When defining a set of generic Vector types, I came across this error after adding an inline function that internally uses an overloaded operator (e.g. op_Addition as in the example below) that take a vector and a constant ('T).

Repro steps

1. Run the following code in Visual Studio (F# Interactive):

type Vector<'T> =
    { X: 'T
      Y: 'T }
    static member inline (+) (u, a) =
        { X = u.X + a
          Y = u.Y + a }

module Vector =
    let inline add (vector: Vector<'T>) amount =
        vector + amount

Expected behavior

Should presumably not throw an internal error.

Actual behavior

F# encounters an internal error:

> error FS0073: internal error: Undefined or unsolved type variable: ^_?16934

Known workarounds

Any of the following steps prevent the internal error from occurring:

• Avoiding the overloaded operator
• Using a unary operator instead of a binary operator
• Removing the inline function
• Constraining the type variable of amount
• Not using a generic type variable for Vector

Related information

• Operating system - Windows 10
• F# 4.7.2
• .NET Runtime kind - .NET Core 3.1
• Editing Tools - Visual Studio 2019 16.5.3

Possibly related issue: #6648