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Fix InvalidProgramException from Seq.collect over a struct seq materialised with List.ofSeq/Seq.toList/Seq.toArray #20227

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Fix InvalidProgramException from Seq.collect over a struct seq materialised with List.ofSeq/Seq.toList/Seq.toArray #20227

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17f0e02
Fix InvalidProgramException from Seq.collect/yield! over struct seq m…
invalid-email-address Aug 6, 2026
d2c6f1e
Regenerate EmittedIL baselines for dead-IL removal + pin struct box/u…
invalid-email-address Aug 6, 2026
9e8690d
Add release note for issue #20203 struct seq InvalidProgramException fix
invalid-email-address Aug 6, 2026
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1 change: 1 addition & 0 deletions docs/release-notes/.FSharp.Compiler.Service/11.0.100.md
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Expand Up @@ -129,6 +129,7 @@
* Fix dot-completion after indexed expressions (`a.[0].Data.`, `a[0].Data.`, `[1;2].Length.`) returning unrelated global completions instead of expression-typings members. ([Issue #4966](https://github.com/dotnet/fsharp/issues/4966), [PR #19934](https://github.com/dotnet/fsharp/pull/19934))
* Quotations of `match s with "" -> _` no longer leak the `s <> null && s.Length = 0` lowering; the empty-string optimization moved from pattern-match compilation to the optimizer so quoted expressions keep `op_Equality(s, "")`. ([Issue #19873](https://github.com/dotnet/fsharp/issues/19873))
* Fix #5795: Allow attributes defined in a `module rec` / `namespace rec` scope to be used on union cases, record fields, and generic type parameters of types in the same recursive scope. ([Issue #5795](https://github.com/dotnet/fsharp/issues/5795), [PR #19744](https://github.com/dotnet/fsharp/pull/19744))
* Fix Release-only `System.InvalidProgramException` when a struct (value-type) collection implementing `seq<'T>` (e.g. `System.Collections.Immutable.ImmutableArray<_>`) is materialised via `Seq.collect`/`yield!` into `List.ofSeq`/`Seq.toList`/`Seq.toArray`. The comprehension lowering now coerces (boxes) the struct sub-collection to `seq<'T>` before `ListCollector`/`ArrayCollector.AddMany`, and uses `unit` as the try/finally result type, so it no longer emits invalid IL (a value type where a reference `IEnumerable` is expected, plus a spurious `ldnull` stored into a struct local). ([Issue #20203](https://github.com/dotnet/fsharp/issues/20203))

### Added

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23 changes: 20 additions & 3 deletions src/Compiler/Optimize/LowerComputedCollections.fs
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Expand Up @@ -64,6 +64,10 @@ let LowerComputedListOrArraySeqExpr tcVal g amap m collectorTy overallSeqExpr =
let infoReader = InfoReader(g, amap)
let collVal, collExpr = mkMutableCompGenLocal m "@collector" collectorTy
//let collExpr = mkValAddr m false (mkLocalValRef collVal)
// The 'seq<'T>' parameter type of AddMany/AddManyAndClose, derived from the
// ListCollector<'T>/ArrayCollector<'T> element type. A struct sub-collection must be
// coerced (boxed) to this so the AddMany call site emits valid IL.
let collectorSeqTy = mkSeqTy g (List.head (argsOfAppTy g collectorTy))
let rec ConvertSeqExprCode isUninteresting isTailcall expr =
match expr with
| SeqYield g (e, m) ->
Expand Down Expand Up @@ -98,7 +102,7 @@ let LowerComputedListOrArraySeqExpr tcVal g amap m collectorTy overallSeqExpr =
let cleanupE = BuildDisposableCleanup tcVal g infoReader m v
let exprR =
mkLet spBind m v resource
(mkTryFinally g (bodyExprR, cleanupE, m, tyOfExpr g bodyExpr, DebugPointAtTry.No, DebugPointAtFinally.No))
(mkTryFinally g (bodyExprR, cleanupE, m, g.unit_ty, DebugPointAtTry.No, DebugPointAtFinally.No))
Result.Ok (false, exprR)
| Result.Error msg -> Result.Error msg

