[GROOVY-9796] Static type checker doesn't backtrack when solving generics

• Type: Bug
• Status: Open
• Priority: Major
• Resolution: Unresolved
• Affects Version/s: 3.0.6
• Component/s: Static Type Checker
• Assignee: Unassigned
• Reporter: Christopher Smith
• Created: 28/Oct/20 20:50
• Updated: 12/Nov/20 21:30

Description

When a method pipeline involves generics, the static type checker is greedy and tends to be overly specific, producing errors when the correct answer involves relaxing to a common bound.

@CompileStatic
Number returnANumber() {
  Number number = Optional.of(1234 as Integer) // should infer Optional<Number>
    orElse(42.0 as Double)
}

[Static type checking] - Cannot find matching method java.util.Optional#orElse(java.lang.Double). Please check if the declared type is correct and if the method exists.

Explicitly inserting a type witness Optional.<Number>of works.

Note: This might be a duplicate of GROOVY-8103, but I'm unable to tell precisely. 3.0.6 was released one week after it was marked resolved. GROOVY-8961 is marked fixed in 3.0.6, but I'm having the problem there.