add FS0067 compiler message (#48442)

Author: StuartMosquera

docs/fsharp/language-reference/compiler-messages/fs0067.md

@@ -0,0 +1,32 @@
+---
+description: "Learn more about: FS0067: This type test or downcast will always hold"
+title: "Compiler error FS0067"
+ms.date: 9/10/2025
+f1_keywords:
+  - "FS0067"
+helpviewer_keywords:
+  - "FS0067"
+---
+
+# FS0067: This type test or downcast will always hold
+
+This message is given when attempting to perform a type test ([:?](../match-expressions.md)) or downcast ([:?>](../casting-and-conversions.md#downcasting)) that will always succeed based on the types involved, making the operation unnecessary.
+
+Redundant Type test:
+
+[!code-fsharp[FS0067-redundant-type-test](~/samples/snippets/fsharp/compiler-messages/fs0067.fsx#L2-L8)]
+
+Redundant Downcast:
+
+[!code-fsharp[FS0067-redundant-downcast](~/samples/snippets/fsharp/compiler-messages/fs0067.fsx#L11-L18)]
+
+The two examples above cause the compiler to display the following message:
+
+```text
+FS0067: This type test or downcast will always hold
+```
+
+The use of `:?` and `:?>` operators is preferable when working with:
+
+- Base classes when the runtime type might differ.
+- Values of type `obj` or interfaces types.

docs/fsharp/language-reference/compiler-messages/toc.yml

@@ -19,5 +19,7 @@
     href: ./fs0037.md
   - name: FS0052 - Defensive copy
     href: ./fs0052.md
+  - name: FS0067 - This type test or downcast will always hold
+    href: ./fs0067.md
   - name: FS0703 - Expected type parameter, not unit-of-measure parameter
     href: ./fs0703.md

samples/snippets/fsharp/compiler-messages/fs0067.fsx

@@ -0,0 +1,18 @@
+(* Redundant Type test *)
+type Dog() =
+    member this.Bark() = printfn "Woof!"
+
+let dog = Dog()
+
+if dog :? Dog then
+    dog.Bark()
+
+(* Redundant Downcast *)
+type Cat(name: string) =
+    member this.Name = name
+
+let cat = Cat("Kitten")
+
+let sameCat = cat :?> Cat
+
+printfn "It's still a %s" sameCat.Name