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cmd/compile: confusing error message for non-bool comparison #41500

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smasher164 opened this issue Sep 19, 2020 · 3 comments
Closed

cmd/compile: confusing error message for non-bool comparison #41500

smasher164 opened this issue Sep 19, 2020 · 3 comments

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@smasher164
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@smasher164 smasher164 commented Sep 19, 2020

What version of Go are you using (go version)?

go version go1.15.2 darwin/amd64

What did you do?

I tried to compile the following program: https://play.golang.org/p/k1xIda2_lTk

package main

type Struct struct {
	Slice []int
}

func main() {
	var x *Struct
	if x == nil || len(x.Slice) {
	}
}

I forgot to add len(x.Slice) == 0.

What did you expect to see?

non-bool len(x.Slice) (type int) used as if condition

What did you see instead?

cannot use x == nil (type untyped bool) as type int
@cuonglm
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@cuonglm cuonglm commented Sep 20, 2020

This is at least, working as intended per Go spec. x == nil yields an untyped bool, len(x.Slice) yields an int, so x == nil is first converted to int, which produces that error.

I'm not sure is there anything we should fix here cc @mdempsky

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@mdempsky
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@mdempsky mdempsky commented Sep 20, 2020

The issue here is when type-checking binary operations like X op Y, the Go compiler:

  1. Checks that X and Y have the same type.
  2. Checks that op is a valid operation on X and Y's type.

As @cuonglm points out, x == nil has type "untyped bool", so we try to convert it to "int". But that's not allowed, hence the error.

In this case though, that leads to a slightly suboptimal error message. Because || and && are only valid on boolean types anyway, so it's perhaps a bit misleading to users to focus on converting "untyped bool" to int, rather than not being able to use int as a bool.

I suspect a check like:

if n.Op == OANDAND || n.Op == OOROR {
    if !n.Left.Type.IsBoolean() {
        yyerror("non-bool %L used as %v operand", n.Left, n.Op)
        n.Type = nil
        break
    }
    if !n.Right.Type.IsBoolean() { ... }
}

before the defaultlit2 call might give an error message similar to the suggested one.

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@gopherbot
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@gopherbot gopherbot commented Sep 21, 2020

Change https://golang.org/cl/255899 mentions this issue: cmd/compile: clearer error when non-bool used as "||" and "&&" operand

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@gopherbot gopherbot closed this in 23573d0 Sep 22, 2020
@golang golang locked and limited conversation to collaborators Sep 22, 2021
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