inline implicit conversion infers `Nothing` for covariant types

[soronpo]

Compiler version

v3.3.1
v3.4.0-RC1

Minimized code

import language.implicitConversions
class Foo[+T]

implicit inline def conv[T, R](from: R): Foo[T] = 
  compiletime.summonInline[T =:= Int]
  new Foo[T]

val f: Foo[Int] = "o" //error

Output

Cannot prove that Nothing =:= Int.

Expectation

No error.

[soronpo]

I started working on this, and after enabling some of the traces, the problem is somewhere in interpolateTypeVars, where the type variables are instantiated in doInstantiate.
I see the following:

adapt to subtype ("o" : String) !<:< Foo[Int]
interpolate conv[T, R]("o"): Foo[T] in TS[131, 1, 0], pt = Foo[Int], owned vars = T, R, qualifying = T, R, previous =  /  uninstantiated variables: T, R
 constrained types: [T, R](from: R): Foo[T]
 bounds:
     T <: Int
     R >: ("o" : String)
 ordering:
 co-deps:
 contra-deps:

interpolate T in TS[131, 1, 0] in conv[T, R]("o"): Foo[T], fromBelow = true,  uninstantiated variables: T, R
 constrained types: [T, R](from: R): Foo[T]
 bounds:
     T <: Int
     R >: ("o" : String)
 ordering:
 co-deps:
 contra-deps:

approx T, from below = true, inst = Nothing
instantiating T with Nothing
interpolate non-occurring R in TS[131, 1, 0] in conv[Nothing, R]("o"): Foo[Nothing], fromBelow = true,  uninstantiated variables: R
 constrained types: [T, R](from: R): Foo[T]
 bounds:
     T := Nothing
     R >: ("o" : String)
 ordering:
 co-deps:
 contra-deps:

[soronpo]

FWIW, the new-style of conversions has the same problem:

import language.implicitConversions
class Foo[+T]

inline given DFBitValConversion[R, T]: Conversion[R, Foo[T]] = 
  compiletime.summonInline[T =:= Int]
  ??? 

val f: Foo[Int] = "o" //error

[nicolasstucki]

The type inference is correct based on the constraints available at call site. In general any type T between Nothing <:< T <:< Int would be ok.

import language.implicitConversions
class Foo[+T]

inline given DFBitValConversion[R, T]: Conversion[R, Foo[T]] =  ???

val f: Foo[Int] = "o" // DFBitValConversion[String, Nothing].apply("o") // inferred
val f2: Foo[Int] = DFBitValConversion[String, Nothing].apply("o") // ok
val f3: Foo[Int] = DFBitValConversion[String, Int].apply("o") // ok