid to return {{null}}.

I understand I can rewrite the method like {{return (b == null) ? b : !b;}}, however, it would be nice if checker could figure out that automatically.

Another example might involve regular generics without Poly annotations:

  public <T> static T of(T a) {
    return a == null ? null : a;
  }

It would be nice if checker could figure out return null is reachable only in the case T is nullable.

Another example:

  /** Converts a JDBC array to a list. */
  public static @PolyNull List arrayToList(final java.sql.@PolyNull Array a) {
    if (a == null) {
      return null; // <- error here
    }
    ...

[vlsi]

One more sample which does not have a nice workaround except castNonNull

  public static @PolyNull Integer plus(@PolyNull Integer b0, @PolyNull Integer b1) {
    return (b0 == null || b1 == null) ? null : (b0 + b1);
  }

[mernst]

The not and plus examples may be related to #3622, because rewriting them as if statements causes the Nullness Checker to accept them.

[mernst]

Followup: the fix for #3622 does not resolve this issue.
There are test cases checked into the repository in file Issue3614.java.