alternative fix for 9565 (#9629)

* fix 9565

* add comment

Author: dsyme

src/fsharp/MethodCalls.fs

@@ -1173,6 +1173,15 @@ let AdjustCallerArgForOptional tcFieldInit eCallerMemberName (infoReader: InfoRe
         if isOptCallerArg then errorR(Error(FSComp.SR.tcFormalArgumentIsNotOptional(), m))
         assignedArg
 
+    // For non-nullable, non-optional arguments no conversion is needed.
+    // We return precisely the assignedArg.  This also covers the case where there
+    // can be a lingering permitted type mismatch between caller argument and called argument, 
+    // specifically caller can by `byref` and called `outref`.  No coercion is inserted in the
+    // expression tree in this case. 
+    | NotOptional when not (isNullableTy g calledArgTy) -> 
+        if isOptCallerArg then errorR(Error(FSComp.SR.tcFormalArgumentIsNotOptional(), m))
+        assignedArg
+
     | _ ->
 
         let callerArgExpr2 = 
@@ -1183,7 +1192,7 @@ let AdjustCallerArgForOptional tcFieldInit eCallerMemberName (infoReader: InfoRe
                 if isNullableTy g calledArgTy then 
                     MakeNullableExprIfNeeded infoReader calledArgTy callerArgTy callerArgExpr m
                 else
-                    callerArgExpr
+                    failwith "unreachable" // see case above
 
             | CallerSide dfltVal -> 
                 let calledArgTy = calledArg.CalledArgumentType