Nullness :: Improvement :: Overrides of generic methods with nullable type instantiations by T-Gro · Pull Request #18337 · dotnet/fsharp · GitHub
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Nullness :: Improvement :: Overrides of generic methods with nullable type instantiations #18337

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Merged
abonie merged 4 commits into from
Feb 25, 2025
Merged

Nullness :: Improvement :: Overrides of generic methods with nullable type instantiations #18337

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issue analysis
T-Gro Feb 21, 2025
8afd0ef
No nullness warning if override passes in explicit nullable type argu…
T-Gro Feb 24, 2025
5e8304d
Merge branch 'main' into 17988-nullness-not-correctly-inferred-for-re…
T-Gro Feb 24, 2025
3dfea19
notes
T-Gro Feb 24, 2025
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1 change: 1 addition & 0 deletions docs/release-notes/.FSharp.Compiler.Service/9.0.300.md
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22 changes: 18 additions & 4 deletions src/Compiler/Checking/Expressions/CheckExpressions.fs
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Original file line number Diff line number Diff line change
Expand Up @@ -1890,7 +1890,11 @@ let FreshenAbstractSlot g amap m synTyparDecls absMethInfo =

// Work out the required type of the member
let argTysFromAbsSlot = argTys |> List.mapSquared (instType typarInstFromAbsSlot)
let retTyFromAbsSlot = retTy |> GetFSharpViewOfReturnType g |> instType typarInstFromAbsSlot

let retTyFromAbsSlot =
retTy
|> GetFSharpViewOfReturnType g
|> instType typarInstFromAbsSlot
typarsFromAbsSlotAreRigid, typarsFromAbsSlot, argTysFromAbsSlot, retTyFromAbsSlot

let CheckRecdExprDuplicateFields (elems: Ident list) =
Expand Down Expand Up @@ -11774,16 +11778,26 @@ and ApplyAbstractSlotInference (cenv: cenv) (envinner: TcEnv) (_: Val option) (a
match uniqueAbstractMethSigs with
| uniqueAbstractMeth :: _ ->

// Overrides can narrow the retTy from nullable to not-null.
// By changing nullness to be variable we do not get in the way of eliminating nullness (=good).
// We only keep a WithNull nullness if it was part of an explicit type instantiation
let canChangeNullableRetTy =
match g.checkNullness, renaming with
| false, _ -> false
| true, [] -> true
| true, _ -> not(uniqueAbstractMeth.HasGenericRetTy())

let uniqueAbstractMeth = uniqueAbstractMeth.Instantiate(cenv.amap, m, renaming)

let typarsFromAbsSlotAreRigid, typarsFromAbsSlot, argTysFromAbsSlot, retTyFromAbsSlot =
FreshenAbstractSlot g cenv.amap m synTyparDecls uniqueAbstractMeth

let declaredTypars = (if typarsFromAbsSlotAreRigid then typarsFromAbsSlot else declaredTypars)

// Overrides can narrow the retTy from nullable to not-null.
// By changing nullness to be variable we do not get in the way of eliminating nullness (=good).
let retTyFromAbsSlot = retTyFromAbsSlot |> changeWithNullReqTyToVariable g
let retTyFromAbsSlot =
if canChangeNullableRetTy then
retTyFromAbsSlot |> changeWithNullReqTyToVariable g
else retTyFromAbsSlot

let absSlotTy = mkMethodTy g argTysFromAbsSlot retTyFromAbsSlot

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14 changes: 14 additions & 0 deletions src/Compiler/Checking/infos.fs
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Original file line number Diff line number Diff line change
Expand Up @@ -1430,6 +1430,20 @@ type MethInfo =
let (ParamAttribs(isParamArrayArg, isInArg, isOutArg, optArgInfo, callerInfo, reflArgInfo)) = info
ParamData(isParamArrayArg, isInArg, isOutArg, optArgInfo, callerInfo, nmOpt, reflArgInfo, pty)))

member x.HasGenericRetTy() =
match x with
| ILMeth(_g, ilminfo, _) -> ilminfo.RawMetadata.Return.Type.IsTypeVar
| FSMeth(g, _, vref, _) ->
let _, _, _, retTy, _ = GetTypeOfMemberInMemberForm g vref
match retTy with
| Some retTy -> isTyparTy g retTy
| None -> false
| MethInfoWithModifiedReturnType(_,retTy) -> false
| DefaultStructCtor _ -> false
#if !NO_TYPEPROVIDERS
| ProvidedMeth(amap, mi, _, m) -> false
#endif

/// Get the ParamData objects for the parameters of a MethInfo
member x.HasParamArrayArg(amap, m, minst) =
x.GetParamDatas(amap, m, minst) |> List.existsSquared (fun (ParamData(isParamArrayArg, _, _, _, _, _, _, _)) -> isParamArrayArg)
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2 changes: 2 additions & 0 deletions src/Compiler/Checking/infos.fsi
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Original file line number Diff line number Diff line change
Expand Up @@ -538,6 +538,8 @@ type MethInfo =
/// Get the signature of an abstract method slot.
member GetSlotSig: amap: ImportMap * m: range -> SlotSig

member HasGenericRetTy: unit -> bool

/// Get the ParamData objects for the parameters of a MethInfo
member HasParamArrayArg: amap: ImportMap * m: range * minst: TType list -> bool

Expand Down
26 changes: 26 additions & 0 deletions tests/FSharp.Compiler.ComponentTests/Language/Nullness/NullableReferenceTypesTests.fs
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