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cmd/compile/internal/types2: "cannot infer T" message confusing at end: "([<nil>])" #49272

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danscales opened this issue Nov 1, 2021 · 2 comments
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@danscales
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danscales commented Nov 1, 2021

On go tip (generics), just a reminder that the "cannot infer T" message is confusing at the end:

./test.go:8:11: cannot infer T (test.go:3:8) ([])

I think this is being printed out from {types2,go/types}/infer.go, so the [] is the value of targs, which is the list of inferred and not-inferred type arguments. But especially when there is only one type argument that cannot be inferred, the string '([])' is not meaningful, and may make the user think there is some bigger problem.

If some of the types can be inferred, then the inferred types are included, as in: "([string ])". So, probably just need to print out the inferred args only (with a preceding label), and print out nothing if no targs were inferred.

Here's the simple example program:

package main

func g[T interface{ int64 | int32 }](x int64) T {
	return T(x)
}

func main() {
	println(g(43))
}

@griesemer @findleyr

@griesemer griesemer self-assigned this Nov 1, 2021
@griesemer griesemer added this to the Go1.18 milestone Nov 1, 2021
@griesemer griesemer added the NeedsFix The path to resolution is known, but the work has not been done. label Nov 1, 2021
@griesemer
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griesemer commented Nov 1, 2021

Fix forthcoming.

@gopherbot
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gopherbot commented Nov 1, 2021

Change https://golang.org/cl/360554 mentions this issue: go/types, types2: remove confusing debugging output in inference error message

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