Andreas Rossberg wrote:
In particular, > > function f(a) { > ... // TDZ > let a = ...; > ... > } > > and > > function g() { > try { > ... > } catch (e) { > ... // TDZ > let e = ...; > ... > } > } > > should be early errors because there's no useful shadowing going on with > 'let' -- the TDZ means the outer binding cannot be used in the commented > places -- but any prior version using 'var' would have worked and possibly > allowed (coverage-dependent) uses of the "outer" (in catch's case, var > hoists across the catch head in sloppy code; in the parameter case there's > only ever one 'a' binding) use in the TDZ. > > So an early error does no actual harm in either case, and helps avoid bugs > slipping past incomplete test coverage.Well, this is a valid concern, but I'd like to point out that it is a flawed argument for defending the status quo. If we really cared about legacy refactoring mistakes going unnoticed, then the suitable (and consistent) semantics would be to make_all_ intra-function shadowing (or more precisely, multiple declarations) an error. Otherwise, we are just picking random cases. For example, I don't see how the above argument can justify the current discrepancy between these two: for (let x ...) { let x; ... } // fine try ... catch(x) { let x; ... } // error The former is just as much a potential bug as the latter, or as all other cases of redeclarations.
You're right, if the issue is shadowing an immediately enclosing binding. The param vs. var-ported-to-let case is different because the var does not shadow and this is observable via sloppy mode arguments object element / formal parameter aliasing.
But even in the catch case, in sloppy code with var x in the catch block, latent bugs are likely to lurk due to hoisting.
So the rationale is not that immediate-shadowing per se should be an error -- it is rather that var-cum-let bindings that tangle with or cross over a legacy danger zone (formal parameter aliased by var; implicit let binding in catch head) should be errors.
Macros and similar systems could very well want immediate let-bound shadowing within a block that is self-contained, in new code. We don't believe all such cases should be errors.
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