Warning 20 range: also walk through let/use bindings (follow-up to #19504)

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

@@ -24,7 +24,8 @@
 * Fix DU case names matching IWSAM member names no longer cause duplicate property entries. (Issue [#14321](https://github.com/dotnet/fsharp/issues/14321), [PR #19341](https://github.com/dotnet/fsharp/pull/19341))
 * Fix DefaultAugmentation(false) duplicate entry in method table. (Issue [#16565](https://github.com/dotnet/fsharp/issues/16565), [PR #19341](https://github.com/dotnet/fsharp/pull/19341))
 * Fix abstract event accessors now have SpecialName flag. (Issue [#5834](https://github.com/dotnet/fsharp/issues/5834), [PR #19341](https://github.com/dotnet/fsharp/pull/19341))
-* Fix warning 20 ("expression is implicitly ignored") pointing at the wrong range when the last expression in a sequential block (e.g. inside `for`, `while` loops) is non-unit. The squiggle now correctly highlights only the offending expression. ([Issue #5735](https://github.com/dotnet/fsharp/issues/5735), [PR #19504](https://github.com/dotnet/fsharp/pull/19504))
+* Fix warning 20 ("expression is implicitly ignored") pointing at the wrong range when the last expression in a sequential block (e.g. inside `for`, `while` loops) is non-unit. The squiggle now correctly highlights only the offending expression. ([Issue #5418](https://github.com/dotnet/fsharp/issues/5418), [PR #19504](https://github.com/dotnet/fsharp/pull/19504))
+* Extend the warning 20 range fix to also walk through `let`/`use` bindings, so the squiggle lands on the offending expression when it is the body of a `let`/`use`. ([Issue #5418](https://github.com/dotnet/fsharp/issues/5418), [PR #19896](https://github.com/dotnet/fsharp/pull/19896))
 * Fix missing "No implementation was given" error when F# class inherits from a C# class with `abstract override` members without providing an implementation. ([Issue #7776](https://github.com/dotnet/fsharp/issues/7776), [PR #19503](https://github.com/dotnet/fsharp/pull/19503))
 * 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))

src/Compiler/Checking/Expressions/CheckExpressions.fs

@@ -5582,12 +5582,10 @@ and TcStmt (cenv: cenv) env tpenv synExpr =
     let g = cenv.g
     let expr, ty, tpenv = TcExprOfUnknownType cenv env tpenv synExpr
 
-    // Use the range of the last expression in a sequential chain for warnings,
-    // so that "expression is ignored" diagnostics point at the offending expression
-    // rather than the entire sequential body. See https://github.com/dotnet/fsharp/issues/5735
     let rec lastExprRange (e: SynExpr) =
         match e with
         | SynExpr.Sequential(expr2 = expr2) -> lastExprRange expr2
+        | SynExpr.LetOrUse({ Body = body }) -> lastExprRange body
         | _ -> e.Range
 
     let m = lastExprRange synExpr

tests/FSharp.Compiler.ComponentTests/ErrorMessages/WarnExpressionTests.fs

@@ -176,7 +176,6 @@ while x < 1 do
         |> withSingleDiagnostic (Warning 20, Line 6, Col 5, Line 6, Col 9,
                                  "The result of this expression has type 'bool' and is implicitly ignored. Consider using 'ignore' to discard this value explicitly, e.g. 'expr |> ignore', or 'let' to bind the result to a name, e.g. 'let result = expr'.")
 
-    // https://github.com/dotnet/fsharp/issues/5735
     [<Fact>]
     let ``Warn On Last Expression In For Loop - int``() =
         FSharp """
@@ -192,7 +191,6 @@ for i in 1 .. 10 do
         |> withSingleDiagnostic (Warning 20, Line 7, Col 5, Line 7, Col 8,
                                  "The result of this expression has type 'int' and is implicitly ignored. Consider using 'ignore' to discard this value explicitly, e.g. 'expr |> ignore', or 'let' to bind the result to a name, e.g. 'let result = expr'.")
 
