[JS/TS] Fix invalid syntax emitted when negating negative literals (#4440)

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: MangelMaxime

src/Fable.Cli/CHANGELOG.md

@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+* [JS/TS] Fix invalid syntax emitted when negating negative literals (fix #4251) (by @MangelMaxime)
 * [Rust] Fixed negative counting in CallInfo.GenericArgs (by @ncave)
 * [JS/TS] Improve `Regex.Escape` and `Regex.Unescape` handling (by @MangelMaxime)
 * [All] Fix allow plugins to target .NET6 target framework (by @MangelMaxime)

src/Fable.Compiler/CHANGELOG.md

@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+* [JS/TS] Fix invalid syntax emitted when negating negative literals (fix #4251) (by @MangelMaxime)
 * [Rust] Fixed negative counting in CallInfo.GenericArgs (by @ncave)
 * [JS/TS] Improve `Regex.Escape` and `Regex.Unescape` handling (by @MangelMaxime)
 * [All] Fix allow plugins to target .NET6 target framework (by @MangelMaxime)

src/Fable.Transforms/BabelPrinter.fs

@@ -1265,14 +1265,30 @@ module PrinterExtensions =
                             operator + " "
                     )
 
+            // When negating a negative numeric literal (e.g. -(-5)), we must wrap
+            // it in parens to avoid emitting --5, which JS parses as a decrement.
+            let needsParens =
+                not isSuffix
+                && operator = "-"
+                && (
+                    match argument with
+                    | Literal(NumericLiteral(value, _)) -> value < 0.0
+                    | UnaryExpression(_, op, isSuffix, _) -> op = "-" && not isSuffix
+                    | _ -> false
+                )
+
             printer.AddLocation(loc)
 
             if isSuffix then
                 printer.ComplexExpressionWithParens(argument)
                 printOp ()
             else
                 printOp ()
-                printer.ComplexExpressionWithParens(argument)
+
+                if needsParens then
+                    printer.WithParens(argument)
+                else
+                    printer.ComplexExpressionWithParens(argument)
 
         member printer.PrintUpdateExpression(prefix, argument, operator, loc) =
             printer.AddLocation(loc)

tests/Js/Main/ArithmeticTests.fs

@@ -33,6 +33,15 @@ let tests =
     testCase "Unary negation with negative literal values works" <| fun () ->
         -literalNegativeValue |> equal 345
 
+    testCase "Negating negative literals inlined in computation expressions works" <| fun () -> // #4251
+        let f x = x
+        let result =
+            [
+                let x = -5.0
+                f (-x)
+            ]
+        result |> equal [5.0]
+
     testCase "Unary negation with integer MinValue works" <| fun () ->
         -(-128y) |> equal SByte.MinValue
         -(-32768s) |> equal Int16.MinValue