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Crash when using code quotations in tests #14225
Description
We do our unit tests using the unquote library to write our assertions. This relies a lot on F# code quotations. Most of the time our tests run just fine but once in a while a test fails with a NullReferenceException. The weird thing is that both the test and the code under test are pure so I find it hard to understand where the flaky behavior comes from.
I've also reported the bug to unquote, but unfortunately the author also doesn't know. Because the stack trace only contains FSharp.Core calls I was hoping maybe someone here might have an idea:
System.NullReferenceException : Object reference not set to an instance of an object.
Stack Trace:
D:\a\_work\1\s\src\FSharp.Core\quotations.fs(112,0): at Microsoft.FSharp.Quotations.FSharpVar..ctor(String name, Type typ, FSharpOption`1 isMutable)
D:\a\_work\1\s\src\FSharp.Core\quotations.fs(2070,0): at Microsoft.FSharp.Quotations.PatternsModule.u_VarDecl@2070.Invoke(BindingEnv env)
D:\a\_work\1\s\src\FSharp.Core\quotations.fs(2048,0): at Microsoft.FSharp.Quotations.PatternsModule.u_Expr@2048-2.Invoke(BindingEnv env)
D:\a\_work\1\s\src\FSharp.Core\local.fs(236,0): at Microsoft.FSharp.Primitives.Basics.List.mapToFreshConsTail[a,b](FSharpList`1 cons, FSharpFunc`2 f, FSharpList`1 x)
D:\a\_work\1\s\src\FSharp.Core\local.fs(246,0): at Microsoft.FSharp.Primitives.Basics.List.map[T,TResult](FSharpFunc`2 mapping, FSharpList`1 x)
D:\a\_work\1\s\src\FSharp.Core\quotations.fs(2031,0): at Microsoft.FSharp.Quotations.PatternsModule.u_Expr@2030.Invoke(BindingEnv env)
D:\a\_work\1\s\src\FSharp.Core\local.fs(245,0): at Microsoft.FSharp.Primitives.Basics.List.map[T,TResult](FSharpFunc`2 mapping, FSharpList`1 x)
D:\a\_work\1\s\src\FSharp.Core\quotations.fs(2031,0): at Microsoft.FSharp.Quotations.PatternsModule.u_Expr@2030.Invoke(BindingEnv env)
D:\a\_work\1\s\src\FSharp.Core\local.fs(245,0): at Microsoft.FSharp.Primitives.Basics.List.map[T,TResult](FSharpFunc`2 mapping, FSharpList`1 x)
D:\a\_work\1\s\src\FSharp.Core\quotations.fs(2031,0): at Microsoft.FSharp.Quotations.PatternsModule.u_Expr@2030.Invoke(BindingEnv env)
D:\a\_work\1\s\src\FSharp.Core\quotations.fs(2059,0): at Microsoft.FSharp.Quotations.PatternsModule.u_Expr@2059-5.Invoke(BindingEnv env)
D:\a\_work\1\s\src\FSharp.Core\quotations.fs(2568,0): at Microsoft.FSharp.Quotations.PatternsModule.deserialize(Type localAssembly, Type[] referencedTypeDefs, Type[] spliceTypes, FSharpExpr[] spliceExprs, Byte[] bytes)
D:\a\_work\1\s\src\FSharp.Core\quotations.fs(2781,0): at Microsoft.FSharp.Quotations.FSharpExpr.Deserialize40(Type qualifyingType, Type[] referencedTypes, Type[] spliceTypes, FSharpExpr[] spliceExprs, Byte[] bytes)
at Tomra.Ts.Core.MapTests.Keys works as expected()
This is the test:
let [<Fact>] ``Keys works as expected``() =
let map = Map.ofList [ (1, "One"); (2, "Two"); (3, "Three") ]
test <@ map |> Map.keys |> List.ofSeq = [1; 2; 3] @>And this is the code being tested:
namespace global
open System
[<RequireQualifiedAccess>]
module Map =
let keys (map: Map<_, _>) =
seq {
for KeyValue(key, _) in map do
yield key
}Related information
I'm using .NET SDK 6.0.300 and originally experienced the problem when using F# 5. This week we were finally able to switch to F# 6 and I was secretly hoping this might fix the problem but unfortunately it did not.
Activity
bartelink commentedon Oct 10, 2026
Still happening in 10.0.303 SwensenSoftware/unquote#160 (comment)
NRE is on call to
getStampfsharp/src/FSharp.Core/quotations.fs
Lines 103 to 109 in a274432
Semi-related: #14362 (re guarantees and/or having warnings re triggering of initialization)
bartelink commentedon Oct 10, 2026
Test rig with a repro and my best [incomplete] ideas for a fix https://gist.github.com/bartelink/283ee9446a46d5b00a30a944501de611
tl;dr would involve stamp generation being spread over 2 files - no good ideas
doing a
static member valwithDefaultValuehas same issues