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Fix nullness flow for type aliases after null pattern #18852

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Fix nullness flow for type aliases after null pattern #18852

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54075c5
Initial plan
Copilot Aug 15, 2025
75947fb
Implement nullness refinement preserving type aliases for match null …
Copilot Aug 15, 2025
f450f32
WIP: Attempt to fix nullness refinement - still failing tests
Copilot Aug 15, 2025
9da5dd9
Fix nullness refinement to preserve type aliases in match expressions
Copilot Aug 15, 2025
6ebd419
Fix nullness flow for type aliases after null pattern - targeted appr…
Copilot Sep 8, 2025
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11 changes: 9 additions & 2 deletions src/Compiler/Checking/Expressions/CheckExpressions.fs
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Original file line number Diff line number Diff line change
Expand Up @@ -10733,8 +10733,15 @@ and TcMatchClause cenv inputTy (resultTy: OverallTy) env isFirst tpenv synMatchC

let inputTypeForNextPatterns=
let removeNull t =
let stripped = stripTyEqns cenv.g t
replaceNullnessOfTy KnownWithoutNull stripped
// Check if this is a type abbreviation that we should preserve
match t with
| TType_app (tcref, tinst, _) when tcref.Deref.IsTypeAbbrev ->
// Preserve the type abbreviation structure while refining nullness
TType_app (tcref, tinst, KnownWithoutNull)
| _ ->
// Use existing logic for non-abbreviation types
let stripped = stripTyEqns cenv.g t
replaceNullnessOfTy KnownWithoutNull stripped
let rec isWild (p:Pattern) =
match p with
| TPat_wild _ -> true
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