[GROOVY-11364] STC loses generic specialization on inheritance

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 4.0.15, 4.0.21
• Fix Version/s: 3.0.22, 5.0.0-alpha-9, 4.0.22
• Component/s: Static Type Checker
• Assignee: Eric Milles
• Reporter: Christopher Smith
• Created: 29/Apr/24 17:25
• Updated: 30/Apr/24 17:20
• Resolved: 30/Apr/24 15:56

Description

When a base class contains a type parameter <N> and a method returns N, the STC believes that the return type is the superclass bound of N and not the actual type of N. Overriding the method in the subclass with the specific type does not fix the problem.

package bug.groovy

import groovy.transform.CompileStatic

@CompileStatic
abstract class BugRepro<N extends Number> {
  protected N process(N input) { input }
}

@CompileStatic
class IntegerRepro extends BugRepro<Integer> {
  static void consume(Optional<Integer> input) {}

  // The compilation fails with or without this override
  @Override
  protected Integer process(Integer input) { super.process(input) }

  void run() {
    consume(Optional.of(42).map(this::process))
  }
}

Error:

Groovy:[Static type checking] - Cannot call bug.groovy.IntegerRepro#consume(java.util.Optional<java.lang.Integer>) with arguments [java.util.Optional<java.lang.Number>]

Expected: The compiler recognizes that N at the call site is Integer.

This bug appears to have been introduced in 4.0.15; the code compiles correctly in 4.0.14. The bug is still present in 4.0.21.