cmd/compile: inconsistent error messages based on type constraints #53692
Description
What version of Go are you using (go version)?
$ go version go version go1.18.3 darwin/arm64
Does this issue reproduce with the latest release?
Yes, including tip as of today
What operating system and processor architecture are you using (go env)?
go env Output
$ go envGO111MODULE="on"
GOARCH="arm64"
GOBIN=""
GOCACHE="/Users/sethvargo/Library/Caches/go-build"
GOENV="/Users/sethvargo/Library/Application Support/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="arm64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/sethvargo/Development/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/sethvargo/Development/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/opt/homebrew/Cellar/go/1.18.3/libexec"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/opt/homebrew/Cellar/go/1.18.3/libexec/pkg/tool/darwin_arm64"
GOVCS=""
GOVERSION="go1.18.3"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/dev/null"
GOWORK=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -arch arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/cp/qb9vbbkx4w36f6dclng481br00gy5b/T/go-build4094322222=/tmp/go-build -gno-record-gcc-switches -fno-common"
What did you do?
I suspect the root cause of my actual issue is #41176, but it would be great if @ianlancetaylor can confirm for me 😄.
Playground link for the code: https://go.dev/play/p/RqzqQxNapkw. Note this code does not compile.
Code:
package main
// Cache is a generic cache implementation.
type Cache[K comparable, V any] interface {
Get(K)
Set(K, V)
}
// LRU is a cache.
type LRU[K comparable, V any] struct{}
func (l *LRU[K, V]) Get(key K) {}
func (l *LRU[K, V]) Set(key K, val V) {}
// WithLocking1 returns a cache that wraps operations in a mutex.
func WithLocking1[K comparable, V any, C Cache[K, V]](cache C) {}
// WithLocking2 returns a cache that wraps operations in a mutex.
func WithLocking2[K comparable, V any](cache Cache[K, V]) {}
// WithLocking3 returns a cache that wraps operations in a mutex.
func WithLocking3[K comparable](cache Cache[K, any]) {}
func main() {
var lru LRU[string, int]
WithLocking1[string, int](&lru) // ok
WithLocking1[string](&lru) // nok: cannot infer V
WithLocking1(&lru) // nok: cannot infer K
WithLocking2[string, int](&lru) // ok
WithLocking2[string](&lru) // nok: does not match Cache[string, V]
WithLocking2(&lru) // nok: cannot infer K and V
WithLocking3[string](&lru) // nok: does not implement Cache[string, any] (wrong type for method Set)
// have Set(key string, val int)
// want Set(string, any)
WithLocking3(&lru) // nok: cannot infer K
}What did you expect to see?
As shown in the comments, the error messages are all subtly different depending on the type declarations on the function. I would expect the same error messages here.
What did you see instead?
Different error messages.
Aside
As an aside, it's unclear to me what the "correct" authorship is here. I really appreciate Go's "do one thing", but there seems to be multiple ways to define generics, all of which are subtly different with unknown consequences.