[GROOVY-10067] STC: Wrong return value type inferred for generic method with upper bound without parameters

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 3.0.8, 4.0.0-alpha-3
• Fix Version/s: 4.0.0-beta-1
• Component/s: Static Type Checker
• Environment: OpenJDK8
• Assignee: Eric Milles
• Reporter: Lyuben Atanasov
• Created: 30/Apr/21 15:05
• Updated: 19/May/22 15:49
• Resolved: 08/May/21 20:16

Description

When static compilation is enabled, and we have a method call for a generic method that returns a generic value with an upper bound, and the method does not have any parameters, the inferred return type is the upper bound instead of the type of the variable to which that value is assigned. To better illustrate the issue, here's an example of what doesn't work and some work-arounds:

class Test {

	<T extends Number> T getValue() {
		return null;
	}
	
	void methodWithIntParam(Integer param) {
		// do nothing
	}
	
	void test() {
		// not working
		Integer int1 = getValue();
		methodWithIntParam(getValue());
		
		// working
		Integer int2 = this.<Integer>getValue();
		Integer int3 = (Integer) getValue();
		methodWithIntParam(this.<Integer>getValue());
		methodWithIntParam((Integer) getValue());
	}
}

Compiling this code with static type checking enabled causes the following errors:

Script_e94ec281a61eb83bce1a38580b498a48.groovy: 13: [Static type checking] - Cannot assign value of type java.lang.Number to variable of type java.lang.Integer
 @ line 13, column 18.
   		Integer int1 = getValue();

Script_e94ec281a61eb83bce1a38580b498a48.groovy: 14: [Static type checking] - Cannot find matching method Test#methodWithIntParam(java.lang.Number). Please check if the declared type is correct and if the method exists.
 @ line 14, column 3.
   		methodWithIntParam(getValue());

Note that if we just a define a generic method without any upper bounds to the generic value, type checking works as expected:

<T> T getNumber() {
    return null;
}

Integer myInt = getNumber(); // this works!

Issue Links

• relates to: GROOVY-10342 STC is unable to infer type parameter of parameterized method call