Concepts with inheritance causing 'SIGSEGV: Illegal storage access ...'

[deech]

Concepts don't seem to handle subtypes very well, the following code:

type
   Foo = object of RootObj
   Bar = object of Foo

   FooOrBar = concept s, type T
     s is T
     f(s) is string

proc f(f: Foo): string = "Foo"

proc conceptf(f : FooOrBar): string =
  f(f)

let bar = Bar()
echo conceptf(bar)

generates the following error:

Hint: system [Processing]
Hint: concepttest [Processing]
SIGSEGV: Illegal storage access. (Attempt to read from nil?)

While Nim should not SIGSEGV when compiling this snippet I think concepts should not try and follow inheritance hierarchies and the line s is T should mean s is exactly T and not s is T or a subtype of T. Coming from Scala my experience is subtype + typeclasses (or concepts) causes horrendous type inference and makes concept resolution very hard to reason about.

TLDR: This compiler should not SIGSEGV on this code but it shouldn't typecheck either because concepts + subtyping leads to bad or hard to understand behavior.

[mratsim]

T is a compile-time type. At compile-time Bar is Foo, since concepts work at compile-time I don't see a way around your proposition, a normal proc would also accept both Foo and Bar.

[narimiran]

Currently (Nim 0.20.99) there is no SIGSEGV, but it produces the following: Error: unhandled exception: types.nim(1126, 10) b.kind in {tyObject} + skipPtrs [AssertionError].

[metagn]

The call to inheritanceDiff which has the assert is here, in transf.