Warn when function type passed to interpolated string

docs/release-notes/.FSharp.Compiler.Service/10.0.300.md

@@ -17,6 +17,8 @@
 
 ### Added
 
+* Added warning FS3882 when a function or delegate value is used as an interpolated string argument. ([PR #19289](https://github.com/dotnet/fsharp/pull/19289))
+
 ### Changed
 
 * Centralized product TFM (Target Framework Moniker) into MSBuild props file `eng/TargetFrameworks.props`. Changing the target framework now only requires editing one file, and it integrates with MSBuild's `--getProperty` for scripts.

docs/release-notes/.Language/preview.md

@@ -1,5 +1,7 @@
 ### Added
 
+* Warn (FS3882) when a function or delegate value is used as an interpolated string argument, since it will be formatted via `ToString` rather than being applied. ([PR #19289](https://github.com/dotnet/fsharp/pull/19289))
+
 ### Fixed
 
 ### Changed

src/Compiler/Checking/Expressions/CheckExpressions.fs

@@ -7466,6 +7466,15 @@ and TcFormatStringExpr cenv (overallTy: OverallTy) env m tpenv (fmtString: strin
         )
 
 /// Check an interpolated string expression
+and [<TailCall>] warnForFunctionValuesInFillExprs (g: TcGlobals) argTys synFillExprs =
+    match argTys, synFillExprs with
+    | argTy :: restTys, (synFillExpr: SynExpr) :: restExprs ->
+        if isFunTy g argTy || isDelegateTy g argTy then
+            warning (Error(FSComp.SR.tcFunctionValueUsedAsInterpolatedStringArg (), synFillExpr.Range))
+
+        warnForFunctionValuesInFillExprs g restTys restExprs
+    | _ -> ()
+
 and TcInterpolatedStringExpr cenv (overallTy: OverallTy) env m tpenv (parts: SynInterpolatedStringPart list) =
     let g = cenv.g
 
@@ -7615,6 +7624,9 @@ and TcInterpolatedStringExpr cenv (overallTy: OverallTy) env m tpenv (parts: Syn
             // Type check the expressions filling the holes
             let fillExprs, tpenv = TcExprsNoFlexes cenv env m tpenv argTys synFillExprs
 
+            if g.langVersion.SupportsFeature LanguageFeature.WarnWhenFunctionValueUsedAsInterpolatedStringArg then
+                warnForFunctionValuesInFillExprs g argTys synFillExprs
+
             // Take all interpolated string parts and typed fill expressions
             // and convert them to typed expressions that can be used as args to System.String.Concat
             // return an empty list if there are some format specifiers that make lowering to not applicable
@@ -7684,6 +7696,9 @@ and TcInterpolatedStringExpr cenv (overallTy: OverallTy) env m tpenv (parts: Syn
         // Type check the expressions filling the holes
         let fillExprs, tpenv = TcExprsNoFlexes cenv env m tpenv argTys synFillExprs
 
+        if g.langVersion.SupportsFeature LanguageFeature.WarnWhenFunctionValueUsedAsInterpolatedStringArg then
+            warnForFunctionValuesInFillExprs g argTys synFillExprs
+
         let fillExprsBoxed = (argTys, fillExprs) ||> List.map2 (mkCallBox g m)
 
         let dotnetFormatStringExpr = mkString g m dotnetFormatString

src/Compiler/FSComp.txt

@@ -1803,5 +1803,7 @@ featureAllowLetOrUseBangTypeAnnotationWithoutParens,"Allow let! and use! type an
 3878,tcAttributeIsNotValidForUnionCaseWithFields,"This attribute is not valid for use on union cases with fields."
 3879,xmlDocNotFirstOnLine,"XML documentation comments should be the first non-whitespace text on a line."
 featureReturnFromFinal,"Support for ReturnFromFinal/YieldFromFinal in computation expressions to enable tailcall optimization when available on the builder."
+featureWarnWhenFunctionValueUsedAsInterpolatedStringArg,"Warn when a function value is used as an interpolated string argument"
 3880,optsLangVersionOutOfSupport,"Language version '%s' is out of support. The last .NET SDK supporting it is available at https://dotnet.microsoft.com/en-us/download/dotnet/%s"
-3881,optsUnrecognizedLanguageFeature,"Unrecognized language feature name: '%s'. Use a valid feature name such as 'NameOf' or 'StringInterpolation'."
+3881,optsUnrecognizedLanguageFeature,"Unrecognized language feature name: '%s'. Use a valid feature name such as 'NameOf' or 'StringInterpolation'."
+3882,tcFunctionValueUsedAsInterpolatedStringArg,"This expression is a function value. When used in an interpolated string it will be formatted using its 'ToString' method, which is likely not the intended behavior. Consider applying the function to its arguments."

