Improve implied lambda and delegate argument names

src/Compiler/Checking/CheckExpressions.fs

@@ -8445,7 +8445,19 @@ and TcUnionCaseOrExnCaseOrActivePatternResultItemThen (cenv: cenv) overallTy env
                 // This is where the constructor expects arguments but is not applied to arguments, hence build a lambda
                 numArgTys,
                 (fun () ->
-                    let vs, args = argTys |> List.mapi (fun i ty -> mkCompGenLocal mItem ("arg" + string i) ty) |> List.unzip
+                    let argNamesIfFeatureEnabled =
+                        if g.langVersion.SupportsFeature LanguageFeature.ImprovedImpliedArgumentNames then
+                            argNames
+                        else
+                            []
+
+                    let vs, args =
+                        argTys
+                        |> List.mapi (fun i ty ->
+                            let argName = argNamesIfFeatureEnabled |> List.tryItem i |> Option.map (fun x -> x.idText) |> Option.defaultWith (fun () -> "arg" + string i)
+                            mkCompGenLocal mItem argName ty)
+                        |> List.unzip
+
                     let constrApp = mkConstrApp mItem args
                     let lam = mkMultiLambda mItem vs (constrApp, tyOfExpr g constrApp)
                     lam)
@@ -9535,7 +9547,13 @@ and TcMethodApplication_CheckArguments
     let denv = env.DisplayEnv
     match curriedCallerArgsOpt with
     | None ->
-        let curriedArgTys, returnTy =
+        let curriedArgTys, curriedArgNamesIfFeatureEnabled, returnTy =
+            let paramNamesIfFeatureEnabled (g: TcGlobals) (meth: MethInfo) =
+                if g.langVersion.SupportsFeature LanguageFeature.ImprovedImpliedArgumentNames then
+                    meth.GetParamNames()
+                else
+                    []
+
             match candidates with
             // "single named item" rule. This is where we have a single accessible method
             //      member x.M(arg1, ..., argN)
@@ -9547,19 +9565,23 @@ and TcMethodApplication_CheckArguments
             // to their default values (for optionals) and be part of the return tuple (for out args).
             | [calledMeth] ->
                 let curriedArgTys, returnTy = UnifyMatchingSimpleArgumentTypes cenv env exprTy.Commit calledMeth mMethExpr mItem
-                curriedArgTys, MustEqual returnTy
+                curriedArgTys, paramNamesIfFeatureEnabled g calledMeth, MustEqual returnTy
             | _ ->
                 let domainTy, returnTy = UnifyFunctionType None cenv denv mMethExpr exprTy.Commit
                 let argTys = if isUnitTy g domainTy then [] else tryDestRefTupleTy g domainTy
                 // Only apply this rule if a candidate method exists with this number of arguments
-                let argTys =
-                    if candidates |> List.exists (CalledMethHasSingleArgumentGroupOfThisLength argTys.Length) then
-                        argTys
-                    else
-                        [domainTy]
-                [argTys], MustEqual returnTy
-
-        let lambdaVarsAndExprs = curriedArgTys |> List.mapiSquared (fun i j ty -> mkCompGenLocal mMethExpr ("arg"+string i+string j) ty)
+                let argTys, argNames =
+                    match candidates |> List.tryFind (CalledMethHasSingleArgumentGroupOfThisLength argTys.Length) with
+                    | Some meth -> argTys, paramNamesIfFeatureEnabled g meth
+                    | None -> [domainTy], [[None]]
+                [argTys], argNames, MustEqual returnTy
+
+        let lambdaVarsAndExprs =
+            curriedArgTys
+            |> List.mapiSquared (fun i j ty ->
+                let argName = curriedArgNamesIfFeatureEnabled |> List.tryItem i |> Option.bind (List.tryItem j) |> Option.flatten |> Option.defaultWith (fun () -> "arg" + string i + string j)
+                mkCompGenLocal mMethExpr argName ty)
+
         let unnamedCurriedCallerArgs = lambdaVarsAndExprs |> List.mapSquared (fun (_, e) -> CallerArg(tyOfExpr g e, e.Range, false, e))
         let namedCurriedCallerArgs = lambdaVarsAndExprs |> List.map (fun _ -> [])
         let lambdaVars = List.mapSquared fst lambdaVarsAndExprs

src/Compiler/Checking/MethodCalls.fs

@@ -1266,8 +1266,23 @@ let BuildNewDelegateExpr (eventInfoOpt: EventInfo option, g, amap, delegateTy, d
             if List.exists (isByrefTy g) delArgTys then
                     error(Error(FSComp.SR.tcFunctionRequiresExplicitLambda(delArgTys.Length), m)) 
 
