Add nullness regression test for recursive inline SRTP memoization

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

@@ -2037,3 +2037,42 @@ if r <> 1 then failwith $"Expected 1 but got {r}"
         |> asExe
         |> compileAndRun
         |> shouldSucceed
+
+    // The domain-order fix keeps the SRTP typar as the unification representative, so a nullable
+    // argument annotation survives the recursive specialization and nullness inference stays intact
+    // under --checknulls+ (no lost or spurious warning, no internal error).
+    [<Fact>]
+    let ``Recursive inline SRTP memoization preserves nullable argument inference under checknulls`` () =
+        FSharp """
+module Test
+open System.Collections.Concurrent
+
+type Default1 = class end
+
+[<Struct>]
+type MemoizationKeyWrapper<'a> = MemoizationKeyWrapper of 'a
+
+type MemoizeN =
+    inherit Default1
+    static member getOrAdd (cd: ConcurrentDictionary<MemoizationKeyWrapper<'a>,'b>) (f: 'a -> 'b) k =
+        cd.GetOrAdd (MemoizationKeyWrapper k, (fun (MemoizationKeyWrapper x) -> x) >> f)
+
+let inline memoizeN (f: ^F) : ^F =
+    let inline call_2 (a: ^MemoizeN, b: ^b) = ((^MemoizeN or ^b) : (static member MemoizeN : ^MemoizeN * 'b -> _ ) (a, b))
+    call_2 (Unchecked.defaultof<MemoizeN>, Unchecked.defaultof< ^F >) f
+
+type MemoizeN with
+    static member        MemoizeN (_: Default1, _:      'a -> 'b) = MemoizeN.getOrAdd (ConcurrentDictionary ())
+    static member inline MemoizeN (_: MemoizeN, _:'t -> 'a -> 'b) = MemoizeN.getOrAdd (ConcurrentDictionary ()) << (<<) memoizeN
+
+let lookup (prefix: int) (index: int) (s: string | null) = s.Length + prefix + index
+let mlookup = memoizeN lookup
+mlookup 1 2 "abc" |> ignore
+"""
+        |> withLangVersion "preview"
+        |> withCheckNulls
+        |> withWarnOn 3261
+        |> compile
+        |> withDiagnostics [
+            Warning 3261, Line 23, Col 60, Line 23, Col 61, "Nullness warning: Possible dereference of a null value when accessing member 'Length' on the nullable value 's' of type 'string | null'."
+        ]