I agree that it is an unfortunate error message, but to be clear, it is
entirely correct - you are defining a new type called `uint8` and it is
indeed not possible to assign an untyped integer constant to that, as its
underlying type is a type parameter (constrained on any).
I'm pretty sure there already is an issue about this, though I can't come
up with helpful search terms to look it up. But the short term fix is
absolutely "don't define types with the same name as predeclared idenfiers".
On Fri, Nov 10, 2023 at 3:25 PM ahuigo <a132...@gmail.com> wrote:
> Btw, please ignore the type logic in my code, I wrote this piece of code
> just to illustrate the oddities of generics in struct methods.
> Regardless of whether its type is int, string, any, *or the exact uint8,
> this error is very strange.*
>
>
> // it doesn't work
> func (c *cachedFn[uint8, V]) Get0() (V, error) {
> var s uint8 = 0
> s = 0 // error: cannot use 0 (untyped int constant) as uint8 value in
> assignment
> fmt.Printf("cache key: %#v, %T\n", s, s) // cache key: 0, uint8
> return c.Get(s)
> }
>
> // it works
> func (c *cachedFn[uint8, V]) Get0() (V, error) {
> var s uint8 = 0
> fmt.Printf("cache key: %#v, %T\n", s, s) // cache key: 0, uint8
> return c.Get(s)
> }
>
> On Friday, November 10, 2023 at 4:34:46 AM UTC+8 Axel Wagner wrote:
>
>> Yes, this has come up before.
>>
>> On Fri, Nov 10, 2023 at 7:09 AM ahuigo <a13...@gmail.com> wrote:
>>
>>> There is an example: https://go.dev/play/p/guzOWRKi-yp
>>>
>>> ```
>>> func (c *cachedFn[string, V]) Get0() (V, error) {
>>> // var s any
>>> var s string
>>> s = "abc" // error: cannot use "abc" (untyped string constant) as string
>>> value in assignment
>>> fmt.Printf("cache key: %#v, %T\n", s, s) // cache key: 0, uint8
>>> return c.Get(s)
>>> }
>>> ```
>>> I find the generic type of the struct method a bit confusing.
>>> 1. The type `cachedFn[string, V]` does not really constrain the type
>>> of `s` to **string**. It's actual type is `uint8`
>>>
>>
>> The type `cachedVn[string, V]` *would* in fact instantiate `cachedVn`
>> with `string` and `V`.
>> But that's not what you are doing. You are writing the receiver type as
>> `fun c(c *cachedFn[string, V])`, which means that "the receiver is the
>> generic type `cachedVn` with two type parameters called `string` and `V`".
>> Predeclared identifiers in Go are not special in any way, you can re-use
>> them for your own variables and types - or type parameters. So what you are
>> doing here is fundamentally similar to this problem:
>> https://go.dev/play/p/lDE-o7fGHi8
>>
>> There probably should be a vet check for using a predeclared identifier
>> as a type parameter name (or maybe even for any re-use of a predeclared
>> identifier).
>>
>> 2. And this error is a bit strange. (`s = "abc" // error: cannot use
>>> "abc" (untyped string constant) as string value in assignment. ` )
>>>
>>>
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