go: Untyped consts should not be automatically type converted

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

$ go version
go version go1.15 linux/amd64

Does this issue reproduce with the latest release?

Yes.

What operating system and processor architecture are you using (`go env`)?

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/.cache/go-build"
GOENV="/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build533260346=/tmp/go-build -gno-record-gcc-switches"

What did you do?

https://play.golang.org/p/iLPNLfxLl5e

What did you expect to see?

All five calls to foo should have been compiler errors due to an implicit conversion from int to time.Duration.

What did you see instead?

If the variable in question is a const, it is automatically type converted. In our case, this lead to a bug.

I understand that this is as per the Go spec:

A value x is assignable to a variable of type T (“x is assignable to T”) if one of the following conditions applies: […] x is an untyped constant representable by a value of type T.

However, since this can easily lead to a bug, it would be nice if go vet at least warned about this situation.

About this issue

Original URL
State: closed
Created 4 years ago
Reactions: 3
Comments: 16 (10 by maintainers)

Most upvoted comments

time.Duration is one particular case. Existing code uses many types that are intended to support conversion from untyped constants, as well as many types that are not intended to support conversion from untyped constants. At this point we can’t arbitrarily declare that all such conversions are invalid. That would break working, correct code. And we can’t make that change in “go vet” either.

ianlancetaylor on Sep 1, 2020

@rittneje Run staticcheck (https://staticcheck.io/). It has checks for many things, including apparent misuses of time.Duration.