Failing constraint printed twice for `type.arguments.not.inferred` error

Example:

import java.util.*;
import org.checkerframework.checker.nullness.qual.*;
class Test {
  public static <T extends @Nullable Object> T firstOrDefault(List<T> list, T defaultValue) {
    return list.isEmpty() ? defaultValue : list.get(0);
  }
  public static <U extends @Nullable Object> List<U> singletonList(U o) {
    throw new RuntimeException();
  }

  String field = firstOrDefault(singletonList(null), "hello");
}

Nullness Checker output:

$ path/to/checker/bin/javac -processor nullness Test.java
Test.java:13: error: [assignment] incompatible types in assignment.
  String field = firstOrDefault(singletonList(null), "hello");
                               ^
  found   : @Initialized @Nullable String
  required: @Initialized @NonNull String
Test.java:13: error: [type.arguments.not.inferred] Could not infer type arguments for Test.firstOrDefault
  String field = firstOrDefault(singletonList(null), "hello");
                               ^
  unsatisfiable constraint: @org.checkerframework.checker.nullness.qual.Nullable <: @org.checkerframework.checker.nullness.qual.NonNull
  @org.checkerframework.checker.nullness.qual.Nullable <: @org.checkerframework.checker.nullness.qual.NonNull
2 errors

The unsat constraint @org.checkerframework.checker.nullness.qual.Nullable <: @org.checkerframework.checker.nullness.qual.NonNull is printed twice.

Start by compiling the Java example with the Nullness Checker and compare the two diagnostics for type.arguments.not.inferred. Trace the diagnostic-generation entry point for that error and identify where the unsatisfiable constraint is emitted more than once. Done means the constraint appears once while the relevant errors remain correctly reported.