Regression with constraints produced by Nullable

[gusty]

Repro steps

Run the following in fsi:

open System

type JsonValue = JNull

type JsonEncoding = JsonEncoding with
    static member nullableE encoder (x: Nullable<'a>) = if x.HasValue then encoder x.Value else JNull
    static member nullable  codec = JsonEncoding.nullableE codec

Expected behavior

Compile OK

Actual behavior

~vs3D37.fsx(7,37): error FS0193: Type constraint mismatch. The type 
    ''a'    
is not compatible with type
    'Nullable<'a>'    

Known workarounds

• Don't upgrade Visual Studio
• Transform the first method to a let bind

Related information

• Operating system: Windows
• Editing Tools (e.g. Visual Studio Version, Visual Studio): Works in VS 16.8.3, doesn't work in VS 16.9.3

[cartermp]

Only speculative, but #10582 may be related, since this was the only tweak that I know of from VS 16.8 until VS 16.9. Confirmed that the old code used to work, but now does not.

[cartermp]

cc @TIHan, @dsyme

[TIHan]

I can confirm that #10582 is what caused this regression. Am looking at this.