cmd/compile: generics + constant cycle causes panic in type checker

[adonovan]

https://go.dev/play/p/xnY6OXjtKPT

$ go version
go version go1.18.1 darwin/arm64

$ cat b.go 
package main

import "unsafe"

type S[T any] struct{}

const c = unsafe.Sizeof(S[[c]byte]{})

$ go tool compile b.go
<unknown line number>: internal compiler error: panic: unimplemented basic: invalid type (kind 0)

Please file a bug report including a short program that triggers the error.
https://go.dev/issue/new

[findleyr]

Huh, this reports an initialization cycle when type-checking. For example, after copying the above into cmd/compile/internal/types2/testdata/manual.go:

> go test -run=TestManual
--- FAIL: TestManual (0.00s)
    check_test.go:161: testdata/manual.go:14:7: initialization cycle for c
                testdata/manual.go:14:7: c refers to
                testdata/manual.go:14:7: c

How does it get past type-checking?

[findleyr]

Aha, full stack is below. It's getting to noder.gcSizes.Sizeof, despite the cycle error. Probably this is actually a types2 bug.

> go tool compile cmd/compile/internal/types2/testdata/manual.go
<unknown line number>: internal compiler error: panic: unimplemented basic: invalid type (kind 0)

goroutine 1 [running]:
runtime/debug.Stack()
        /home/rfindley/src/go/src/runtime/debug/stack.go:24 +0x65
cmd/compile/internal/base.FatalfAt({0x1?, 0x0?}, {0xd039b2, 0x9}, {0xc0001383d8, 0x1, 0x1})
        /home/rfindley/src/go/src/cmd/compile/internal/base/print.go:227 +0x1d7
cmd/compile/internal/base.Fatalf(...)
        /home/rfindley/src/go/src/cmd/compile/internal/base/print.go:196
cmd/compile/internal/gc.handlePanic()
        /home/rfindley/src/go/src/cmd/compile/internal/gc/main.go:48 +0x85
panic({0xc727e0, 0xc00010ac60})
        /home/rfindley/src/go/src/runtime/panic.go:866 +0x212
cmd/compile/internal/types2.(*Checker).handleBailout(0xc000432000, 0xc000139848)
        /home/rfindley/src/go/src/cmd/compile/internal/types2/check.go:295 +0x8b
panic({0xc727e0, 0xc00010ac60})
        /home/rfindley/src/go/src/runtime/panic.go:866 +0x212
cmd/compile/internal/noder.(*gcSizes).Sizeof(0xc000432000?, {0xe6b678, 0x12edb00})
        /home/rfindley/src/go/src/cmd/compile/internal/noder/sizes.go:96 +0x3e7
cmd/compile/internal/types2.(*Config).sizeof(0xe6b678?, {0xe6b678?, 0x12edb00?})
        /home/rfindley/src/go/src/cmd/compile/internal/types2/sizes.go:261 +0x31
cmd/compile/internal/types2.(*Checker).builtin(0xc000432000, 0xc00041b7c0, 0xc00041b700, 0x12)
        /home/rfindley/src/go/src/cmd/compile/internal/types2/builtins.go:716 +0x2adb
cmd/compile/internal/types2.(*Checker).callExpr(0xc000432000, 0xc00041b7c0, 0xc00041b700)
        /home/rfindley/src/go/src/cmd/compile/internal/types2/call.go:155 +0xcc9
cmd/compile/internal/types2.(*Checker).exprInternal(0xc000432000, 0xc00041b7c0, {0xe6c560?, 0xc00041b700}, {0x0?, 0x0?})
        /home/rfindley/src/go/src/cmd/compile/internal/types2/expr.go:1636 +0xbf1
cmd/compile/internal/types2.(*Checker).rawExpr(0xc000432000, 0xc00041b7c0, {0xe6c560?, 0xc00041b700?}, {0x0?, 0x0?}, 0x0)
        /home/rfindley/src/go/src/cmd/compile/internal/types2/expr.go:1251 +0x1a5
cmd/compile/internal/types2.(*Checker).expr(0xb4a998?, 0xc000432000?, {0xe6c560?, 0xc00041b700?})
        /home/rfindley/src/go/src/cmd/compile/internal/types2/expr.go:1793 +0x35
cmd/compile/internal/types2.(*Checker).constDecl(0xc000432000, 0xc0004087e0, {0x0?, 0x0?}, {0xe6c560, 0xc00041b700}, 0x0)
        /home/rfindley/src/go/src/cmd/compile/internal/types2/decl.go:387 +0x2ee
cmd/compile/internal/types2.(*Checker).objDecl(0xc000432000, {0xe70878?, 0xc0004087e0?}, 0x0?)
        /home/rfindley/src/go/src/cmd/compile/internal/types2/decl.go:191 +0x9b9
cmd/compile/internal/types2.(*Checker).packageObjects(0xc000432000)
        /home/rfindley/src/go/src/cmd/compile/internal/types2/resolver.go:685 +0x285
cmd/compile/internal/types2.(*Checker).checkFiles(0xc000432000, {0xc00011c460, 0x1, 0x1})
        /home/rfindley/src/go/src/cmd/compile/internal/types2/check.go:325 +0x14e
cmd/compile/internal/types2.(*Checker).Files(...)
        /home/rfindley/src/go/src/cmd/compile/internal/types2/check.go:300
cmd/compile/internal/types2.(*Config).Check(0xc000117908?, {0xd0685a?, 0x1?}, {0xc00011c460, 0x1, 0x1}, 0x0?)
        /home/rfindley/src/go/src/cmd/compile/internal/types2/api.go:417 +0x70
cmd/compile/internal/noder.checkFiles({0xc00011c438, 0x1, 0x203000?})
        /home/rfindley/src/go/src/cmd/compile/internal/noder/irgen.go:64 +0x45c
cmd/compile/internal/noder.check2({0xc00011c438, 0x1, 0x1})
        /home/rfindley/src/go/src/cmd/compile/internal/noder/irgen.go:77 +0x46
cmd/compile/internal/noder.LoadPackage({0xc000124010, 0x1, 0x1})
        /home/rfindley/src/go/src/cmd/compile/internal/noder/noder.go:84 +0x49c
cmd/compile/internal/gc.Main(0xd338f0)
        /home/rfindley/src/go/src/cmd/compile/internal/gc/main.go:190 +0xb13
main.main()
        /home/rfindley/src/go/src/cmd/compile/main.go:55 +0xdd

[griesemer]

This is a compiler bug: The compiler supplies its own size computation function to the type checker, and this size computation function assumes that a type cannot be invalid, which it is in this case. In contrast, the types2 size computation function has a catch-all for cases like these.

We just need to decide if a catch-all or adding the invalid case (adding an entry basicSizes in noder/sizes.go) to is the better solution.

[gopherbot]

Change https://go.dev/cl/404894 mentions this issue: cmd/compile: don't crash in size computation for invalid type