[GROOVY-10623] Type mismatch when encountering unconditional flow typing and null

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 4.0.5, 3.0.20
• Component/s: Static Type Checker
• Assignee: Eric Milles
• Reporter: Thodoris Sotiropoulos
• Created: 13/May/22 13:37
• Updated: 30/Nov/23 17:18
• Resolved: 15/May/22 18:45

Description

I don't know if this is a bug or a feature

I have the following program

class A {}

class Test {
  void test() {
    def x = new A() // type of x is A
    x = null; // type of x becomes Object

    A y = x
  }

  void test2() {
    def x = new A()
    if (true) {
      x = null
    }
    A y = x // compiles as expected

  }
}

Actual behavior

org.codehaus.groovy.control.MultipleCompilationErrorsException: startup failed:
test.groovy: 9: [Static type checking] - Cannot assign value of type java.lang.Object to variable of type A
 @ line 9, column 11.
       A y = x
             ^

1 error

Expected behavior

Compile successfully

The issue here is that when the compiler encounters a `null` assignment to a `def` variable, it changes the previously-inferred type of the variable to `Object`. Probably, this is not necessary as `null` is assignable to any type.

Tested against master (commit: ee12bb52381e8f0583c61fc25d43de1f55b80a87)