Expand Down Expand Up @@ -126,7 +130,10 @@ let LowerComputedListOrArraySeqExpr tcVal g amap m collectorTy overallSeqExpr =
(callNonOverloadedILMethod g amap mIn "get_Current" inpEnumTy [enumve]))
bodyExprR, mIn),
cleanupE,
mFor, tyOfExpr g bodyExpr, DebugPointAtTry.No, DebugPointAtFinally.No))
// The lowered body is a unit-typed collector call, so the try/finally result
// type is unit; using the original (possibly struct) body type made the optimizer
// emit 'ldnull; stloc <struct>' - invalid IL.
mFor, g.unit_ty, DebugPointAtTry.No, DebugPointAtFinally.No))
|> addForDebugPoint
Result.Ok (false, exprR)
| Result.Error msg -> Result.Error msg
Expand All @@ -136,7 +143,7 @@ let LowerComputedListOrArraySeqExpr tcVal g amap m collectorTy overallSeqExpr =
match resBody with
| Result.Ok (_, bodyExprR) ->
let exprR =
mkTryFinally g (bodyExprR, compensation, m, tyOfExpr g bodyExpr, spTry, spFinally)
mkTryFinally g (bodyExprR, compensation, m, g.unit_ty, spTry, spFinally)
Result.Ok (false, exprR)
| Result.Error msg -> Result.Error msg

Expand Down Expand Up @@ -201,6 +208,16 @@ let LowerComputedListOrArraySeqExpr tcVal g amap m collectorTy overallSeqExpr =
// printfn "FAILED - not worth compiling an unrecognized Seq.toList at %s " (stringOfRange m)
Result.Error ()
else
// A struct sub-collection (a value-type seq<'T>, e.g. ImmutableArray<_>) is never
// implicitly upcast, so coerce it to the collector's seq<'T> param type. GenCoerce emits
// nothing for reference upcasts and 'box; unbox.any' for structs (matching the yield! path);
// without this, AddMany/AddManyAndClose receive a value type where a ref IEnumerable is
// expected, producing invalid IL.
let srcTy = tyOfExpr g arbitrarySeqExpr
let arbitrarySeqExpr =
if typeEquiv g srcTy collectorSeqTy then arbitrarySeqExpr
else mkCoerceExpr (arbitrarySeqExpr, collectorSeqTy, m, srcTy)

// If we're the final in a sequential chain then we can AddMany, Close and return
if isTailcall then
let exprR = mkCallCollectorAddManyAndClose tcVal (g: TcGlobals) infoReader m collExpr arbitrarySeqExpr
Expand Down
6 changes: 6 additions & 0 deletions tests/FSharp.Compiler.ComponentTests/EmittedIL/ComputedCollections/ComputedCollections.fs
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Original file line number Diff line number Diff line change
Expand Up @@ -85,3 +85,9 @@ module ComputedCollections =
compilation
|> getCompilation
|> verifyCompilation

[<Theory; FileInlineData("StructSeqCollectToList.fs", Realsig = BooleanOptions.Both, Optimize = BooleanOptions.True)>]
let ``StructSeqCollectToList_fs`` compilation =
compilation
|> getCompilation
|> verifyCompilation
136 changes: 63 additions & 73 deletions .../FSharp.Compiler.ComponentTests/EmittedIL/ComputedCollections/ForNInRangeArrays.fs.il.bsl
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Original file line number Diff line number Diff line change
Expand Up @@ -523,9 +523,8 @@
.maxstack 5
.locals init (valuetype [FSharp.Core]Microsoft.FSharp.Core.CompilerServices.ArrayCollector`1<int32> V_0,
class [runtime]System.Collections.Generic.IEnumerator`1<int32> V_1,
class [runtime]System.Collections.Generic.IEnumerable`1<int32> V_2,
int32 V_3,
class [runtime]System.IDisposable V_4)
int32 V_2,
class [runtime]System.IDisposable V_3)
IL_0000: nop
IL_0001: ldc.i4.1
IL_0002: ldc.i4.1
Expand All @@ -541,17 +540,17 @@

