cmd/compile: misleading compiler error #51233
Comments
ALTree
added
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NeedsInvestigation
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There is no form of type inference that will figure out the additional type argument for That said I agree that the error message is pretty bad. |
ianlancetaylor
changed the title
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@ianlancetaylor something that's a bit confusing about this is that it only seems to be a compile error when I actually call Is this expected behavior? It's a bit weird to me that changing something outside of the struct definition can magically cause a compilation error in a struct definition that hasn't changed. |
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I just looked at this briefly. It looks like the bug is in the order of instantiation: where we try to substitute in the underlying of The error is not coming from type inference, which is why it only occurs if the field is used. |
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Change https://go.dev/cl/387918 mentions this issue: |
dmitshur
added
NeedsFix
What version of Go are you using (
go version)?Does this issue reproduce with the latest release?
Yes
What did you do?
https://go.dev/play/p/PzbJGM7V9dn?v=gotip
package main type RC[RG any] interface { ~[]RG } type Fn[RCT RC[RG], RG any] func(RCT) type FFn[RCT RC[RG], RG any] func() Fn[RCT] type F[RCT RC[RG], RG any] interface { Fn() Fn[RCT] } type concreteF[RCT RC[RG], RG any] struct { - makeFn FFn[RCT] + makeFn FFn[RCT, RG] } func (c *concreteF[RCT, RG]) Fn() Fn[RCT] { return c.makeFn() } func main() {}What did you expect to see?
Nothing.
What did you see instead?
If I change the line from the red version to the green version (adding the explicit type), then the compilation error disappears. I expected the compiler to figure out
RGsince it was part of the definition ofRCT.If type inference isn't able to figure this out, then it would be good to fix the compiler error to be less confusing.
makeFnis a function, the error is actually that the compiler can't infer how to instantiate it.Notes
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