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cmd/compile: compiler does not reject overflowing type-parameter conversion #49247

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ALTree opened this issue Oct 31, 2021 · 3 comments
Closed

cmd/compile: compiler does not reject overflowing type-parameter conversion #49247

ALTree opened this issue Oct 31, 2021 · 3 comments
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FrozenDueToAge NeedsFix The path to resolution is known, but the work has not been done. release-blocker
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Go1.18

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@ALTree
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ALTree commented Oct 31, 2021 

$ gotip version
go version devel go1.18-8e3d5f0bb3 Sun Oct 31 08:29:02 2021 +0000 linux/amd64

According to my reading of the type parameters proposal, specifically this section:

Untyped constants

Some functions use untyped constants. An untyped constant is permitted with a value of a type parameter if it is permitted with every type in the type set of the type parameter's constraint.

// This function is INVALID.
func Add1024[T integer](s []T) {
  for i, v := range s {
  	s[i] = v + 1024 // INVALID: 1024 not permitted by int8/uint8
  }
}

This program should be rejected by the compiler:

package main

func f[T interface{ int8 }]() {
	println(T(1024))
}

func main() {
	f[int8]()
}

but it isn't, and it prints 0. If T is manually replaced with int8, the compiler errors with:

cannot convert 1024 (untyped int constant) to int8

cc @griesemer @ianlancetaylor
@ALTree ALTree added NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. release-blocker labels Oct 31, 2021
@ALTree ALTree added this to the Go1.18 milestone Oct 31, 2021
@griesemer griesemer self-assigned this Oct 31, 2021
@griesemer griesemer added NeedsDecision Feedback is required from experts, contributors, and/or the community before a change can be made. 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. release-blocker NeedsDecision Feedback is required from experts, contributors, and/or the community before a change can be made. labels Oct 31, 2021
@griesemer
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This is just a bug. Note that this code:

package p
func _[T ~byte]() {
	var _ T = 256 // ERROR: cannot use 256 (untyped int constant) as T value in variable declaration
}

is correctly rejected, but:

package p
func _[T ~byte]() {
	var _ T = T(256)
}

with an explicit conversion is accepted. The reason is that a value of type parameter type cannot be a constant value, so the conversion makes T(256) a non-constant value. But the switch from constant to non-constant happens too early, and then 256 is not correctly rejected.

@griesemer
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Hm. This will require a bit of code shuffling to get right, after all. Marking as release blocker again.

@gopherbot
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Change https://golang.org/cl/360396 mentions this issue: cmd/compile/internal/types2: fix conversions of constants to type parameter

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