Type argument inference crashed

[cushon]

$ cat CrashRepro.java
import java.util.Optional;

public class CrashRepro {
  void test(Optional<Object> optional) {
    Optional<String> x = optional.flatMap((Object unused) -> Optional.of(""));
  }
}

$ ~/checker-framework-4.0.0/checker/bin/javac -processor nullness CrashRepro.java 
CrashRepro.java:5: error: [type.argument.inference.crashed] Type argument inference crashed for Optional.flatMap
    Optional<String> x = optional.flatMap((Object unused) -> Optional.of(""));
                                         ^
  error: An exception occurred:  False bound for: Constraint: inference type: capture#235 of ? extends U <: String Result: FALSE
  inference type: capture#235 of ? extends U <: String
  use of U from optional.flatMap((Object unused)->Optional.of("")) <: String
  From complementary bound.: use of U from optional.flatMap((Object unused)->Optional.of("")) <= String
  inference type: java.util.Optional<U> <: Optional<String>
  inference type: java.util.Optional<U> -> Optional<String>
  From: Constraint between method call type and target type for method call:optional.flatMap((Object unused)->Optional.of(""))
CrashRepro.java:5: error: [type.argument.inference.crashed] Type argument inference crashed for Optional.flatMap
    Optional<String> x = optional.flatMap((Object unused) -> Optional.of(""));
                                         ^
  error: An exception occurred:  False bound for: Constraint: inference type: capture#218 of ? extends U <: @Initialized @NonNull String Result: FALSE
  inference type: capture#218 of ? extends U <: @Initialized @NonNull String
  use of U from optional.flatMap((Object unused)->Optional.of("")) <: @Initialized @NonNull String
  From complementary bound.: use of U from optional.flatMap((Object unused)->Optional.of("")) <= @Initialized @NonNull String
  inference type: java.util.Optional<U> <: @UnknownInitialization @Nullable Optional<@Initialized @NonNull String>
  inference type: java.util.Optional<U> -> @UnknownInitialization @Nullable Optional<@Initialized @NonNull String>
  From: Constraint between method call type and target type for method call:optional.flatMap((Object unused)->Optional.of(""))
2 errors

[cushon]

We're seeing a number of False bound for errors, it's not clear which are the same and which are different, so I just provided this report for that category.