Can't use generic function for initializing field of generic object

[tsoj]

Description

The following snippet doesn't compile:

func what[T](): T =
    discard

type MyObject[T] = object
    f: T = what[T]()

var m: MyObject[float] = MyObject[float]()
echo m

However, this one works fine:

func what[T](): T =
    discard

type MyObject = object
    f: float = what[float]()

var m: MyObject = MyObject()
echo m

Nim Version

Nim Compiler Version 1.9.3 [Linux: amd64]
Compiled at 2023-05-27
Copyright (c) 2006-2023 by Andreas Rumpf

git hash: 2beea72
active boot switches: -d:release

Current Output

/tmp/testtest/main.nim(5, 12) Error: cannot instantiate: 'what[T]'

Expected Output

(vec: 0.0)

# or an easier to understand error message

Possible Solution

No response

Additional Information

No response

[Araq]

We only want to allow constant values in object field initializers anyway. Ping @ringabout

[ringabout]

f: T = what[T]() cannot be evaluated at the compile time when the object is defined, which might be improved by a better error message.

[tsoj]

Does this mean that even the second example with

type MyObject = object
    f: float = what[float]()

shouldn't compile, or is it just that any generic parameter can't be used for default initialization?

[ringabout]

The default value of object fields is handled by semConstExpr when it semchecks the definition of an object. The second example works because what[float]() is known at the compile time and is instantiated when it is evaluated. It is not the case of what[T]().

[beef331]

We only want to allow constant values in object field initializers anyway.

@Araq what about when it comes to things like mathematical vectors? If only constant values and not constant expressions are allowed anything that has it's own constructors procedures cannot be used.

[arnetheduck]

We only want to allow constant values in object field initializers anyway.

this seems like an unnecessary limitation, ie anything that evaluates to a constant should be fair game - there's no fundamental reason func f(): float = 42.0 shouldn't work as an initializer - generic or not.

[metagn]

Now crashes the compiler with Error: VM problem: dest register is not set, probably since #24010