Allow subtyping on optional operands #271
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With this change, is the obvious bottom-up algo described here valid? If not, I'd be interested to know what are the cases which differ. |
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@lukewagner, yes, that's the idea: you simply do a join of all branches targeting the block, wrt to the trivial subtyping semi-lattice that has Void as its top type. |
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Thank you. |
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Great, lgtm |
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Landing with LGTM and no objections |
ngzhian
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This reverses #180, and adds a number of corresponding tests.
Motivation: not allowing optional operands to be implicitly dropped (1) reduced the compositionality of the language, (2) made it harder to allow multiple targets (like with a br_if also returning its operand), and (3) put an extra burden on bottom-up type checkers, where the arity might not be known upfront. And with multiple value, we eventually would want to lift this restriction again anyway.
This should address issues #227, and enable WebAssembly/design#539.
@jcbeyler, you filed #179, which lead to #180. Are you okay with reverting to status ante quo?