src/Compiler/Facilities/LanguageFeatures.fs

@@ -104,6 +104,7 @@ type LanguageFeature =
     | ErrorOnInvalidDeclsInTypeDefinitions
     | AllowTypedLetUseAndBang
     | ReturnFromFinal
+    | WarnWhenFunctionValueUsedAsInterpolatedStringArg
 
 /// LanguageVersion management
 type LanguageVersion(versionText, ?disabledFeaturesArray: LanguageFeature array) =
@@ -245,6 +246,7 @@ type LanguageVersion(versionText, ?disabledFeaturesArray: LanguageFeature array)
 
                 // F# 11.0
                 // Put stabilized features here for F# 11.0 previews via .NET SDK preview channels
+                LanguageFeature.WarnWhenFunctionValueUsedAsInterpolatedStringArg, languageVersion110
 
                 // Difference between languageVersion110 and preview - 11.0 gets turned on automatically by picking a preview .NET 11 SDK
                 // previewVersion is only when "preview" is specified explicitly in project files  and users also need a preview SDK
@@ -440,6 +442,8 @@ type LanguageVersion(versionText, ?disabledFeaturesArray: LanguageFeature array)
         | LanguageFeature.ErrorOnInvalidDeclsInTypeDefinitions -> FSComp.SR.featureErrorOnInvalidDeclsInTypeDefinitions ()
         | LanguageFeature.AllowTypedLetUseAndBang -> FSComp.SR.featureAllowLetOrUseBangTypeAnnotationWithoutParens ()
         | LanguageFeature.ReturnFromFinal -> FSComp.SR.featureReturnFromFinal ()
+        | LanguageFeature.WarnWhenFunctionValueUsedAsInterpolatedStringArg ->
+            FSComp.SR.featureWarnWhenFunctionValueUsedAsInterpolatedStringArg ()
 
     /// Get a version string associated with the given feature.
     static member GetFeatureVersionString feature =

src/Compiler/Facilities/LanguageFeatures.fsi

@@ -95,6 +95,7 @@ type LanguageFeature =
     | ErrorOnInvalidDeclsInTypeDefinitions
     | AllowTypedLetUseAndBang
     | ReturnFromFinal
+    | WarnWhenFunctionValueUsedAsInterpolatedStringArg
 
 /// LanguageVersion management
 type LanguageVersion =

src/Compiler/Service/FSharpProjectSnapshot.fs

@@ -101,7 +101,7 @@ type FSharpFileSnapshot(FileName: string, Version: string, GetSource: unit -> Ta
                     task {
                         match! f fileName |> Async.StartAsTask with
                         | Some source -> return SourceTextNew.ofISourceText source
-                        | None -> return failwith $"Couldn't get source for file {f}"
+                        | None -> return failwith $"Couldn't get source for file {fileName}"
                     }
             )
 

src/Compiler/Utilities/DependencyGraph.fs

@@ -329,22 +329,22 @@ type GraphExtensions =
     static member Unpack(node: 'NodeValue, unpacker) =
         match unpacker node with
         | Some value -> value
-        | None -> failwith $"Expected {unpacker} but got: {node}"
+        | None -> failwith $"Expected unpacker to match but got: {node}"
 
     [<Extension>]
     static member UnpackOne(dependencies: 'NodeValue seq, unpacker: 'NodeValue -> 'UnpackedDependency option) =
         dependencies
         |> Seq.tryExactlyOne
         |> Option.bind unpacker
         |> Option.defaultWith (fun () ->
-            failwith $"Expected exactly one dependency matching {unpacker} but got: %A{dependencies |> Seq.toArray}")
+            failwith $"Expected exactly one dependency matching unpacker but got: %A{dependencies |> Seq.toArray}")
 
     [<Extension>]
     static member UnpackMany(dependencies: 'NodeValue seq, unpacker) =
         let results = dependencies |> Seq.choose unpacker
 
         if dependencies |> Seq.length <> (results |> Seq.length) then
-            failwith $"Expected all dependencies to match {unpacker} but got: %A{dependencies |> Seq.toArray}"
+            failwith $"Expected all dependencies to match unpacker but got: %A{dependencies |> Seq.toArray}"
 
         results
 
@@ -361,7 +361,7 @@ type GraphExtensions =
             | None, None -> extras.Add dependency |> ignore
 
         match oneResult with
-        | None -> failwith $"Expected exactly one dependency matching {oneUnpacker} but didn't find any"
+        | None -> failwith $"Expected exactly one dependency matching oneUnpacker but didn't find any"
         | Some head ->
             if extras.Count > 0 then
                 failwith $"Found extra dependencies: %A{extras.ToArray()}"