Report a missing return statement in a closure or function

[bentsherman]

Close #7737

The type checker turns the trailing statement of each code path into an implicit return, but never checked that every path actually yields a value. So this passed, and the closure emits null whenever the condition is false:

channel.of(1, 2, 3).map { x ->
    if( x > 1 ) {
        return x * 2
    }
}

Now, when a function or closure must return a value, every code path must end with an explicit return or a trailing expression statement (or a throw). Otherwise it reports Missing return statement on the statement where the path ends. That covers:

• an if with no else, or an empty branch (reported on the if)
• a branch that ends with a declaration or assignment, such as def y = 'a' or y = 'a', which were previously treated as implicit returns
• a try/catch where the try block or a catch block doesn't return. Before, a trailing try/catch was never checked at all

The check only applies when a value is required: a non-void declared return type, an unresolved generic such as the R in map, or a def function where some path returns a value. Void functions and closures are unaffected.

Because predicate closures must return a value (#7714), a filter closure that only returns inside an if is also reported, even though the fall-through null would be treated as false.

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[jorgee]

Looks fine to me. I ran an agent to look for false positives in common patterns (>> publish closures, process and workflow bodies, each/subscribe, if / else if / else chains, try/catch and throw), and it found none.

One corner case, non-blocking: an empty body reports the error without a source position.

channel.of(1).map { x -> }
def f(x: Integer) -> String { }

Error main.nf:-1:-1: Missing return statement

The top-level addReturnsIfNeeded(node, node) uses the empty block as its own fallback, and an empty block has no position. Passing the function or closure node as the fallback would fix it. It's a rare case, so up to you whether to fix it here or wait until someone reports it.