[Repo Assist] [Python] Fix TCO context adding unnecessary default params to nested lambdas#4426
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dbrattli merged 4 commits intoMar 21, 2026
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#3877) When a function is tail-call optimised, all nested functions/lambdas created within its body previously inherited all TCO variables as default parameters (e.g. `_arg: Any=_arg`), even when the nested function doesn't reference those variables at all. Root cause: `transformFunction` computes `tcArgs` from the outer `TailCallOpportunity` in context and appends them unconditionally to every nested function. The capture is needed for closures that *do* reference a mutated TCO variable (Python captures by reference, so defaults bind the current value). But it was applied indiscriminately. Fix: before appending a TCO arg as a default, check whether the function body actually references that variable using `isIdentUsed`. Variables not referenced in the body are excluded from the captured defaults. This eliminates spurious parameters like `_arg: Any=_arg` on lambdas like `fun () -> g()` that don't use the outer TCO variable, and keeps the capture for lambdas like `fun () -> i` that do. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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Summary
transformFunctioninFable2Python.Transforms.fsunconditionally appended all TCO args as default parameters to every nested lambda, regardless of whether the lambda body references those variablesisIdentUsed name bodybefore adding them as defaultsTest plan
test Passing a function reference in a TCO context does not capture unneeded outer argsadded totests/Python/TestTailCall.fs./build.sh test python --skip-fable-libraryto verify🤖 Generated with [Claude Code]((claude.com/redacted)