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cmd/compile: constraints that are impossible to satisfy are not caught #67025

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mertcandav opened this issue Apr 24, 2024 · 2 comments
Open

cmd/compile: constraints that are impossible to satisfy are not caught #67025

mertcandav opened this issue Apr 24, 2024 · 2 comments
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compiler/runtime Issues related to the Go compiler and/or runtime. NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one.

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

$ go version
go version go1.20.3 darwin/arm64

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="arm64"
GOBIN=""
GOCACHE="/Users/mertcandav/Library/Caches/go-build"
GOENV="/Users/mertcandav/Library/Application Support/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="arm64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/mertcandav/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/mertcandav/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_arm64"
GOVCS=""
GOVERSION="go1.20.3"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/dev/null"
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 arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/jq/_8zwvbpd0qd5sd2yk4ypk7w40000gn/T/go-build1983794575=/tmp/go-build -gno-record-gcc-switches -fno-common"
GOROOT/bin/go version: go version go1.20.3 darwin/arm64
GOROOT/bin/go tool compile -V: compile version go1.20.3
uname -v: Darwin Kernel Version 23.4.0: Fri Mar 15 00:12:49 PDT 2024; root:xnu-10063.101.17~1/RELEASE_ARM64_T6020
ProductName:		macOS
ProductVersion:		14.4.1
BuildVersion:		23E224
lldb --version: lldb-1500.0.404.7
Apple Swift version 5.10 (swiftlang-5.10.0.13 clang-1500.3.9.4)

What did you do?

I used generic types that refer to each other. This will result in a cycle that is impossible to satisfy (please correct me if I am wrong). From what I've seen the compiler doesn't complain about this loop.

This isn't something I actually do in practice, I was just curious. I don't know if this is legal or reported before. I just thought it would be a good idea to let you know.

Here is Go Playground.

What did you expect to see?

I expect the compiler to complain that this isn't possible.

What did you see instead?

A compiler error regarding types incompatibility. It is not possible to fix this, but the compiler seems to leave the impression that it is possible.
@seankhliao seankhliao changed the title compiler: constraints that are impossible to satisfy are not catched cmd/compile: constraints that are impossible to satisfy are not catched Apr 24, 2024
@gopherbot gopherbot added the compiler/runtime Issues related to the Go compiler and/or runtime. label Apr 24, 2024
@seankhliao seankhliao changed the title cmd/compile: constraints that are impossible to satisfy are not catched cmd/compile: constraints that are impossible to satisfy are not caught Apr 24, 2024
@seankhliao seankhliao added the NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. label Apr 24, 2024
@randall77
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We do not promise to detect unsatisfiable type parameters. See https://go.googlesource.com/proposal/+/refs/heads/master/design/43651-type-parameters.md#empty-type-sets
My understanding is that is because the problem is in general NP-complete.
Your case does seem a simple case that could be caught. Probably not a high priority though, as any use is guaranteed to fail at compile time.

@go101
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go101 commented Apr 26, 2024

Spec mentions:

Implementation restriction: A compiler need not report an error if an operand's type is a type parameter with an empty type set. Functions with such type parameters cannot be instantiated; any attempt will lead to an error at the instantiation site.

See the end of this article for more empty type set examples.

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@randall77 @gopherbot @seankhliao @go101 @mertcandav