Editorial: non-syntactic async/generator functions use NormalCompletion #3606
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bakkot
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editor call
AsyncBlockStart needs to get a return completion when evaluating concise async arrows so that it knows to use the value instead of *undefined*, so we need to return a return completion in the evaluation semantics for ExpressionBody and then in the non-async case unwrap that to a normal completion in EvaluateConciseBody (which is not used in the async case).
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This moves the machinery for handling implicit returns in syntactic async/generator functions into the branches for evaluating parse nodes, instead of also using that logic for evaluating Abstract Closures.
It assumes that non-syntactic async/generator functions always internally return ThrowCompletions or NormalCompletions. Right now they can use both ReturnCompletions or NormalCompletions, which provides an unnecessary degree of freedom. The alternative is to enforce that non-syntactic async/generator functions return ThrowCompletions or ReturnCompletions, for which see #3605.
This makes non-syntactic async/generator functions match non-syntactic ordinary functions, which similarly always return ThrowCompletions or NormalCompletions. For that reason, I prefer this PR over #3605. (Edit: in the editor call today, the other editors agreed.)
In addition, it moves the handling of ReturnCompletions in evaluation of ordinary functions to immediately after the evaluation of the function body, to match async/generator functions as of this PR. This means EvaluateBody and its callees now return NormalCompletions.
Commits should be reviewed individually.
(This PR would also require changing the existing built-in generators/async functions to never return ReturnCompletions. I'll push up a commit doing that if we go this route.)