[GROOVY-7713] CompileStatic checking fails with null returns

• Type: Bug
• Status: Closed
• Priority: Minor
• Resolution: Fixed
• Affects Version/s: 2.4.4, 2.4.5
• Fix Version/s: 2.4.8
• Component/s: Static Type Checker
• Assignee: John Wagenleitner
• Reporter: Scott Douglas
• Created: 17/Dec/15 22:51
• Updated: 01/Feb/17 23:18
• Resolved: 19/Aug/16 02:38

Description

---------
TEST 1
---------

package test

import groovy.transform.CompileStatic

class TestClass {

	@CompileStatic
	void doTest() {
		Closure<String> closure = {
			return "foo";
		}
	}

}

Compiles fine.

---------
TEST 2
---------

package test

import groovy.transform.CompileStatic

class TestClass {

	@CompileStatic
	void doTest() {
		Closure<String> closure = {
			if ("bah".length() == 3) {
				return null
			}
			return "foo";
		}
	}

}

...gives...

groovyc Test2.groovy
org.codehaus.groovy.control.MultipleCompilationErrorsException: startup failed:
Test2.groovy: 9: [Static type checking] - Incompatible generic argument types. Cannot assign groovy.lang.Closure <java.lang.Object> to: groovy.lang.Closure <String>
 @ line 9, column 29.
   		Closure<String> closure = {
                               ^

1 error

---------
TEST 3
---------

package test

import groovy.transform.CompileStatic

class TestClass {

	@CompileStatic
	void doTest() {
		Closure<String> closure = {
			return null;
		}
	}

}

...gives...

groovyc Test3.groovy
org.codehaus.groovy.control.MultipleCompilationErrorsException: startup failed:
Test3.groovy: 9: [Static type checking] - Incompatible generic argument types. Cannot assign groovy.lang.Closure <java.lang.Object> to: groovy.lang.Closure <String>
 @ line 9, column 29.
   		Closure<String> closure = {
                               ^

1 error

---------
TEST 4
---------

package test

import groovy.transform.CompileStatic

class TestClass {

	@CompileStatic
	void doTest() {
		Closure<String> closure = {

		}
	}

}

Compiles fine.

---------
COMMENTS
---------
All files were compiled with 'groovyc Test[1234].groovy'. I expected all tests to compile. I wouldn't have thought returning null would cause a static compilation failure since null is a valid String. Also, I expected test 3 and test 4 to be equivalent.

The workaround appears to be to cast the closure, like:

Closure<String> closure = (Closure<String>) {
    ...
}

Issue Links

• links to: GitHub Pull Request #384