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Type checker: recover on checking binding parameter constraints #19046

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Type checker: recover on checking binding parameter contraints
auduchinok Nov 3, 2025
e74b246
Release notes
auduchinok Nov 3, 2025
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auduchinok Nov 3, 2025
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2 changes: 2 additions & 0 deletions docs/release-notes/.FSharp.Compiler.Service/11.0.0.md
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Expand Up @@ -10,6 +10,8 @@

### Added

* Type checker: recover on checking binding parameter constraints ([#19046](https://github.com/dotnet/fsharp/pull/19046))

### Changed

* Parallel compilation stabilised and enabled by default ([PR #18998](https://github.com/dotnet/fsharp/pull/18998))
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14 changes: 8 additions & 6 deletions src/Compiler/Checking/Expressions/CheckExpressions.fs
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Expand Up @@ -11561,12 +11561,14 @@ and TcLetBinding (cenv: cenv) isUse env containerInfo declKind tpenv (synBinds,

// Canonicalize constraints prior to generalization
let denv = env.DisplayEnv
CanonicalizePartialInferenceProblem cenv.css denv synBindsRange
(checkedBinds |> List.collect (fun tbinfo ->
let (CheckedBindingInfo(_, _, _, _, explicitTyparInfo, _, _, _, tauTy, _, _, _, _, _)) = tbinfo
let (ExplicitTyparInfo(_, declaredTypars, _)) = explicitTyparInfo
let maxInferredTypars = (freeInTypeLeftToRight g false tauTy)
declaredTypars @ maxInferredTypars))
try
CanonicalizePartialInferenceProblem cenv.css denv synBindsRange
(checkedBinds |> List.collect (fun tbinfo ->
let (CheckedBindingInfo(_, _, _, _, explicitTyparInfo, _, _, _, tauTy, _, _, _, _, _)) = tbinfo
let (ExplicitTyparInfo(_, declaredTypars, _)) = explicitTyparInfo
let maxInferredTypars = (freeInTypeLeftToRight g false tauTy)
declaredTypars @ maxInferredTypars))
with RecoverableException _ -> ()
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@T-Gro T-Gro Nov 6, 2025

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Please add more explanation as code comment why it is fine to gave an all-catching try block here, this might not be obvious for future readers.

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@auduchinok auduchinok Nov 6, 2025

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It's just like in other places where it felt safe to recover and continue To be honest, I don't even know what to write in such a comment so it wouldn't be just stating that we're recovering...


let lazyFreeInEnv = lazy (GeneralizationHelpers.ComputeUngeneralizableTypars env)

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33 changes: 32 additions & 1 deletion tests/FSharp.Compiler.Service.Tests/TypeChecker/TypeCheckerRecoveryTests.fs
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@@ -1,9 +1,20 @@
module FSharp.Compiler.Service.Tests.TypeChecker.TypeCheckerRecoveryTests

open FSharp.Compiler.Service.Tests
open FSharp.Compiler.Text
open FSharp.Test.Assert
open Xunit

let assertHasSymbolUsageAtCaret name source =
let context, checkResults = Checker.getCheckedResolveContext source

getSymbolUses checkResults
|> Seq.exists (fun symbolUse ->
Range.rangeContainsPos symbolUse.Range context.Pos &&
symbolUse.Symbol.DisplayNameCore = name
)
|> shouldEqual true

[<Fact>]
let ``Let 01`` () =
let _, checkResults = getParseAndCheckResults """
Expand Down Expand Up @@ -49,4 +60,24 @@ Math.Max(a,b,)
"(4,0--4,14)", 503
]

assertHasSymbolUsages ["Max"] checkResults
assertHasSymbolUsages ["Max"] checkResults

module Constraints =
[<Fact>]
let ``Type 01`` () =
assertHasSymbolUsageAtCaret "f" """
let f (x: string) =
x + 1

{caret}f ""
"""

