Fix generalization of call-argument lambdas causing type errors#9243
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Fix generalization of call-argument lambdas causing type errors#9243
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Lambdas passed as call arguments (e.g. `list.fold_rev(dest, |item, acc| ...)`) were independently generalized at an inner rank, setting their type vars to rank 0. When those vars were then unified with outer scope vars via Rank.min in merge, the outer vars were pulled to rank 0, preventing the enclosing function from generalizing. This caused spurious TYPE MISMATCH errors when a polymorphic function with nested lambdas was tested from within its own module (via expect), while working correctly from external modules. Skip generalization for standalone lambdas that are direct call arguments, since they are consumed immediately and don't need independent generalization. Their type vars remain at the enclosing scope's rank and generalize together with the enclosing function. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
The existing test only covered cross-module usage (importing the function). Add an expect inside the module itself and a CLI test that runs `roc test` on it, covering the case that was previously broken. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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See https://roc.zulipchat.com/#narrow/channel/231634-beginners/topic/Unexpected.20type.20mismatch.20with.20parameterised.20function/near/577730388
Lambdas passed as call arguments (e.g.
list.fold_rev(dest, |item, acc| ...))were independently generalized at an inner rank, setting their type vars to
rank 0. When those vars were then unified with outer scope vars via Rank.min
in merge, the outer vars were pulled to rank 0, preventing the enclosing
function from generalizing. This caused spurious TYPE MISMATCH errors when
a polymorphic function with nested lambdas was tested from within its own
module (via expect), while working correctly from external modules.
Skip generalization for standalone lambdas that are direct call arguments,
since they are consumed immediately and don't need independent generalization.
Their type vars remain at the enclosing scope's rank and generalize together
with the enclosing function.
Co-Authored-By: Claude Opus 4.6 noreply@anthropic.com