Disallow calling abstract methods directly on interfaces

docs/release-notes/.FSharp.Compiler.Service/8.0.300.md

@@ -1,5 +1,6 @@
 ### Fixed
 
+* Disallow calling abstract methods directly on interfaces. ([PR #16319](https://github.com/dotnet/fsharp/pull/16319))
 * Fix a false positive of the `[<TailCall>]` analysis in combination with `yield!`. ([PR #16933](https://github.com/dotnet/fsharp/pull/16933))
 * Improve error reporting: ambiguous override method in object expression. ([PR #16985](https://github.com/dotnet/fsharp/pull/16985))
 * Don't blow the stack when traversing deeply nested sequential expressions. ([PR #16882](https://github.com/dotnet/fsharp/pull/16882))

src/Compiler/Checking/CheckExpressions.fs

@@ -9955,9 +9955,9 @@ and TcMethodApplication
     let finalCalledMeth =
 
         let callerArgs = { Unnamed = unnamedCurriedCallerArgs ; Named = namedCurriedCallerArgs }
-
         let postArgumentTypeCheckingCalledMethGroup =
-            preArgumentTypeCheckingCalledMethGroup |> List.map (fun (minfo, minst, pinfoOpt, usesParamArrayConversion) ->
+            preArgumentTypeCheckingCalledMethGroup
+            |> List.map (fun (minfo, minst, pinfoOpt, usesParamArrayConversion) ->
                 let callerTyArgs =
                     match tyArgsOpt with
                     | Some tyargs -> minfo.AdjustUserTypeInstForFSharpStyleIndexedExtensionMembers tyargs

src/Compiler/Checking/ConstraintSolver.fs

@@ -2944,8 +2944,17 @@ and ResolveOverloading
     let calledMethOpt, errors, calledMethTrace = 
 
         match calledMethGroup, candidates with 
-        | _, [calledMeth] when not isOpConversion -> 
-            Some calledMeth, CompleteD, NoTrace
+        | _, [calledMeth] when not isOpConversion ->
+            // See what candidates we have based on name and arity, and check for static/virtual/abstract
+            // OK: static virtual TResult operator checked i.e. IAdditionOperators.op_CheckedAddition
+            // Error: static abstract TResult operator i.e. IAdditionOperators.op_Addition
+            match calledMeth.Method with
+            | ILMeth(ilMethInfo= ilMethInfo) when ilMethInfo.IsStatic && ilMethInfo.IsVirtual && methodName.Contains("op_") ->
+                if ilMethInfo.IsAbstract then
+                    None, ErrorD (Error (FSComp.SR.csMethodNotFound(methodName), m)), NoTrace
+                else
+                    Some calledMeth, CompleteD, NoTrace
+            |  _ ->  Some calledMeth, CompleteD, NoTrace
 
         | [], _ when not isOpConversion -> 
             None, ErrorD (Error (FSComp.SR.csMethodNotFound(methodName), m)), NoTrace

tests/FSharp.Compiler.ComponentTests/Conformance/Types/TypeConstraints/IWSAMsAndSRTPs/IWSAMsAndSRTPsTests.fs

@@ -1183,4 +1183,33 @@ let execute = IPrintable.Say("hello")
          |> withOptions [ "--nowarn:3536" ; "--nowarn:3535" ]
          |> withLangVersion80
          |> typecheck
-         |> shouldSucceed
+         |> shouldSucceed
+
+    [<FactForNETCOREAPP>]
+    let ``Accessing to IWSAM(System.Numerics non virtual) produces a compilation error`` () =
+        Fsx """
+open System.Numerics
+
+IAdditionOperators.op_Addition (3, 6)
+        """
+         |> withOptions [ "--nowarn:3536" ; "--nowarn:3535" ]
+         |> withLangVersion80
+         |> compile
+         |> shouldFail
+         |> withSingleDiagnostic (Error 509, Line 4, Col 1, Line 4, Col 38, "Method or object constructor 'op_Addition' not found")
+
+    [<FactForNETCOREAPP>]
+    let ``Accessing to IWSAM(System.Numerics virtual member) compiles and runs`` () =
+        Fsx """
+open System.Numerics
+
+let res = IAdditionOperators.op_CheckedAddition (3, 6)
+
+printf "%A" res"""
+        |> withOptions [ "--nowarn:3536" ; "--nowarn:3535" ]
+        |> withLangVersion80
+        |> asExe
+        |> compile
+        |> shouldSucceed
+        |> run
+        |> verifyOutput "9"