go/types: issue with using const with implicit unint64

[alanyee]

Go version

go version go1.22.2 darwin/amd64

Output of go env in your module/workspace:

GO111MODULE='on'
GOARCH='amd64'
GOBIN='/Users/admin/go_repos/bin'
GOCACHE='/Users/admin/Library/Caches/go-build'
GOENV='/Users/admin/Library/Application Support/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='amd64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMODCACHE='/Users/admin/go_repos/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='darwin'
GOPATH='/Users/admin/go_repos'
GOPRIVATE=''
GOPROXY=''
GOROOT='/usr/local/Cellar/go/1.22.2/libexec'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/usr/local/Cellar/go/1.22.2/libexec/pkg/tool/darwin_amd64'
GOVCS=''
GOVERSION='go1.22.2'
GCCGO='gccgo'
GOAMD64='v1'
AR='ar'
CC='cc'
CXX='c++'
CGO_ENABLED='1'
GOMOD='/Users/admin/go_something/go.mod'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -arch x86_64 -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -ffile-prefix-map=/var/folders/wq/5g8tq0nx0tn8rwrmbwg5fxzw0000gp/T/go-build1921388423=/tmp/go-build -gno-record-gcc-switches -fno-common'

What did you do?

1. Create a const with a value that makes it so that it has to be uint64 but do not explicitly state that it is uint64: https://go.dev/play/p/hB59P8SZ0Nd
2. Try to use the constant value directly

What did you see happen?

https://go.dev/play/p/hB59P8SZ0Nd does not compile:

./prog.go:13:36: cannot use MaxUint64 (untyped int constant 18446744073709551615) as int value in argument to fmt.Printf (overflows)

What did you expect to see?

I expected the code to compile and to see the test output of:

Type: uint64 Value 18446744073709551615
Type: uint64 Value 18446744073709551615

If you do not use the constant value directly, the code compiles, and the business logic works fine: https://go.dev/play/p/gTNfesGnMyj

[dmitshur]

CC @griesemer, @findleyr.

[findleyr]

This is working as intended. The default an untyped int is int, and so when MaxUint64 (which, despite its name, is an untyped int) is used as an argument to Printf, it is converted to an int.

By converting to a uint64 first, the code compiles:
https://go.dev/play/p/Zo6ge6kcksL

The relevant section of the spec describing this behavior is here:
https://go.dev/ref/spec#Constants

So it looks like go/types and the compiler are correctly implementing the spec.