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go/types: string(1 << s) should be an error #26096

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griesemer opened this issue Jun 27, 2018 · 4 comments
Open

go/types: string(1 << s) should be an error #26096

griesemer opened this issue Jun 27, 2018 · 4 comments
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NeedsFix The path to resolution is known, but the work has not been done.
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Unplanned

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@griesemer
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package main

func main() {
	var s uint
	_ = string(1 << s)
}

(https://play.golang.org/p/owHsmdZd32v) is an error: The 1 in 1 << s assumes the type it would have without the shift, which is string. Both cm/compile and gccgo correct report an error.

go/types appears to accept it.
@griesemer griesemer added the NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. label Jun 27, 2018
@griesemer griesemer added this to the Go1.12 milestone Jun 27, 2018
@griesemer griesemer self-assigned this Jun 27, 2018
@griesemer griesemer added the early-in-cycle A change that should be done early in the 3 month dev cycle. label Jun 27, 2018
@griesemer
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In this case (string(number)) where the string conversion is not simply changing the type of the argument but actively changing its value and representation, it would make sense for the shift rules to not look at the context. Or turning it around, maybe this kind of conversion should not be permitted in the first place, as proposed via #3939.

@griesemer griesemer mentioned this issue Jun 27, 2018
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@griesemer
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griesemer commented Jun 27, 2018 via email

@griesemer griesemer mentioned this issue Jun 27, 2018
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@griesemer
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Also related: #21982

@griesemer
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On (go/types) conversions.go:61 we select untyped int (the current type of 1 in string(1 << s)) as type for 1 (i.e., we don't change it). If we don't do anything special in this case (integer to string conversions), the type of 1 will become srtring and we get the expected error message.

Unfortunately, doing nothing there causes regular conversions (such as string('a')) to break because the argument to the conversion is not compatible with string anymore.

Fixing this requires a bit more subtle changes. Postponing for now since this is a) unlikely to occur in real programs, b) there are easy work-arounds, and c) this doesn't prevent go/types from accepting legal programs.

The issue may also become moot if we remove string(int/rune) conversions (#3939).

@griesemer griesemer added NeedsFix The path to resolution is known, but the work has not been done. and removed NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. early-in-cycle A change that should be done early in the 3 month dev cycle. labels Sep 12, 2018
@griesemer griesemer modified the milestones: Go1.12, Unplanned Sep 12, 2018
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