Fix NRE when calling virtual Object methods on value types via inline SRTP (#8098)

When an inline SRTP function calls a virtual Object method (ToString,
GetHashCode, Equals) on a value type, the compiler emits a constrained
callvirt instruction. ECMA-335 requires the receiver of a constrained
call to be a managed pointer (&), but the codegen was pushing the raw
value (or reference) directly onto the stack.

Root cause: MakeMethInfoCall (used by SRTP trait resolution) computes
PossibleConstrainedCall but never takes the receiver's address — unlike
TakeObjAddrForMethodCall which handles this correctly in the
type-checker path for direct calls.

Fix: In MakeMethInfoCall, when ComputeConstrainedCallInfo returns Some
(indicating a constrained call is needed), take the address of the
receiver via mkExprAddrOfExpr before building the IL call expression.
This mirrors TakeObjAddrForMethodCall and ensures the expression tree
contains the proper address-of wrapper, so GenILCall receives a managed
pointer on the stack as ECMA-335 requires.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: T-Gro

docs/release-notes/.FSharp.Compiler.Service/11.0.100.md

@@ -1,5 +1,6 @@
 ### Fixed
 
+* Fix NullReferenceException when calling virtual Object methods (ToString, GetHashCode, Equals) on value types through inline SRTP functions. The codegen now ensures the receiver is passed as a managed pointer for constrained callvirt instructions, as required by ECMA-335. ([Issue #8098](https://github.com/dotnet/fsharp/issues/8098), [PR #NNNN](https://github.com/dotnet/fsharp/pull/NNNN))
 * Fix CLIEvent properties to be correctly recognized as events: `IsEvent` returns `true` and `XmlDocSig` uses `E:` prefix instead of `P:`. ([Issue #10273](https://github.com/dotnet/fsharp/issues/10273), [PR #18584](https://github.com/dotnet/fsharp/pull/18584))
 * Fix extra sequence point at the end of match expressions. ([Issue #12052](https://github.com/dotnet/fsharp/issues/12052), [PR #19278](https://github.com/dotnet/fsharp/pull/19278))
 * Fix wrong sequence point range for `return`/`yield`/`return!`/`yield!` inside computation expressions. ([Issue #19248](https://github.com/dotnet/fsharp/issues/19248), [PR #19278](https://github.com/dotnet/fsharp/pull/19278))

src/Compiler/Checking/MethodCalls.fs

@@ -2242,9 +2242,13 @@ let GenWitnessExpr amap g m (traitInfo: TraitConstraintInfo) argExprs =
             | Some r -> r :: convertedArgs
             | None -> convertedArgs
 
-        // Fix bug 1281: If we resolve to an instance method on a struct and we haven't yet taken 
-        // the address of the object then go do that 
-        if minfo.IsStruct && minfo.IsInstance then 
+        // Fix bug 1281 / issue #8098: If the receiver needs its address taken for a
+        // constrained call, go do that and re-resolve via TraitCall with the byref receiver.
+        let needsAddrTaken =
+            minfo.IsInstance &&
+            (minfo.IsStruct || (ComputeConstrainedCallInfo g amap m staticTyOpt argExprs minfo).IsSome)
+
+        if needsAddrTaken then 
             match argExprs with
             | h :: t when not (isByrefTy g (tyOfExpr g h)) ->
                 let wrap, h', _readonly, _writeonly = mkExprAddrOfExpr g true false PossiblyMutates h None m 

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

@@ -1831,3 +1831,103 @@ let _x3 = curryN f3 1 2 3
         FSharp "module T\nopen CsLib\nlet _ = IP.Get()" |> asExe |> withOptions iwsamWarnings |> withReferences [csLib]
         |> compileAndRun |> shouldSucceed
 
+    // https://github.com/dotnet/fsharp/issues/8098
+    [<Fact>]
+    let ``Issue 8098 - ToString on int via inline SRTP does not throw NRE`` () =
+        FSharp """
+module Test
+
+let inline toString (x: ^a) = (^a : (member ToString : unit -> string) x)
+
+[<EntryPoint>]
+let main _ =
+    let s = toString 123
+    if s <> "123" then failwith (sprintf "Expected '123' but got '%s'" s)
+    0
+        """
+        |> asExe
+        |> compileExeAndRun
+        |> shouldSucceed
+
+    // https://github.com/dotnet/fsharp/issues/8098
+    [<Fact>]
+    let ``Issue 8098 - GetHashCode on int via inline SRTP does not throw NRE`` () =
+        FSharp """
+module Test
+
+let inline getHash (x: ^a) = (^a : (member GetHashCode : unit -> int) x)
+
+[<EntryPoint>]
+let main _ =
+    let h = getHash 42
+    if h <> 42 then failwith (sprintf "Expected 42 but got %d" h)
+    0
+        """
+        |> asExe
+        |> compileExeAndRun
+        |> shouldSucceed
+
+    // https://github.com/dotnet/fsharp/issues/8098
+    [<Fact>]
+    let ``Issue 8098 - ToString on custom struct via inline SRTP does not throw NRE`` () =
+        FSharp """
+module Test
+
+[<Struct>]
+type MyPoint = { X: int; Y: int }
+
+let inline toString (x: ^a) = (^a : (member ToString : unit -> string) x)
+
+[<EntryPoint>]
+let main _ =
+    let p = { X = 1; Y = 2 }
+    let s = toString p
+    if s = null then failwith "Got null"
+    0
+        """
+        |> asExe
+        |> compileExeAndRun
+        |> shouldSucceed
+
+    // https://github.com/dotnet/fsharp/issues/8098
+    [<Fact>]
+    let ``Issue 8098 - ToString on reference type via inline SRTP still works`` () =
+        FSharp """
+module Test
+
+let inline toString (x: ^a) = (^a : (member ToString : unit -> string) x)
+
+[<EntryPoint>]
+let main _ =
+    let s = toString "hello"
+    if s <> "hello" then failwith (sprintf "Expected 'hello' but got '%s'" s)
+    0
+        """
+        |> asExe
+        |> compileExeAndRun
+        |> shouldSucceed
+
+    // https://github.com/dotnet/fsharp/issues/8098
+    [<Fact>]
+    let ``Issue 8098 - ToString on struct without override via inline SRTP does not throw NRE`` () =
+        FSharp """
+module Test
+
+[<Struct>]
+type EmptyStruct =
+    val X: int
+    new(x) = { X = x }
+    // No ToString override — inherits Object.ToString()
+
+let inline toString (x: ^a) = (^a : (member ToString : unit -> string) x)
+
+[<EntryPoint>]
+let main _ =
+    let s = toString (EmptyStruct(42))
+    if s = null then failwith "Got null"
+    0
+        """
+        |> asExe
+        |> compileExeAndRun
+        |> shouldSucceed
+