Indexer property should not allowed to have mismatched value type

src/Compiler/Checking/PostInferenceChecks.fs

@@ -2414,11 +2414,14 @@ let CheckEntityDefn cenv env (tycon: Entity) =
 
             // Check to see if the signatures of the both getter and the setter imply the same property type
 
-            if pinfo.HasGetter && pinfo.HasSetter && not pinfo.IsIndexer then
+            if pinfo.HasGetter && pinfo.HasSetter then
                 let ty1 = pinfo.DropSetter().GetPropertyType(cenv.amap, m)
                 let ty2 = pinfo.DropGetter().GetPropertyType(cenv.amap, m)
                 if not (typeEquivAux EraseNone cenv.amap.g ty1 ty2) then
-                    errorR(Error(FSComp.SR.chkGetterAndSetterHaveSamePropertyType(pinfo.PropertyName, NicePrint.minimalStringOfType cenv.denv ty1, NicePrint.minimalStringOfType cenv.denv ty2), m))
+                    if g.langVersion.SupportsFeature(LanguageFeature.WarningIndexedPropertiesGetSetSameType) && pinfo.IsIndexer then
+                        warning(Error(FSComp.SR.chkIndexedGetterAndSetterHaveSamePropertyType(pinfo.PropertyName, NicePrint.minimalStringOfType cenv.denv ty1, NicePrint.minimalStringOfType cenv.denv ty2), m))
+                    if not pinfo.IsIndexer then
+                        errorR(Error(FSComp.SR.chkGetterAndSetterHaveSamePropertyType(pinfo.PropertyName, NicePrint.minimalStringOfType cenv.denv ty1, NicePrint.minimalStringOfType cenv.denv ty2), m))
 
             hashOfImmediateProps[nm] <- pinfo :: others
 

src/Compiler/FSComp.txt

@@ -1588,6 +1588,7 @@ featureChkNotTailRecursive,"Raises warnings if a member or function has the 'Tai
 featureWhileBang,"'while!' expression"
 featureExtendedFixedBindings,"extended fixed bindings for byref and GetPinnableReference"
 featurePreferStringGetPinnableReference,"prefer String.GetPinnableReference in fixed bindings"
+featureWarningIndexedPropertiesGetSetSameType,"Indexed properties getter and setter must have the same type"
 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."
 3355,tcNotAnIndexerNamedIndexingNotYetEnabled,"The value '%s' is not a function and does not support index notation."
@@ -1724,6 +1725,7 @@ featureUnmanagedConstraintCsharpInterop,"Interop between C#'s and F#'s unmanaged
 3578,chkCopyUpdateSyntaxInAnonRecords,"This expression is an anonymous record, use {{|...|}} instead of {{...}}."
 3579,alwaysUseTypedStringInterpolation,"Interpolated string contains untyped identifiers. Adding typed format specifiers is recommended."
 3580,tcUnexpectedFunTypeInUnionCaseField,"Unexpected function type in union case field definition. If you intend the field to be a function, consider wrapping the function signature with parens, e.g. | Case of a -> b into | Case of (a -> b)."
+3581,chkIndexedGetterAndSetterHaveSamePropertyType,"An indexed property's getter and setter must have the same type. Property '%s' has getter of type '%s' but setter of type '%s'."
 3582,tcInfoIfFunctionShadowsUnionCase,"This is a function definition that shadows a union case. If this is what you want, ignore or suppress this warning. If you want it to be a union case deconstruction, add parentheses."
 3583,unnecessaryParentheses,"Parentheses can be removed."
 3584,tcDotLambdaAtNotSupportedExpression,"Shorthand lambda syntax is only supported for atomic expressions, such as method, property, field or indexer on the implied '_' argument. For example: 'let f = _.Length'."

src/Compiler/Facilities/LanguageFeatures.fs

@@ -79,6 +79,7 @@ type LanguageFeature =
     | WhileBang
     | ExtendedFixedBindings
     | PreferStringGetPinnableReference
+    | WarningIndexedPropertiesGetSetSameType
 
 /// LanguageVersion management
 type LanguageVersion(versionText) =
@@ -185,6 +186,7 @@ type LanguageVersion(versionText) =
                 // F# preview
                 LanguageFeature.FromEndSlicing, previewVersion
                 LanguageFeature.UnmanagedConstraintCsharpInterop, previewVersion
+                LanguageFeature.WarningIndexedPropertiesGetSetSameType, previewVersion
             ]
 
     static let defaultLanguageVersion = LanguageVersion("default")
@@ -321,6 +323,7 @@ type LanguageVersion(versionText) =
         | LanguageFeature.WhileBang -> FSComp.SR.featureWhileBang ()
         | LanguageFeature.ExtendedFixedBindings -> FSComp.SR.featureExtendedFixedBindings ()
         | LanguageFeature.PreferStringGetPinnableReference -> FSComp.SR.featurePreferStringGetPinnableReference ()
+        | LanguageFeature.WarningIndexedPropertiesGetSetSameType -> FSComp.SR.featureWarningIndexedPropertiesGetSetSameType ()
 
     /// Get a version string associated with the given feature.
     static member GetFeatureVersionString feature =

src/Compiler/Facilities/LanguageFeatures.fsi

@@ -69,6 +69,7 @@ type LanguageFeature =
     | WhileBang
     | ExtendedFixedBindings
     | PreferStringGetPinnableReference
+    | WarningIndexedPropertiesGetSetSameType
 
 /// LanguageVersion management
 type LanguageVersion =