IL_0012: ldloc.1
IL_0013: callvirt instance !0 class [runtime]System.Collections.Generic.IEnumerator`1<int32>::get_Current()
IL_0018: stloc.3
IL_0018: stloc.2
IL_0019: ldarg.0
IL_001a: ldnull
IL_001b: callvirt instance !1 class [FSharp.Core]Microsoft.FSharp.Core.FSharpFunc`2<class [FSharp.Core]Microsoft.FSharp.Core.Unit,class [FSharp.Core]Microsoft.FSharp.Core.Unit>::Invoke(!0)
IL_0020: pop
IL_0021: ldloca.s V_0
IL_0023: ldloc.3
IL_0023: ldloc.2
IL_0024: call instance void valuetype [FSharp.Core]Microsoft.FSharp.Core.CompilerServices.ArrayCollector`1<int32>::Add(!0)
IL_0029: nop
IL_002a: ldloca.s V_0
IL_002c: ldloc.3
IL_002c: ldloc.2
IL_002d: ldc.i4.1
IL_002e: add
IL_002f: call instance void valuetype [FSharp.Core]Microsoft.FSharp.Core.CompilerServices.ArrayCollector`1<int32>::Add(!0)
Expand All @@ -560,29 +559,25 @@
IL_0036: callvirt instance bool [runtime]System.Collections.IEnumerator::MoveNext()
IL_003b: brtrue.s IL_0012

IL_003d: ldnull
IL_003e: stloc.2
IL_003f: leave.s IL_0056
IL_003d: leave.s IL_0051

}
finally
{
IL_0041: ldloc.1
IL_0042: isinst [runtime]System.IDisposable
IL_0047: stloc.s V_4
IL_0049: ldloc.s V_4
IL_004b: brfalse.s IL_0055

IL_004d: ldloc.s V_4
IL_004f: callvirt instance void [runtime]System.IDisposable::Dispose()
IL_0054: endfinally
IL_0055: endfinally
IL_003f: ldloc.1
IL_0040: isinst [runtime]System.IDisposable
IL_0045: stloc.3
IL_0046: ldloc.3
IL_0047: brfalse.s IL_0050

IL_0049: ldloc.3
IL_004a: callvirt instance void [runtime]System.IDisposable::Dispose()
IL_004f: endfinally
IL_0050: endfinally
}
IL_0056: ldloc.2
IL_0057: pop
IL_0058: ldloca.s V_0
IL_005a: call instance !0[] valuetype [FSharp.Core]Microsoft.FSharp.Core.CompilerServices.ArrayCollector`1<int32>::Close()
IL_005f: ret
IL_0051: ldloca.s V_0
IL_0053: call instance !0[] valuetype [FSharp.Core]Microsoft.FSharp.Core.CompilerServices.ArrayCollector`1<int32>::Close()
IL_0058: ret
}

.method public static int32[] f0000() cil managed
Expand Down Expand Up @@ -1024,12 +1019,11 @@
.maxstack 5
.locals init (valuetype [FSharp.Core]Microsoft.FSharp.Core.CompilerServices.ArrayCollector`1<int32> V_0,
class [runtime]System.Collections.Generic.IEnumerator`1<int32> V_1,
class [runtime]System.Collections.Generic.IEnumerable`1<int32> V_2,
int32 V_3,
int32 V_2,
valuetype [FSharp.Core]Microsoft.FSharp.Core.CompilerServices.ArrayCollector`1<int32>& V_3,
valuetype [FSharp.Core]Microsoft.FSharp.Core.CompilerServices.ArrayCollector`1<int32>& V_4,
valuetype [FSharp.Core]Microsoft.FSharp.Core.CompilerServices.ArrayCollector`1<int32>& V_5,
valuetype [FSharp.Core]Microsoft.FSharp.Core.CompilerServices.ArrayCollector`1<int32>& V_6,
class [runtime]System.IDisposable V_7)
class [runtime]System.IDisposable V_6)
IL_0000: nop
IL_0001: ldc.i4.1
IL_0002: ldc.i4.1
Expand All @@ -1041,60 +1035,56 @@
IL_000f: stloc.1
.try
{
IL_0010: br.s IL_004c
IL_0010: br.s IL_004a