+            let delFuncArgNamesIfFeatureEnabled =
+                match delFuncExpr with
+                | Expr.Val (valRef = vref) when g.langVersion.SupportsFeature LanguageFeature.ImprovedImpliedArgumentNames ->
+                    match vref.ValReprInfo with
+                    | Some repr when repr.ArgNames.Length = delArgTys.Length -> Some repr.ArgNames
+                    | _ -> None
+                | _ -> None
+
             let delArgVals =
-                delArgTys |> List.mapi (fun i argTy -> fst (mkCompGenLocal m ("delegateArg" + string i) argTy)) 
+                delArgTys
+                |> List.mapi (fun i argTy ->
+                    let argName =
+                        match delFuncArgNamesIfFeatureEnabled with
+                        | Some argNames -> argNames[i]
+                        | None -> "delegateArg" + string i
+
+                    fst (mkCompGenLocal m argName argTy)) 
 
             let expr = 
                 let args = 

src/Compiler/Checking/infos.fs

@@ -1100,6 +1100,21 @@ type MethInfo =
     member x.GetFSharpReturnType(amap, m, minst) =
         x.GetCompiledReturnType(amap, m, minst) |> GetFSharpViewOfReturnType amap.g
 
+    member x.GetParamNames() =
+        match x with
+        | FSMeth (g, _, vref, _) ->
+            ParamNameAndType.FromMember x.IsCSharpStyleExtensionMember g vref |> List.mapSquared (fun (ParamNameAndType (name, _)) -> name |> Option.map (fun x -> x.idText))
+        | ILMeth (ilMethInfo = ilminfo) ->
+            // A single group of tupled arguments
+            [ ilminfo.ParamMetadata |> List.map (fun x -> x.Name) ]
+#if !NO_TYPEPROVIDERS
+        | ProvidedMeth (_, mi, _, m) ->
+            // A single group of tupled arguments
+            [ [ for p in mi.PApplyArray((fun mi -> mi.GetParameters()), "GetParameters", m) do
+                yield p.PUntaint((fun p -> Some p.Name), m) ] ]
+#endif
+        | _ -> []
+
     /// Get the parameter types of a method info
     member x.GetParamTypes(amap, m, minst) =
         match x with

src/Compiler/Checking/infos.fsi

@@ -517,6 +517,9 @@ type MethInfo =
     /// Get the ParamData objects for the parameters of a MethInfo
     member GetParamDatas: amap: ImportMap * m: range * minst: TType list -> ParamData list list
 
+    /// Get the parameter names of a MethInfo
+    member GetParamNames: unit -> string option list list
+
     /// Get the parameter types of a method info
     member GetParamTypes: amap: ImportMap * m: range * minst: TType list -> TType list list
 

src/Compiler/FSComp.txt

@@ -1573,6 +1573,7 @@ featureNonInlineLiteralsAsPrintfFormat,"String values marked as literals and IL
 featureNestedCopyAndUpdate,"Nested record field copy-and-update"
 featureExtendedStringInterpolation,"Extended string interpolation similar to C# raw string literals."
 featureWarningWhenMultipleRecdTypeChoice,"Raises warnings when multiple record type matches were found during name resolution because of overlapping field names."
+featureImprovedImpliedArgumentNames,"Improved implied argument names"
 3353,fsiInvalidDirective,"Invalid directive '#%s %s'"
 3354,tcNotAFunctionButIndexerNamedIndexingNotYetEnabled,"This value supports indexing, e.g. '%s.[index]'. The syntax '%s[index]' requires /langversion:preview. See https://aka.ms/fsharp-index-notation."
 3354,tcNotAFunctionButIndexerIndexingNotYetEnabled,"This expression supports indexing, e.g. 'expr.[index]'. The syntax 'expr[index]' requires /langversion:preview. See https://aka.ms/fsharp-index-notation."

