jdaugherty commented on PR #2690:
URL: https://github.com/apache/groovy/pull/2690#issuecomment-4957813540
On the drop return type note:
Because that "tie" is exactly the failure. Sorting only by name + parameters
means a bridge method and the method it bridges compare equal (return 0).
Arrays.sort is stable — TimSort — so equal elements keep their input order. And
the input order is getDeclaredMethods(), the very thing that's nondeterministic
and varies between JVM runs. So the tie doesn't get resolved; it silently
inherits the nondeterminism you're trying to eliminate.
Concretely, covariant returns / generic erasure produce this:
class Base<T> { T get() { ... } }
class Sub extends Base<String> {
String get() { ... } // real method
// Object get() // synthetic bridge — same name, same (empty)
params
}
Sub has two get methods: same name, identical parameter types, differing
only in return type (String vs Object). With a name+params comparator they tie,
and whichever the JVM happened to enumerate first wins — per build. Adding the
return-type key breaks that last tie, so no two distinct methods ever compare
equal.
That's what "total order" buys: the JVM forbids two methods in one class
sharing a name and descriptor, and the descriptor is precisely parameters +
return type. Compare on the full descriptor and every element has a unique sort
key, so the output is fully determined no matter what order
getDeclaredMethods() hands back. Leave out the return type and the ordering is
merely partial — total everywhere except the bridge pairs, which are the one
place reproducibility actually needed it.
(Constructors don't need it: name is always <init> and return type always
void, so parameters alone are a total order there — which is why
getDeclaredConstructorsSorted stops at compareParameterTypes.)
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