Can't use addition (substruction) multiple times in member for constrained to `INumber<>` generic

[gurustron]

Steps required to reproduce the problem

type Test<'T when 'T:> INumber<'T>> = 
 { One: 'T;
 Two: 'T; 
 Three: 'T } with
 member this.ThreeSum = this.One + this.Two + this.Three 

Expected behavior

Compiles

Actual behavior

error FS0670: This code is not sufficiently generic. The type variable ^T when ^T :> INumber<^T> could not be generalized because it would escape its scope.

Known workarounds

Intermediate member or using type-defined methods:

type Test<'T when 'T:> INumber<'T>> = 
 { One: 'T;
 Two: 'T; 
 Three: 'T } with
 member this.TwoSum = this.One + this.Two
 member this.ThreeSum = this.TwoSum + this.Three
 member this.ThreeSum1 = 'T.op_CheckedAddition('T.op_CheckedAddition(this.One, this.Two), this.Three)

Related information

Provide any related information (optional):

• Operating system: Windows 11
• .NET Runtime kind (.NET Core, .NET Framework, Mono)
  .NET SDK:
  Version: 8.0.100
  Commit: 57efcf1350
  Workload version: 8.0.100-manifests.8d38d0cc
• Editing Tools Rider/CLI

Extra notes

inline member with + constraint works:

type Test1<'T when 'T : (static member (+) : 'T -> 'T -> 'T)> = 
 { One: 'T;
 Two: 'T; 
 Three: 'T } with
 member inline this.TwoSum = this.One + this.Two + this.Three

But similar approach with INumber<> fails:

type Test<'T when 'T:> INumber<'T>> = 
 { One: 'T;
 Two: 'T; 
 Three: 'T } with
 member inline this.ThreeSum1 = this.One + this.Two - this.Three

error FS0339: The signature and implementation are not compatible because the type parameter in the class/signature has a different compile-time requirement to the one in the member/implementation

[gurustron]

From SO question

[github-actions]

🤖 This is an automated response from Repo Assist.

This bug is still present in F# 10.0 (fsc 15.2.201.0, SDK 10.0.201, Linux/net10.0).

Exact repro:

open System.Numerics

type Test<'T when 'T :> INumber<'T>> = 
 { One: 'T; Two: 'T; Three: 'T } with
 member this.ThreeSum = this.One + this.Two + this.Three
```

**Error:**
```
error FS0670: This code is not sufficiently generic. The type variable ^T when ^T :> INumber<^T>
could not be generalized because it would escape its scope.

The + operator on INumber<'T> is a static abstract member, so it goes through SRTP. The first this.One + this.Two produces an intermediate SRTP call result whose type variable is constrained to ^T. When the second + this.Three is applied, the type variable for the intermediate result can no longer be generalised — the two chained + calls create a type variable that "escapes" the member's scope.

The workaround (using an intermediate let binding or chaining via 'T.op_CheckedAddition(...)) avoids the issue but shouldn't be necessary.

Generated by Repo Assist · ◷

To install this agentic workflow, run

gh aw add githubnext/agentics/workflows/repo-assist.md@9135cdfde26838a01779aa966628308404ec1f02