Compiler bug or feature?

[Jens]

I think this old thread is related.

It includes similar (single file) examples like:

func foo() -> Int {
    return x
}
let x = foo()
print(x)

which still compiles, and prints 0.
And if you change Int to [Int] it will crash at runtime.

Here's one of the replies in that old thread:

@jrose

Yep, it really is a long-standing bug. Script-mode top-level locals are treated as globals (module-scope bindings) by the compiler, but their initial bindings are evaluated eagerly instead of lazily (as you’d want in a script). Taken together, this means that you can get this completely unsafe behavior.

So, why is ‘a’ accepted but ‘b’ not in your original example?

func foo() -> Int { return b }
let a = 1
let b = 2
print(foo())

The secret to the current behavior is that …

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Some possibly related bugs:

[SR-2730] Failure to initialize global variable

[SR-2727] Script mode rules for top-level bindings are confusing and broken

[SR-284] Strange error caused by let/var at specific place in main file.