Internal error FS0073 "Undefined or unsolved type variable" with nested inline SRTP and multiple overloads

[gusty]

When an inline SRTP function dispatches through a type with multiple overloads where a method type parameter only appears in a "don't care" wildcard position (e.g. _ in App<LA, _>), the compiler crashes with:

error FS0073: internal error: Undefined or unsolved type variable: '?NNNNN'

This occurs during IL code generation when the optimizer's compulsory inlining phase resolves trait calls via CodegenWitnessExprForTraitConstraint. The bug requires two or more overloads on the dispatch type. A single overload compiles fine.

Repro steps

type App<'F, 'a> = | App of 'F * 'a

type LA = LA
type LB = LB

type D =
    static member inline Pur(_witness: App<LA, _>, x: 'a) : App<LA, 'a> = App(LA, x)
    static member inline Pur(_witness: App<LB, _>, x: 'a) : App<LB, list<'a>> = App(LB, [x])

let inline pur_impl (_mthd: ^M, output: ^F, x: 'a) : ^F
    when (^M or ^F) : (static member Pur : ^F * 'a -> ^F) =
    ((^M or ^F) : (static member Pur : ^F * 'a -> ^F) (output, x))

let inline pur (x: 'a) : ^F =
    pur_impl (Unchecked.defaultof<D>, Unchecked.defaultof< ^F>, x)

let result : App<LA, int> = pur 1

Removing either Pur overload makes it compile. The _ wildcard in App<LA, _> is key. It introduces an anonymous type parameter that never gets solved.

Expected behavior

The code should compile and run correctly.

Actual behavior

error FS0073: internal error: Undefined or unsolved type variable: '?31223'

Known workarounds

Remove one of the overloads, or avoid wildcard _ in positions that introduce method type parameters.

Related information

Not specific to any .NET version. Reproducilble in F# Interactive version 15.2.202.0 for F# 10.0