Warn when 'unit' is passed to an 'obj' argument

docs/release-notes/.FSharp.Compiler.Service/9.0.300.md

@@ -26,6 +26,7 @@
 * Update `Obsolete` attribute checking to account for `DiagnosticId` and `UrlFormat` properties. ([PR #18224](https://github.com/dotnet/fsharp/pull/18224))
 * Remove `Cancellable.UsingToken` from tests ([PR #18276](https://github.com/dotnet/fsharp/pull/18276))
 * Added nullability annotations to `.Using` builder method for `async`, `task` and compiler-internal builders ([PR #18292](https://github.com/dotnet/fsharp/pull/18292))
+* Warn when `unit` is passed to an `obj`-typed argument  ([PR #18330](https://github.com/dotnet/fsharp/pull/18330))
 * Warning for "useless null handling" works with piped syntax constructs now ([PR #18331](https://github.com/dotnet/fsharp/pull/18331))
 
 ### Breaking Changes

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

@@ -4,6 +4,7 @@
 * Deprecate places where `seq` can be omitted. ([Language suggestion #1033](https://github.com/fsharp/fslang-suggestions/issues/1033), [PR #17772](https://github.com/dotnet/fsharp/pull/17772))
 * Added type conversions cache, only enabled for compiler runs ([PR#17668](https://github.com/dotnet/fsharp/pull/17668))
 * Support ValueOption + Struct attribute as optional parameter for methods ([Language suggestion #1136](https://github.com/fsharp/fslang-suggestions/issues/1136), [PR #18098](https://github.com/dotnet/fsharp/pull/18098))
+* Warn when `unit` is passed to an `obj`-typed argument  ([PR #18330](https://github.com/dotnet/fsharp/pull/18330))
 
 ### Fixed
 

src/Compiler/Checking/ConstraintSolver.fs

@@ -3177,6 +3177,7 @@ and ArgsMustSubsumeOrConvert
     trackErrors {
         let g = csenv.g
         let m = callerArg.Range
+        let callerTy = callerArg.CallerArgumentType
         let calledArgTy, usesTDC, eqn = AdjustCalledArgType csenv.InfoReader ad isConstraint enforceNullableOptionalsKnownTypes calledArg callerArg
 
         match eqn with 
@@ -3188,8 +3189,10 @@ and ArgsMustSubsumeOrConvert
         match usesTDC with 
         | TypeDirectedConversionUsed.Yes(warn, _, _) -> do! WarnD(warn csenv.DisplayEnv)
         | TypeDirectedConversionUsed.No -> ()
-        do! SolveTypeSubsumesTypeWithReport csenv ndeep m trace cxsln (Some calledArg.CalledArgumentType) calledArgTy callerArg.CallerArgumentType
-        if calledArg.IsParamArray && isArray1DTy g calledArgTy && not (isArray1DTy g callerArg.CallerArgumentType) then 
+        do! SolveTypeSubsumesTypeWithReport csenv ndeep m trace cxsln (Some calledArg.CalledArgumentType) calledArgTy callerTy
+        if g.langVersion.SupportsFeature(LanguageFeature.WarnWhenUnitPassedToObjArg) && isUnitTy g callerTy && isObjTyAnyNullness g calledArgTy then
+            do! WarnD(Error(FSComp.SR.tcUnitToObjSubsumption(), m))
+        if calledArg.IsParamArray && isArray1DTy g calledArgTy && not (isArray1DTy g callerTy) then 
             return! ErrorD(Error(FSComp.SR.csMethodExpectsParams(), m))
         else 
             return usesTDC

src/Compiler/FSComp.txt

@@ -1683,6 +1683,7 @@ forFormatInvalidForInterpolated4,"Interpolated strings used as type IFormattable
 3394,parsNewExprMemberAccess,"This member access is ambiguous. Please use parentheses around the object creation, e.g. '(new SomeType(args)).MemberName'"
 3395,tcImplicitConversionUsedForMethodArg,"This expression uses the implicit conversion '%s' to convert type '%s' to type '%s'."
 3396,tcLiteralAttributeCannotUseActivePattern,"A [<Literal>] declaration cannot use an active pattern for its identifier"
+3397,tcUnitToObjSubsumption,"This expression uses 'unit' for an 'obj'-typed argument. This will lead to passing 'null' at runtime. This warning may be disabled using '#nowarn \"3397\"."
 3401,ilxgenInvalidConstructInStateMachineDuringCodegen,"The resumable code construct '%s' may only be used in inlined code protected by 'if __useResumableCode then ...' and the overall composition must form valid resumable code."
 3402,tcInvalidResumableConstruct,"The construct '%s' may only be used in valid resumable code."
 3501,tcResumableCodeFunctionMustBeInline,"Invalid resumable code. Any method of function accepting or returning resumable code must be marked 'inline'"
@@ -1794,3 +1795,4 @@ featureDontWarnOnUppercaseIdentifiersInBindingPatterns,"Don't warn on uppercase
 3874,tcExpectedTypeParamMarkedWithUnitOfMeasureAttribute,"Expected unit-of-measure type parameter must be marked with the [<Measure>] attribute."
 featureDeprecatePlacesWhereSeqCanBeOmitted,"Deprecate places where 'seq' can be omitted"
 featureSupportValueOptionsAsOptionalParameters,"Support ValueOption as valid type for optional member parameters"
+featureSupportWarnWhenUnitPassedToObjArg,"Warn when unit is passed to a member accepting `obj` argument, e.g. `Method(o:obj)` will warn if called via `Method()`."

src/Compiler/Facilities/LanguageFeatures.fs

@@ -98,6 +98,7 @@ type LanguageFeature =
     | UseTypeSubsumptionCache
     | DeprecatePlacesWhereSeqCanBeOmitted
     | SupportValueOptionsAsOptionalParameters
+    | WarnWhenUnitPassedToObjArg
 
 /// LanguageVersion management
 type LanguageVersion(versionText) =
@@ -227,6 +228,7 @@ type LanguageVersion(versionText) =
                 LanguageFeature.DontWarnOnUppercaseIdentifiersInBindingPatterns, previewVersion
                 LanguageFeature.DeprecatePlacesWhereSeqCanBeOmitted, previewVersion
                 LanguageFeature.SupportValueOptionsAsOptionalParameters, previewVersion
+                LanguageFeature.WarnWhenUnitPassedToObjArg, previewVersion
             ]
 
     static let defaultLanguageVersion = LanguageVersion("default")
@@ -388,6 +390,7 @@ type LanguageVersion(versionText) =
         | LanguageFeature.UseTypeSubsumptionCache -> FSComp.SR.featureUseTypeSubsumptionCache ()
         | LanguageFeature.DeprecatePlacesWhereSeqCanBeOmitted -> FSComp.SR.featureDeprecatePlacesWhereSeqCanBeOmitted ()
         | LanguageFeature.SupportValueOptionsAsOptionalParameters -> FSComp.SR.featureSupportValueOptionsAsOptionalParameters ()
+        | LanguageFeature.WarnWhenUnitPassedToObjArg -> FSComp.SR.featureSupportWarnWhenUnitPassedToObjArg ()
 
     /// Get a version string associated with the given feature.
     static member GetFeatureVersionString feature =

src/Compiler/Facilities/LanguageFeatures.fsi

@@ -89,6 +89,7 @@ type LanguageFeature =
     | UseTypeSubsumptionCache
     | DeprecatePlacesWhereSeqCanBeOmitted
     | SupportValueOptionsAsOptionalParameters
+    | WarnWhenUnitPassedToObjArg
 
 /// LanguageVersion management
 type LanguageVersion =