[<Fact>]
let ``Type 02`` () =
assertHasSymbolUsageAtCaret "M" """
type T =
static member M(x: string) =
x + 1

T.M{caret} ""
"""
4 changes: 3 additions & 1 deletion tests/fsharp/typecheck/sigs/neg68.bsl
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@@ -1,2 +1,4 @@

neg68.fsx(71,46,71,47): parse error FS0010: Unexpected symbol ')' in binding. Expected incomplete structured construct at or before this point or other token.
neg68.fsx(138,15,138,16): typecheck info FS3370: The use of '!' from the F# library is deprecated. See https://aka.ms/fsharp-refcell-ops. For example, please change '!cell' to 'cell.Value'.

neg68.fsx(158,24,158,26): typecheck info FS3370: The use of ':=' from the F# library is deprecated. See https://aka.ms/fsharp-refcell-ops. For example, please change 'cell := expr' to 'cell.Value <- expr'.
2 changes: 1 addition & 1 deletion tests/fsharp/typecheck/sigs/neg68.fsx
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Expand Up @@ -68,7 +68,7 @@ type sRealTime
[<Measure>]
type pixels

let G = 6.67e-11<m ^ 3 / (kg s^2))>
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@T-Gro T-Gro Nov 6, 2025

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Its not really a "neg" test anymore, isn't it?
Imo you might as well remove it, as I do not see it it testing anything.

If you kept the superfluous paren to maintain this being a neg test, is the recovery same/better ?

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@auduchinok auduchinok Nov 6, 2025

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Yeah, it felt like this test doesn't test anything useful given how the errors flow in that file. I can remove it for sure.

let G = 6.67e-11<m ^ 3 / (kg s^2)>
let m_per_AU = 149597870691.0<m/AU>
let AU_per_m = 1.0/m_per_AU
let Pixels_per_AU = 200.0<pixels/AU>
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27 changes: 2 additions & 25 deletions tests/fsharp/typecheck/sigs/neg68.vsbsl
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@@ -1,26 +1,3 @@
neg68.fsx(138,15,138,16): typecheck info FS3370: The use of '!' from the F# library is deprecated. See https://aka.ms/fsharp-refcell-ops. For example, please change '!cell' to 'cell.Value'.

neg68.fsx(71,46,71,47): parse error FS0010: Unexpected symbol ')' in binding. Expected incomplete structured construct at or before this point or other token.

neg68.fsx(71,32,71,33): typecheck error FS0062: This construct is for ML compatibility. Consider using the '+' operator instead. This may require a type annotation to indicate it acts on strings. This message can be disabled using '--nowarn:62' or '#nowarn "62"'.

neg68.fsx(71,30,71,46): typecheck error FS0001: This expression was expected to have type
'float'
but here has type
'string'

neg68.fsx(71,30,71,31): typecheck error FS1133: No constructors are available for the type 'm'

neg68.fsx(71,43,71,44): typecheck error FS0062: This construct is for ML compatibility. Consider using the '+' operator instead. This may require a type annotation to indicate it acts on strings. This message can be disabled using '--nowarn:62' or '#nowarn "62"'.

neg68.fsx(71,39,71,45): typecheck error FS0001: The type 'string' does not match the type 'int'

neg68.fsx(71,39,71,41): typecheck error FS1133: No constructors are available for the type 'kg'

neg68.fsx(71,44,71,45): typecheck error FS0001: This expression was expected to have type
'string'
but here has type
'int'

neg68.fsx(123,40,123,41): typecheck error FS0001: The type 'bool' does not match the type 'float<'u>'

neg68.fsx(123,38,123,39): typecheck error FS0043: The type 'bool' does not match the type 'float<'u>'
neg68.fsx(158,24,158,26): typecheck info FS3370: The use of ':=' from the F# library is deprecated. See https://aka.ms/fsharp-refcell-ops. For example, please change 'cell := expr' to 'cell.Value <- expr'.
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