Java: Add test showing missing dispatch for incomplete parameterised type

[aschackmull]

Cf. #19538

The test is failing for a couple of reasons. The most important is that MethodCall.getMethod() has no results for the call, i.e. the extractor has not populated callableBinding. It's unclear what exactly we'd want the target callable to be, but it's clear what its getSourceDeclaration() should be and that's what we really need. If we fix that somehow, then we'll also need to allow ErrorType in Unification.qll, but for now that won't do anything.

[Copilot]

Pull Request Overview

Adds a reproducer for missing virtual dispatch on error-containing generics and wires up a CodeQL query to detect it.

• Introduce a GenericClass<T> and testDispatch method in Test.java to trigger dispatch on both valid and error types.
• Add Dispatch.ql to query viableImpl of MethodCall.
• Provide expected output in Dispatch.expected for the successful dispatch case.

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
java/ql/test/library-tests/errortype/Test.java	Added nested GenericClass<T> and testDispatch method to exercise dispatch logic
java/ql/test/library-tests/errortype/Dispatch.ql	New query selecting virtual dispatch (viableImpl) matches
java/ql/test/library-tests/errortype/Dispatch.expected	Expected single dispatch result for the String instantiation

Comments suppressed due to low confidence (1)

java/ql/test/library-tests/errortype/Test.java:14

• The method testDispatch is missing a return type. Update the signature to something like public void testDispatch() so the code compiles.

public testDispatch(){

[hvitved]

FWIW, the QL-based type inference/method resolution implementation should be able to handle cases like these.

[aschackmull]

That's good to know.

[smowton]

Suggest trying resolving this by https://github.com/github/semmle-code/tree/smowton/wip/fix-missing-methods

That fixes the database inconsistency so MethodCall.getCallee works, but it is not picked up by viableCallable yet.