Warn when function type passed to interpolated string

src/Compiler/Checking/Expressions/CheckExpressions.fs

@@ -7592,6 +7592,13 @@ and TcInterpolatedStringExpr cenv (overallTy: OverallTy) env m tpenv (parts: Syn
     if argTys.Length <> synFillExprs.Length then
         error (Error(FSComp.SR.tcInterpolationMixedWithPercent(), m))
 
+    let warnForFunctionValuesInFillExprs () =
+        if g.langVersion.SupportsFeature LanguageFeature.WarnWhenFunctionValueUsedAsInterpolatedStringArg then
+            (argTys, synFillExprs)
+            ||> List.iter2 (fun argTy synFillExpr ->
+                if isFunTy g argTy then
+                    warning (Error(FSComp.SR.tcFunctionValueUsedAsInterpolatedStringArg (), synFillExpr.Range)))
+
     match stringKind with
 
     // The case for $"..." used as type string and $"...%d{x}..." used as type PrintfFormat - create a PrintfFormat that captures
@@ -7615,6 +7622,8 @@ 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
 
+            warnForFunctionValuesInFillExprs ()
+
             // 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 +7693,8 @@ 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
 
+        warnForFunctionValuesInFillExprs ()
+
         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 =