cmd/compile: panic when using typeparams

[imjasonh]

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

$ go version
go version devel go1.17-5b1120fac7 Fri May 21 23:25:07 2021 +0000 darwin/amd64

Compiled from source from the dev.typeparams branch.

Does this issue reproduce with the latest release?

No; support for typeparams is not yet released

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

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/jasonhall/Library/Caches/go-build"
GOENV="/Users/jasonhall/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/jasonhall/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/jasonhall/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/Users/jasonhall/git/goroot"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/Users/jasonhall/git/goroot/pkg/tool/darwin_amd64"
GOVCS=""
GOVERSION="devel go1.17-5b1120fac7 Fri May 21 23:25:07 2021 +0000"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/jasonhall/git/client-go2/go.mod"
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 x86_64 -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/bb/l0yfqc6d6kb5zm5f9ktyx0nw0000gn/T/go-build4116573109=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

I attempted to write a Go program using type params. This example is distilled from real-world code to attempt to isolate the bug.

package main

func main() {
        _ = New[*foo]()
}

type i interface { Foo() }
type foo struct {}
func (f *foo) Foo() {} // satisfy interface

func New[T i]() str[T] {
        return str[T]{}
}

type str[T i] struct {
        s string
}

(running with go run -gcflags=-G=3 test.go as recommended in #46346)

What did you expect to see?

I expected to see a successfully compiled program that exits ~immediately.

What did you see instead?

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0xa0 pc=0x11a91c2]

goroutine 1 [running]:
cmd/compile/internal/ir.(*Name).TypeDefn(0x18da9c0)
	/Users/jasonhall/git/goroot/src/cmd/compile/internal/ir/name.go:146 +0x22
cmd/compile/internal/types.findTypeLoop(0xc0000ad0a0, 0xc00011d540)
	/Users/jasonhall/git/goroot/src/cmd/compile/internal/types/size.go:239 +0x290
cmd/compile/internal/types.reportTypeLoop(0xc0000ad0a0)
	/Users/jasonhall/git/goroot/src/cmd/compile/internal/types/size.go:277 +0x65
cmd/compile/internal/types.CalcSize(0xc0000ad0a0)
	/Users/jasonhall/git/goroot/src/cmd/compile/internal/types/size.go:432 +0x68b
cmd/compile/internal/walk.convas(0xc0003d19f0, 0xc00011d810)
	/Users/jasonhall/git/goroot/src/cmd/compile/internal/walk/walk.go:96 +0x27b
cmd/compile/internal/walk.ascompatee(0x16, {0xc000094530, 0x0, 0x8}, {0xc000094540, 0x380bc90, 0xc0000c3218})
	/Users/jasonhall/git/goroot/src/cmd/compile/internal/walk/assign.go:372 +0x6e6
cmd/compile/internal/walk.walkAssignList(0xc00011dad8, 0xc000093680)
	/Users/jasonhall/git/goroot/src/cmd/compile/internal/walk/assign.go:146 +0x12f
cmd/compile/internal/walk.walkExpr1({0x1a4a760, 0xc000093680}, 0xc00011dad8)
	/Users/jasonhall/git/goroot/src/cmd/compile/internal/walk/expr.go:179 +0x2fc
cmd/compile/internal/walk.walkExpr({0x1a4a760, 0xc000093680}, 0xc00011dad8)
	/Users/jasonhall/git/goroot/src/cmd/compile/internal/walk/expr.go:55 +0x3a5
cmd/compile/internal/walk.walkStmt({0x1a4a760, 0xc000093680})
	/Users/jasonhall/git/goroot/src/cmd/compile/internal/walk/stmt.go:59 +0x57e
cmd/compile/internal/walk.walkStmtList({0xc0003e0b80, 0x6, 0xc000096420})
	/Users/jasonhall/git/goroot/src/cmd/compile/internal/walk/stmt.go:169 +0x5b
cmd/compile/internal/walk.Walk(0xc000096420)
	/Users/jasonhall/git/goroot/src/cmd/compile/internal/walk/walk.go:43 +0xef
cmd/compile/internal/gc.prepareFunc(0xc000096420)
	/Users/jasonhall/git/goroot/src/cmd/compile/internal/gc/compile.go:92 +0x6d
cmd/compile/internal/gc.enqueueFunc(0xc000096420)
	/Users/jasonhall/git/goroot/src/cmd/compile/internal/gc/compile.go:66 +0x2f7
cmd/compile/internal/gc.Main(0x1913c48)
	/Users/jasonhall/git/goroot/src/cmd/compile/internal/gc/main.go:281 +0xe05
main.main()
	/Users/jasonhall/git/goroot/src/cmd/compile/main.go:55 +0xdd

---

Perhaps interestingly, slight variations on the code seem to compile without issue. Any of these changes result in successful compilation:

1. Removing the s string field from the str[T i] type.
2. Bypassing the New method and calling _ = str[*foo]{} directly instead.
3. Making foo implement i instead of *foo, and calling _ = New[foo]().

cc @ianlancetaylor @griesemer

[ianlancetaylor]

Thanks. The type parameters work is under very active development by a number of people. I don't think it is useful at this time to file issues about problems using type parameters.

[imjasonh]

Good to know. Thanks! Feel free to close.

…

On Mon, May 24, 2021 at 5:38 PM Ian Lance Taylor ***@***.***> wrote: Thanks. The type parameters work is under very active development by a number of people. I don't think it is useful at this time to file issues about problems using type parameters. — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub <#46353 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AABTOMOJ6LZIETD6YHQ7XE3TPLBOPANCNFSM45N4G7NA> .