IL_0012: ldloc.1
IL_0013: callvirt instance !0 class [runtime]System.Collections.Generic.IEnumerator`1<int32>::get_Current()
IL_0018: stloc.3
IL_0018: stloc.2
IL_0019: ldloca.s V_0
IL_001b: stloc.s V_4
IL_001d: ldloc.s V_4
IL_001f: ldarg.0
IL_0020: ldnull
IL_0021: callvirt instance !1 class [FSharp.Core]Microsoft.FSharp.Core.FSharpFunc`2<class [FSharp.Core]Microsoft.FSharp.Core.Unit,int32>::Invoke(!0)
IL_0026: call instance void valuetype [FSharp.Core]Microsoft.FSharp.Core.CompilerServices.ArrayCollector`1<int32>::Add(!0)
IL_002b: nop
IL_002c: ldloca.s V_0
IL_002e: stloc.s V_5
IL_0030: ldloc.s V_5
IL_0032: ldarg.1
IL_0033: ldnull
IL_0034: callvirt instance !1 class [FSharp.Core]Microsoft.FSharp.Core.FSharpFunc`2<class [FSharp.Core]Microsoft.FSharp.Core.Unit,int32>::Invoke(!0)
IL_0039: call instance void valuetype [FSharp.Core]Microsoft.FSharp.Core.CompilerServices.ArrayCollector`1<int32>::Add(!0)
IL_003e: nop
IL_003f: ldloca.s V_0
IL_0041: stloc.s V_6
IL_0043: ldloc.s V_6
IL_0045: ldloc.3
IL_0046: call instance void valuetype [FSharp.Core]Microsoft.FSharp.Core.CompilerServices.ArrayCollector`1<int32>::Add(!0)
IL_004b: nop
IL_004c: ldloc.1
IL_004d: callvirt instance bool [runtime]System.Collections.IEnumerator::MoveNext()
IL_0052: brtrue.s IL_0012

IL_0054: ldnull
IL_0055: stloc.2
IL_0056: leave.s IL_006d
IL_001b: stloc.3
IL_001c: ldloc.3
IL_001d: ldarg.0
IL_001e: ldnull
IL_001f: callvirt instance !1 class [FSharp.Core]Microsoft.FSharp.Core.FSharpFunc`2<class [FSharp.Core]Microsoft.FSharp.Core.Unit,int32>::Invoke(!0)
IL_0024: call instance void valuetype [FSharp.Core]Microsoft.FSharp.Core.CompilerServices.ArrayCollector`1<int32>::Add(!0)
IL_0029: nop
IL_002a: ldloca.s V_0
IL_002c: stloc.s V_4
IL_002e: ldloc.s V_4
IL_0030: ldarg.1
IL_0031: ldnull
IL_0032: callvirt instance !1 class [FSharp.Core]Microsoft.FSharp.Core.FSharpFunc`2<class [FSharp.Core]Microsoft.FSharp.Core.Unit,int32>::Invoke(!0)
IL_0037: call instance void valuetype [FSharp.Core]Microsoft.FSharp.Core.CompilerServices.ArrayCollector`1<int32>::Add(!0)
IL_003c: nop
IL_003d: ldloca.s V_0
IL_003f: stloc.s V_5
IL_0041: ldloc.s V_5
IL_0043: ldloc.2
IL_0044: call instance void valuetype [FSharp.Core]Microsoft.FSharp.Core.CompilerServices.ArrayCollector`1<int32>::Add(!0)
IL_0049: nop
IL_004a: ldloc.1
IL_004b: callvirt instance bool [runtime]System.Collections.IEnumerator::MoveNext()
IL_0050: brtrue.s IL_0012

IL_0052: leave.s IL_0069

}
finally
{
IL_0058: ldloc.1
IL_0059: isinst [runtime]System.IDisposable
IL_005e: stloc.s V_7
IL_0060: ldloc.s V_7
IL_0062: brfalse.s IL_006c

IL_0064: ldloc.s V_7
IL_0066: callvirt instance void [runtime]System.IDisposable::Dispose()
IL_006b: endfinally
IL_006c: endfinally
IL_0054: ldloc.1
IL_0055: isinst [runtime]System.IDisposable
IL_005a: stloc.s V_6
IL_005c: ldloc.s V_6
IL_005e: brfalse.s IL_0068

IL_0060: ldloc.s V_6
IL_0062: callvirt instance void [runtime]System.IDisposable::Dispose()
IL_0067: endfinally
IL_0068: endfinally
}
IL_006d: ldloc.2
IL_006e: pop
IL_006f: ldloca.s V_0
IL_0071: call instance !0[] valuetype [FSharp.Core]Microsoft.FSharp.Core.CompilerServices.ArrayCollector`1<int32>::Close()
IL_0076: ret
IL_0069: ldloca.s V_0
IL_006b: call instance !0[] valuetype [FSharp.Core]Microsoft.FSharp.Core.CompilerServices.ArrayCollector`1<int32>::Close()
IL_0070: ret
}

.method public static int32[] f1() cil managed
Expand Down
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