Compilation error when using type-generic proc in type definition

[rotu]

What happened?

Using prod in a type definition causes the compiler to fail with an uninformative error message.

import std/math

type Foo[S:static[array[2, int]]] = object
  values: array[prod(S), float]

echo sizeof(Foo[[4,5]])

Curiously, replacing prod with sum causes a different outcome. The below program erroneously prints 1:

import std/math

type Foo[S:static[array[2, int]]] = object
  values: array[sum(S), float]

echo sizeof(Foo[[4,5]])

Nim Version

Nim Compiler Version 1.7.3 [Windows: amd64]
Compiled at 2022-10-07
Copyright (c) 2006-2022 by Andreas Rumpf

active boot switches: -d:release

Current Standard Output Logs

C:\Users\dan\.choosenim\toolchains\nim-#devel\lib\pure\math.nim(357, 3) Error: cannot generate VM code for result

Expected Standard Output Logs

No response

Possible Solution

No response

Additional Information

No response

[rotu]

possibly addressed by #20423?

[beef331]

It's not ideal but a common workaround is to use a template to intercept the typed code so it can actually compile as such:

import std/math

template arr(a: typed, b: typedesc): auto = array[a, b]

type Foo[S:static[array[2, int]]] = object
  values: arr(prod(S), float)

echo sizeof(Foo[[4,5]])

[rotu]

@beef331 Thanks! Any insight as to why the compiler chokes in the first place?

I wound up wrapping prod instead, which seems to also work:

proc intProd(s:seq[int]):int = prod[int](s)

type Foo[S:static[array[2, int]]] = object
  values: array[intProd(S), float]

[bung87]

possibly addressed by #20423?

this one may not related to my PR, that one main problem is inner generic doesn't get static T as outter gets, it only get T

[metagn]

Related #19916 (generic object, static parameter, array index)