How can I create a void return statement in a Groovy AST transformation?

Question

I am building an AST transformation that generates a void method. I want to check whether the value passed in is already equal to another value, and if so, exit early. The code would ordinarily look like this:

if(param.is existing) {
    return
}

The ReturnStatement class has a property returningNullOrVoid that checks to see whether the return expression is null , so I tried the obvious approach:

ifS(sameX(paramEx, existingEx), returnS(constX(null))

This produced an exception when compiling the transformed class:

BUG! exception in phase 'instruction selection' in source unit 'Annotated.groovy' Unexpected return statement at -1:-1 return null

How can I insert the return statement for the early exit?

Answer 1

The ReturnStatement class has a constant named RETURN_NULL_OR_VOID :

/**
 * Only used for synthetic return statements emitted by the compiler.
 * For comparisons use isReturningNullOrVoid() instead.
 */
public static final ReturnStatement RETURN_NULL_OR_VOID = new ReturnStatement(ConstantExpression.NULL);

The Groovy compiler checks for this specific instance to generate a void return; . When creating an AST statement block that includes a return statement, your statement is "a synthetic return statement emitted by the compiler", and you should use that constant:

ifS(sameX(paramEx, existingEx), ReturnStatement.RETURN_NULL_OR_VOID)