src/Compiler/Facilities/LanguageFeatures.fs

@@ -68,6 +68,7 @@ type LanguageFeature =
     | NestedCopyAndUpdate
     | ExtendedStringInterpolation
     | WarningWhenMultipleRecdTypeChoice
+    | ImprovedImpliedArgumentNames
 
 /// LanguageVersion management
 type LanguageVersion(versionText) =
@@ -159,6 +160,7 @@ type LanguageVersion(versionText) =
                 LanguageFeature.NestedCopyAndUpdate, previewVersion
                 LanguageFeature.ExtendedStringInterpolation, previewVersion
                 LanguageFeature.WarningWhenMultipleRecdTypeChoice, previewVersion
+                LanguageFeature.ImprovedImpliedArgumentNames, previewVersion
 
             ]
 
@@ -282,6 +284,7 @@ type LanguageVersion(versionText) =
         | LanguageFeature.NestedCopyAndUpdate -> FSComp.SR.featureNestedCopyAndUpdate ()
         | LanguageFeature.ExtendedStringInterpolation -> FSComp.SR.featureExtendedStringInterpolation ()
         | LanguageFeature.WarningWhenMultipleRecdTypeChoice -> FSComp.SR.featureWarningWhenMultipleRecdTypeChoice ()
+        | LanguageFeature.ImprovedImpliedArgumentNames -> FSComp.SR.featureImprovedImpliedArgumentNames ()
 
     /// Get a version string associated with the given feature.
     static member GetFeatureVersionString feature =

src/Compiler/Facilities/LanguageFeatures.fsi

@@ -58,6 +58,7 @@ type LanguageFeature =
     | NestedCopyAndUpdate
     | ExtendedStringInterpolation
     | WarningWhenMultipleRecdTypeChoice
+    | ImprovedImpliedArgumentNames
 
 /// LanguageVersion management
 type LanguageVersion =

src/Compiler/xlf/FSComp.txt.cs.xlf

@@ -272,6 +272,11 @@
         <target state="translated">implicitní yield</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureImprovedImpliedArgumentNames">
+        <source>Improved implied argument names</source>
+        <target state="new">Improved implied argument names</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureIndexerNotationWithoutDot">
         <source>expr[idx] notation for indexing and slicing</source>
         <target state="translated">Notace expr[idx] pro indexování a vytváření řezů</target>

src/Compiler/xlf/FSComp.txt.de.xlf

@@ -272,6 +272,11 @@
         <target state="translated">implizite yield-Anweisung</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureImprovedImpliedArgumentNames">
+        <source>Improved implied argument names</source>
+        <target state="new">Improved implied argument names</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureIndexerNotationWithoutDot">
         <source>expr[idx] notation for indexing and slicing</source>
         <target state="translated">expr[idx]-Notation zum Indizieren und Aufteilen</target>

src/Compiler/xlf/FSComp.txt.es.xlf

@@ -272,6 +272,11 @@
         <target state="translated">elemento yield implícito</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureImprovedImpliedArgumentNames">
+        <source>Improved implied argument names</source>
+        <target state="new">Improved implied argument names</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureIndexerNotationWithoutDot">
         <source>expr[idx] notation for indexing and slicing</source>
         <target state="translated">Notación para indexación y segmentación expr[idx]</target>

src/Compiler/xlf/FSComp.txt.fr.xlf

@@ -272,6 +272,11 @@
         <target state="translated">yield implicite</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureImprovedImpliedArgumentNames">
+        <source>Improved implied argument names</source>
+        <target state="new">Improved implied argument names</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureIndexerNotationWithoutDot">
         <source>expr[idx] notation for indexing and slicing</source>
         <target state="translated">Notation expr[idx] pour l’indexation et le découpage</target>

