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go/types, types2: improved constant handling guarded by go1.21 but other inference improvements are not #60566

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griesemer opened this issue Jun 1, 2023 · 4 comments
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FrozenDueToAge NeedsDecision Feedback is required from experts, contributors, and/or the community before a change can be made.
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@griesemer
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griesemer commented Jun 1, 2023

The Go 1.21 compiler implements significantly more powerful and precise type inference than Go 1.20. One of the improvements made is for untyped constants (#58671). That change was implemented based on the language version: -lang must be >= go1.21.

func f[P any](...P) {}

func main() {
	f(1, 2.0)
}

produces an error (default type float64 of 2.0 does not match inferred type int for P) with -lang=go1.20 but runs fine with -lang >= go1.21.

However, there are plenty of other inference improvements that that were made for 1.21 which are accessible even if -lang < go1.21. For instance:

package main

func f[T any](interface{ m(T) }) {}

type S struct{}

func (S) m(int) {}

func main() {
	f(S{})
}

In this case the change and error is not fundamentally different from above: in Go 1.20 we couldn't infer a type argument and now we can. In both these cases one can provide explicit type arguments and make the code compile. Also, all inference improvements are fully backwards-compatible.

I propose that we remove the -lang check for the more lenient constant handling for consistency with the other inference improvements. (Making all improvements -lang dependent is not practical.)

@griesemer griesemer added the NeedsDecision Feedback is required from experts, contributors, and/or the community before a change can be made. label Jun 1, 2023
@griesemer griesemer added this to the Go1.21 milestone Jun 1, 2023
@griesemer griesemer self-assigned this Jun 1, 2023
@griesemer
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cc: @findleyr @ianlancetaylor for feedback

@griesemer griesemer changed the title go/types, types2: improved constant handling guarded by go1.21 but other inference imrovements are not go/types, types2: improved constant handling guarded by go1.21 but other inference improvements are not Jun 1, 2023
@gopherbot
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Change https://go.dev/cl/499997 mentions this issue: go/types, types2: remove version check for more lenient constant handling in inference

@ianlancetaylor
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I am OK with this.

@findleyr
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findleyr commented Jun 2, 2023

I am fine with this too. If it proves to be a problem, we can always put back the -lang check: making -lang more compatible with the released Go version should always be OK.

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