Keep struct in a when ^T struct static optimization

CHANGELOG.md

@@ -5,6 +5,8 @@
 ### Fixed
 
 - Exception abbreviation loses its right-hand side. [#3511](https://github.com/fsprojects/fantomas/issues/3511)
+- Static optimization when ^T struct loses struct. [#3514](https://github.com/fsprojects/fantomas/issues/3514)
+- Static optimization loses the `and` between its conditions, and reverses its conditions and `when` clauses. [#3516](https://github.com/fsprojects/fantomas/pull/3516)
 
 ## [8.0.6] - 2026-10-01
 

src/Fantomas.Core.SnapshotTests/cases/oak/Expr/LibraryOnlyStaticOptimization/conditions-joined-by-and.fs

@@ -0,0 +1 @@
+let inline f (x: ^T) (y: ^U) = 0 when ^T : int and ^U struct = 1

src/Fantomas.Core.SnapshotTests/cases/oak/Expr/LibraryOnlyStaticOptimization/conditions-joined-by-and.gold.fs

@@ -0,0 +1 @@
+let inline f (x: ^T) (y: ^U) = 0 when ^T: int and ^U struct = 1

src/Fantomas.Core.SnapshotTests/cases/oak/Expr/LibraryOnlyStaticOptimization/several-when-clauses.fs

@@ -0,0 +1 @@
+let inline f (x: ^T) = 0 when ^T : int = 1 when ^T : float = 2

src/Fantomas.Core.SnapshotTests/cases/oak/Expr/LibraryOnlyStaticOptimization/several-when-clauses.gold.fs

@@ -0,0 +1 @@
+let inline f (x: ^T) = 0 when ^T: int = 1 when ^T: float = 2

src/Fantomas.Core.SnapshotTests/cases/oak/StaticOptimizationConstraint/WhenTyparIsStruct/negative/3514-when-a-type-parameter-is-a-struct.fs

@@ -0,0 +1 @@
+let inline retype (x: ^T) : ^U = (# "" x : ^U #) when ^T struct = 0

src/Fantomas.Core/ASTTransformer.fs

@@ -1923,29 +1923,42 @@ let mkExpr (creationAide: CreationAide) (e: SynExpr) : Expr =
         ExprSetNode(mkExpr creationAide e1, mkExpr creationAide e2, exprRange)
         |> Expr.Set
 
-    | SynExpr.LibraryOnlyStaticOptimization(constraints, e, optExpr, _) ->
-        let constraints =
-            constraints
-            |> List.map (
-                function
-                | SynStaticOptimizationConstraint.WhenTyparTyconEqualsTycon(t1, t2, _) ->
-                    StaticOptimizationConstraintWhenTyparTyconEqualsTyconNode(
-                        mkSynTypar t1,
-                        mkType creationAide t2,
-                        unionRanges t1.Range t2.Range
+    | SynExpr.LibraryOnlyStaticOptimization _ ->
+        let mkConstraint (c: SynStaticOptimizationConstraint) : StaticOptimizationConstraint =
+            match c with
+            | SynStaticOptimizationConstraint.WhenTyparTyconEqualsTycon(t1, t2, _) ->
+                StaticOptimizationConstraintWhenTyparTyconEqualsTyconNode(
+                    mkSynTypar t1,
+                    mkType creationAide t2,
+                    unionRanges t1.Range t2.Range
+                )
+                |> StaticOptimizationConstraint.WhenTyparTyconEqualsTycon
+            | SynStaticOptimizationConstraint.WhenTyparIsStruct(t, _) ->
+                mkSynTypar t |> StaticOptimizationConstraint.WhenTyparIsStruct
+
+        // The parser nests `e when A = a when B = b` as A(a, B(b, e)), with the first clause outermost, while the
+        // Oak puts the last clause outermost: B(b, A(a, e)). `wrap` builds the clauses seen so far around `e`.
+        let rec collect (wrap: Expr -> Expr) (expr: SynExpr) : Expr =
+            match expr with
+            | SynExpr.LibraryOnlyStaticOptimization(constraints, e, optExpr, _) ->
+                collect
+                    (fun inner ->
+                        let optimizedExpr: Expr = wrap inner
+                        let expr: Expr = mkExpr creationAide e
+
+                        ExprLibraryOnlyStaticOptimizationNode(
+                            optimizedExpr,
+                            // The parser keeps the conditions of a `when` in reverse.
+                            constraints |> List.rev |> List.map mkConstraint,
+                            expr,
+                            unionRanges (Expr.Node optimizedExpr).Range (Expr.Node expr).Range
+                        )
+                        |> Expr.LibraryOnlyStaticOptimization
                     )
-                    |> StaticOptimizationConstraint.WhenTyparTyconEqualsTycon
-                | SynStaticOptimizationConstraint.WhenTyparIsStruct(t, _) ->
-                    mkSynTypar t |> StaticOptimizationConstraint.WhenTyparIsStruct
-            )
+                    optExpr
+            | _ -> wrap (mkExpr creationAide expr)
 
-        ExprLibraryOnlyStaticOptimizationNode(
-            mkExpr creationAide optExpr,
-            constraints,
-            mkExpr creationAide e,
-            exprRange
-        )
-        |> Expr.LibraryOnlyStaticOptimization
+        collect id e
     | SynExpr.InterpolatedString(parts, _, _) ->
         let lastIndex = parts.Length - 1
 

src/Fantomas.Core/CodePrinter.fs

@@ -2339,10 +2339,10 @@ let genExpr (e: Expr) =
         genExpr node.OptimizedExpr
         +> onlyIfNot node.Constraints.IsEmpty (!-" when ")
         +> col
-            sepSpace
+            (sepSpace +> wordAndFixed +> sepSpace)
             node.Constraints
             (function
-            | StaticOptimizationConstraint.WhenTyparIsStruct t -> genSingleTextNode t
+            | StaticOptimizationConstraint.WhenTyparIsStruct t -> genSingleTextNode t +> !-" struct"
             | StaticOptimizationConstraint.WhenTyparTyconEqualsTycon n ->
                 genSingleTextNode n.TypeParameter +> sepColon +> sepSpace +> genType n.Type
             )