ADDITIONAL SYSTEM INFORMATION :
javac 15-ea build 21

A DESCRIPTION OF THE PROBLEM :
When a method accepting a functional interfarce gets a generic method reference, the type inference may not work correctly.

In the following example, the first examples do not compile, producing "error: no suitable method found for thenComparing(Comparator<T#1>)". Please note, that using a non-generic method reference, as explicitly specifying the type, works.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Compile the attached snippet.


---------- BEGIN SOURCE ----------
import java.util.Map;
import java.util.Comparator;


public class Foo {

    private static final Map<String, Integer> FREQUENCES = Map.of("a", 2, "b", 3);

    public static void test() {
        // Failing:
        // 1a: When chained
        Comparator<String> c1 = Comparator.comparingInt(FREQUENCES::get)
                .thenComparing(Comparator.naturalOrder()); // <- 'error: no suitable method found for thenComparing(Comparator<T#1>)'
        // 1b: With LVTI
        var compByFreqAsc = Comparator.comparingInt(FREQUENCES::get);
        Comparator<String> c2 = compByFreqAsc.thenComparing(Comparator.naturalOrder()); // <- 'error: no suitable method found for thenComparing(Comparator<T#1>)'

        // Compiling:
        // 2a: With non-generic method reference
        Comparator<String> c5 = Comparator.comparingInt(Foo::frequency)
                .thenComparing(Comparator.naturalOrder());
        // 2b: With lambda parameter type declaration
        Comparator<String> c3 = Comparator.comparingInt((String s) -> FREQUENCES.get(s))
                .thenComparing(Comparator.naturalOrder());
        // 2c: With generic method explicit type parameter declaration
        Comparator<String> c4 = Comparator.<String>comparingInt(FREQUENCES::get)
                .thenComparing(Comparator.naturalOrder());
        // 2d: With intermediate variable
        Comparator<String> compByFreqAsc2 = Comparator.comparingInt(FREQUENCES::get);
        Comparator<String> c6 = compByFreqAsc2.thenComparing(Comparator.naturalOrder());
    }

    private static int frequency(String s) {
        return FREQUENCES.get(s);
    }
}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
The workarounds (also shown in the code):
- Use non-generic method reference, if possible (2a)
- Replace with a lambda, *and* specify the parameter type (2b)
- Specify the generic method type (2c)
- Use an intermediate variable with explicit type declaration (2d)

FREQUENCY : always