Fix unreachable br_on_cast parse bug - #8948
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The br_on_cast* family of instructions take input and output type immediates. We do not store the input type in the IR, so IRBuilder has to check the type of the `ref` operand against the input type before the input type is lost. For br_on_cast_desc_eq{_fail} instructions with unreachable descriptor operands, we had a bug that would allow us to pop a `ref` operand with an incorrect type, which would then immediately fail that type check. This was incorrect because the unreachable `desc` operand should have created a polymorphic stack that allowed popping any valid type. Fix the bug by threading the expected input type through IRBuilder to ChildTyper, where we can note the proper type constraint on the `ref` operand.
tlively
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aheejin and
kripken
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July 29, 2026 01:05
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The br_on_cast* family of instructions take input and output type immediates. We do not store the input type in the IR, so IRBuilder has to check the type of the
refoperand against the input type before the input type is lost. For br_on_cast_desc_eq{_fail} instructions with unreachable descriptor operands, we had a bug that would allow us to pop arefoperand with an incorrect type, which would then immediately fail that type check. This was incorrect because the unreachabledescoperand should have created a polymorphic stack that allowed popping any valid type. Fix the bug by threading the expected input type through IRBuilder to ChildTyper, where we can note the proper type constraint on therefoperand.