Fix #18841: let _ = &expr now compiles like let x = &expr

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

@@ -1,5 +1,6 @@
 ### Fixed
 
+* Fix FS0421 "The address of the variable cannot be used at this point" incorrectly raised for the discard pattern `let _ = &expr` when `let x = &expr` compiles. ([Issue #18841](https://github.com/dotnet/fsharp/issues/18841), [PR #19811](https://github.com/dotnet/fsharp/pull/19811))
 * Honor `--nowarn` and `--warnaserror` for warnings emitted during command-line option parsing ([Issue #19576](https://github.com/dotnet/fsharp/issues/19576), [PR #19776](https://github.com/dotnet/fsharp/pull/19776))
 * Fix `[<return: X>]` prefix attributes being silently dropped on class members, and fix false-positive `AllowMultiple=false` errors when `[<X>]` and `[<return: X>]` are applied to the same binding. ([Issue #17904](https://github.com/dotnet/fsharp/issues/17904), [Issue #19020](https://github.com/dotnet/fsharp/issues/19020), [PR #19738](https://github.com/dotnet/fsharp/pull/19738))
 * Fix attributes on return type of unparenthesized tuple methods being silently dropped from IL. ([Issue #462](https://github.com/dotnet/fsharp/issues/462), [PR #19714](https://github.com/dotnet/fsharp/pull/19714))

src/Compiler/Checking/Expressions/CheckExpressions.fs

@@ -11863,9 +11863,14 @@ and TcLetBinding (cenv: cenv) isUse env containerInfo declKind tpenv (synBinds,
         let rhsExpr = mkTypeLambda m generalizedTypars (rhsExpr, tauTy)
 
         match checkedPat with
-        // Don't introduce temporary or 'let' for 'match against wild' or 'match against unit'
-
-        | TPat_wild _ | TPat_const (Const.Unit, _) when not isUse && not isFixed && isNil generalizedTypars ->
+        // Don't introduce temporary or 'let' for 'match against wild' or 'match against unit',
+        // unless the RHS is a byref-like value (e.g. `let _ = &s`). For byref-like RHS we
+        // must keep a real `Expr.Let` so that PostInferenceChecks treats the binding as
+        // permitting byref expressions, matching the behaviour of `let v = &s`.
+        // See issue dotnet/fsharp#18841.
+
+        | TPat_wild _ | TPat_const (Const.Unit, _)
+            when not isUse && not isFixed && isNil generalizedTypars && not (isByrefLikeTy g m tauTy) ->
             let mkSequentialBind (tm, tmty) = mkSequential m rhsExpr tm, tmty
             (buildExpr >> mkSequentialBind, env, tpenv)
         | _ ->

tests/FSharp.Compiler.ComponentTests/Conformance/Expressions/DataExpressions/AddressOf/AddressOf.fs

@@ -27,3 +27,123 @@ module AddressOf =
         |> shouldFail
         |> withErrorCode 431
         |> withDiagnosticMessageMatches "byref"
+
+    // Issue #18841: let _ = &expr should compile (parity with let x = &expr)
+
+    [<Fact>]
+    let ``Issue 18841 - let discard with address-of struct value compiles`` () =
+        Fsx """
+module Test
+
+type S =
+    struct
+        val Field: int
+    end
+
+let test () =
+    let s = S()
+    let _ = &s
+    let a = &s
+    ignore a
+        """
+        |> typecheck
+        |> shouldSucceed
+
+    [<Fact>]
+    let ``Issue 18841 - let discard with address-of mutable local compiles`` () =
+        Fsx """
+module Test
+let test () =
+    let mutable x = 42
+    let _ = &x
+    ()
+        """
+        |> typecheck
+        |> shouldSucceed
+
+    [<Fact>]
+    let ``Issue 18841 - let discard with address-of struct field compiles`` () =
+        Fsx """
+module Test
+
+[<Struct>]
+type S = { mutable Field: int }
+
+let test () =
+    let mutable s = { Field = 0 }
+    let _ = &s.Field
+    ()
+        """
+        |> typecheck
+        |> shouldSucceed
+
+    [<Fact>]
+    let ``Issue 18841 - let discard with parenthesized address-of compiles`` () =
+        Fsx """
+module Test
+
+type S =
+    struct
+        val Field: int
+    end
+
+let test () =
+    let s = S()
+    let _ = &(s)
+    ()
+        """
+        |> typecheck
+        |> shouldSucceed
+
+    [<Fact>]
+    let ``Issue 18841 - let discard with address-of inside function parameter compiles`` () =
+        Fsx """
+module Test
+
+type S =
+    struct
+        val Field: int
+    end
+
+let f (s: S) =
+    let _ = &s
+    ()
+        """
+        |> typecheck
+        |> shouldSucceed
+
+    [<Fact>]
+    let ``Issue 18841 - named binding let x = &s still compiles (regression guard)`` () =
+        Fsx """
+module Test
+
+type S =
+    struct
+        val Field: int
+    end
+
+let test () =
+    let s = S()
+    let a = &s
+    ignore a
+        """
+        |> typecheck
+        |> shouldSucceed
+
+    [<Fact>]
+    let ``Issue 18841 - plain let discard with no address-of still compiles`` () =
+        Fsx """
+module Test
+
+type S =
+    struct
+        val Field: int
+    end
+
+let test () =
+    let s = S()
+    let _ = s
+    ()
+        """
+        |> typecheck
+        |> shouldSucceed