-    // https://github.com/dotnet/fsharp/issues/5735
     [<Fact>]
     let ``Warn On Last Expression In For Loop - string``() =
         FSharp """
@@ -205,7 +203,6 @@ for i in 1 .. 10 do
         |> withSingleDiagnostic (Warning 20, Line 4, Col 5, Line 4, Col 12,
                                  "The result of this expression has type 'string' and is implicitly ignored. Consider using 'ignore' to discard this value explicitly, e.g. 'expr |> ignore', or 'let' to bind the result to a name, e.g. 'let result = expr'.")
 
-    // https://github.com/dotnet/fsharp/issues/5735
     [<Fact>]
     let ``Warn On Last Expression In Integer For Loop``() =
         FSharp """
@@ -218,7 +215,6 @@ for i = 1 to 10 do
         |> withSingleDiagnostic (Warning 20, Line 4, Col 5, Line 4, Col 7,
                                  "The result of this expression has type 'int' and is implicitly ignored. Consider using 'ignore' to discard this value explicitly, e.g. 'expr |> ignore', or 'let' to bind the result to a name, e.g. 'let result = expr'.")
 
-    // https://github.com/dotnet/fsharp/issues/5735
     [<Fact>]
     let ``Warn On Last Expression In While Loop - non-bool``() =
         FSharp """
@@ -233,6 +229,61 @@ while x < 1 do
         |> withSingleDiagnostic (Warning 20, Line 6, Col 5, Line 6, Col 8,
                                  "The result of this expression has type 'int' and is implicitly ignored. Consider using 'ignore' to discard this value explicitly, e.g. 'expr |> ignore', or 'let' to bind the result to a name, e.g. 'let result = expr'.")
 
+    [<Fact>]
+    let ``Warn On Last Expression In For Loop - non-unit after let binding``() =
+        FSharp """
+for _ in [] do
+    let x = 1
+    x
+        """
+        |> typecheck
+        |> shouldFail
+        |> withSingleDiagnostic (Warning 20, Line 4, Col 5, Line 4, Col 6,
+                                 "The result of this expression has type 'int' and is implicitly ignored. Consider using 'ignore' to discard this value explicitly, e.g. 'expr |> ignore', or 'let' to bind the result to a name, e.g. 'let result = expr'.")
+
+    [<Fact>]
+    let ``Warn On Last Expression In For Loop - non-unit after nested let bindings``() =
+        FSharp """
+for _ in 1 .. 3 do
+    let a = 1
+    let b = 2
+    a + b
+        """
+        |> typecheck
+        |> shouldFail
+        |> withSingleDiagnostic (Warning 20, Line 5, Col 5, Line 5, Col 10,
+                                 "The result of this expression has type 'int' and is implicitly ignored. Consider using 'ignore' to discard this value explicitly, e.g. 'expr |> ignore', or 'let' to bind the result to a name, e.g. 'let result = expr'.")
+
+    [<Fact>]
+    let ``Warn On Last Expression In For Loop - non-unit after use binding``() =
+        FSharp """
+type D() =
+    interface System.IDisposable with
+        member _.Dispose() = ()
+    member _.Value = 1
+for _ in [] do
+    use d = new D()
+    d.Value
+        """
+        |> typecheck
+        |> shouldFail
+        |> withSingleDiagnostic (Warning 20, Line 8, Col 5, Line 8, Col 12,
+                                 "The result of this expression has type 'int' and is implicitly ignored. Consider using 'ignore' to discard this value explicitly, e.g. 'expr |> ignore', or 'let' to bind the result to a name, e.g. 'let result = expr'.")
+
+    [<Fact>]
+    let ``Warn On Last Expression In While Loop - non-unit after let binding``() =
+        FSharp """
+let mutable x = 0
+while x < 3 do
+    let y = x + 1
+    x <- y
+    y
+        """
+        |> typecheck
+        |> shouldFail
+        |> withSingleDiagnostic (Warning 20, Line 6, Col 5, Line 6, Col 6,
+                                 "The result of this expression has type 'int' and is implicitly ignored. Consider using 'ignore' to discard this value explicitly, e.g. 'expr |> ignore', or 'let' to bind the result to a name, e.g. 'let result = expr'.")
+
     [<Fact>]
     let ``Warn If Possible Property Setter``() =
         FSharp """