Null value returned from trait call

There are some cases where an invalid value is returned from a trait call.

Repro steps

Step A

try in fsi

type A = A with
    static member ($) (A, a: float  ) = 0.0
    static member ($) (A, a: decimal) = 0M
    static member ($) (A, a: 't     ) = 0

let inline call x = ($) A x

send it

Step B

Now try this call 42.

Expected behavior

val it : float = 0.0

Actual behavior

val it : obj = <null>

Known workarounds

I haven't found one so far.

Operating system: Windows
FSI: F# Interactive version 11.4.2.0 for F# 5.0
Editing Tools: Visual Studio 2019

I didn't try in older versions, but I'm under the impression that this is a regression.

And here's a repro to send to FSI in one shot

type A = A with
    static member ($) (A, a: float  ) = 0.0
    static member ($) (A, a: decimal) = 0M
    static member ($) (A, a: 't     ) = 0

let inline call x = ($) A x


call 42.  + 1.1

Which results in

type A =
  | A
  with
    static member ( $ ) : A:A * a:'t -> int + 2 overloads
  end
val inline call :
  x: ^a ->  ^_arg3
    when (A or  ^a) : (static member ( $ ) : A *  ^a ->  ^_arg3)
val it : float = nan

when the expeced result should be 1.1

@gusty

@gusty I have tried today with on a script and on fsi and seems to be fixed now :)
FYI @vzarytovskii

Yes, it works for me as well.
I wonder what was the fixing PR.

The issue seems to be reproducing per comments above. Should we add regression tests for the cases reported there instead of closing the issue?

@T-Gro

I can confirm, it still repro in F# 10

@T-Gro please re-open

Add regression test for #12386: SRTP trait call correct overload reso…

Fix SRTP overload resolution across FSI submissions (#12386)

The bot was wrong (the repro needed a multi-submit FSI session, it did not reproduce in any other constellation).
The solution is to treat interactive similar to how lib+app scenario works by having fresh solutions to type parameter constraints (lib+app scenario handles this via pickling that always refreshes).

Good news is that fix is on the way.

🤖 This is an automated response from Repo Assist.

Removing the AI-thinks-issue-fixed label. The previous Repo Assist comment was incorrect — the original repro (multi-submit FSI session) was not properly tested. As @auduchinok and @gusty confirmed, the issue still reproduces in F# 10/11. @T-Gro has identified the root cause and a fix is in progress (#19471).

Apologies for the false positive.

Tested on: d507366 (2026-04-01), F# Interactive 15.2.100.0 for F# 11.0, FSharp.Core 11.0.100.

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Fix SRTP overload resolution across FSI submissions (#12386)

Add regression test for #12386: SRTP trait call correct overload reso…

Fix dotnet#12386, SRTP trait call correct overload resolution in mult…

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Null value returned from trait call #12386

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gusty commented on Nov 13, 2021

edgarfgp commented on Sep 19, 2022

gusty commented on Sep 19, 2022

9 remaining items

github-actions commented on Mar 22, 2026

auduchinok commented on Mar 23, 2026

gusty commented on Mar 23, 2026

T-Gro commented on Mar 26, 2026

github-actions commented on Apr 1, 2026

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