src/Compiler/xlf/FSComp.txt.it.xlf

@@ -272,6 +272,11 @@
         <target state="translated">istruzione yield implicita</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureImprovedImpliedArgumentNames">
+        <source>Improved implied argument names</source>
+        <target state="new">Improved implied argument names</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureIndexerNotationWithoutDot">
         <source>expr[idx] notation for indexing and slicing</source>
         <target state="translated">Notazione expr[idx] per l'indicizzazione e il sezionamento</target>

src/Compiler/xlf/FSComp.txt.ja.xlf

@@ -272,6 +272,11 @@
         <target state="translated">暗黙的な yield</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureImprovedImpliedArgumentNames">
+        <source>Improved implied argument names</source>
+        <target state="new">Improved implied argument names</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureIndexerNotationWithoutDot">
         <source>expr[idx] notation for indexing and slicing</source>
         <target state="translated">インデックス作成とスライス用の expr[idx] 表記</target>

src/Compiler/xlf/FSComp.txt.ko.xlf

@@ -272,6 +272,11 @@
         <target state="translated">암시적 yield</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureImprovedImpliedArgumentNames">
+        <source>Improved implied argument names</source>
+        <target state="new">Improved implied argument names</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureIndexerNotationWithoutDot">
         <source>expr[idx] notation for indexing and slicing</source>
         <target state="translated">인덱싱 및 슬라이싱을 위한 expr[idx] 표기법</target>

src/Compiler/xlf/FSComp.txt.pl.xlf

@@ -272,6 +272,11 @@
         <target state="translated">niejawne słowo kluczowe yield</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureImprovedImpliedArgumentNames">
+        <source>Improved implied argument names</source>
+        <target state="new">Improved implied argument names</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureIndexerNotationWithoutDot">
         <source>expr[idx] notation for indexing and slicing</source>
         <target state="translated">notacja wyrażenia expr[idx] do indeksowania i fragmentowania</target>

src/Compiler/xlf/FSComp.txt.pt-BR.xlf

@@ -272,6 +272,11 @@
         <target state="translated">yield implícito</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureImprovedImpliedArgumentNames">
+        <source>Improved implied argument names</source>
+        <target state="new">Improved implied argument names</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureIndexerNotationWithoutDot">
         <source>expr[idx] notation for indexing and slicing</source>
         <target state="translated">notação expr[idx] para indexação e fatia</target>

src/Compiler/xlf/FSComp.txt.ru.xlf

@@ -272,6 +272,11 @@
         <target state="translated">неявное использование yield</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureImprovedImpliedArgumentNames">
+        <source>Improved implied argument names</source>
+        <target state="new">Improved implied argument names</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureIndexerNotationWithoutDot">
         <source>expr[idx] notation for indexing and slicing</source>
         <target state="translated">expr[idx] для индексации и среза</target>

src/Compiler/xlf/FSComp.txt.tr.xlf

@@ -272,6 +272,11 @@
         <target state="translated">örtük yield</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureImprovedImpliedArgumentNames">
+        <source>Improved implied argument names</source>
+        <target state="new">Improved implied argument names</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureIndexerNotationWithoutDot">
         <source>expr[idx] notation for indexing and slicing</source>
         <target state="translated">Dizin oluşturma ve dilimleme için expr[idx] gösterimi</target>

src/Compiler/xlf/FSComp.txt.zh-Hans.xlf

@@ -272,6 +272,11 @@
         <target state="translated">隐式 yield</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureImprovedImpliedArgumentNames">
+        <source>Improved implied argument names</source>
+        <target state="new">Improved implied argument names</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureIndexerNotationWithoutDot">
         <source>expr[idx] notation for indexing and slicing</source>
         <target state="translated">用于索引和切片的 expr[idx] 表示法</target>

src/Compiler/xlf/FSComp.txt.zh-Hant.xlf

@@ -272,6 +272,11 @@
         <target state="translated">隱含 yield</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureImprovedImpliedArgumentNames">
+        <source>Improved implied argument names</source>
+        <target state="new">Improved implied argument names</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureIndexerNotationWithoutDot">
         <source>expr[idx] notation for indexing and slicing</source>
         <target state="translated">用於編製索引和分割的 expr[idx] 註釋</target>