go/types, types2: panic with generic alias

[griesemer]

The following code:

type T[P T[P]] = int

leads to a panic in the type checker. This requires tip allowing cycles through type parameters.

[odeke-em]

Thanks for the filing Robert! Just to note, this doesn't reproduce for Go1.25 hence is a new regression given

package main

type T[P T[P]] = int

func main() {}

which previously failed with

main.go:3:6: invalid recursive type: T refers to itself

now crashes as per commit 1abc6b0 with

$ go tool compile main.go 
The following panic happened checking types near:
	main.go:3:6
<unknown line number>: internal compiler error: panic: nil

goroutine 1 [running]:
runtime/debug.Stack()
	/Users/emmanuelodeke/go/src/go.googlesource.com/go/src/runtime/debug/stack.go:26 +0x5e
cmd/compile/internal/base.FatalfAt({0x7b64bd18?, 0x3a31?}, {0x10f36821, 0x9}, {0x3a317b64bd48, 0x1, 0x1})
	/Users/emmanuelodeke/go/src/go.googlesource.com/go/src/cmd/compile/internal/base/print.go:230 +0x18c
cmd/compile/internal/base.Fatalf(...)
	/Users/emmanuelodeke/go/src/go.googlesource.com/go/src/cmd/compile/internal/base/print.go:195
cmd/compile/internal/gc.handlePanic()
	/Users/emmanuelodeke/go/src/go.googlesource.com/go/src/cmd/compile/internal/gc/main.go:54 +0x88
panic({0x111214e0?, 0x111e45c0?})
	/Users/emmanuelodeke/go/src/go.googlesource.com/go/src/runtime/panic.go:858 +0x13a
cmd/compile/internal/types2.(*Checker).handleBailout(0x3a317ba50000, 0x3a317b64d158)
	/Users/emmanuelodeke/go/src/go.googlesource.com/go/src/cmd/compile/internal/types2/check.go:442 +0x2a7
panic({0x111214e0?, 0x111e45c0?})
	/Users/emmanuelodeke/go/src/go.googlesource.com/go/src/runtime/panic.go:858 +0x13a
cmd/compile/internal/types2.(*Checker).objDecl.func1()
	/Users/emmanuelodeke/go/src/go.googlesource.com/go/src/cmd/compile/internal/types2/decl.go:55 +0x5c
panic({0x111214e0?, 0x111e45c0?})
	/Users/emmanuelodeke/go/src/go.googlesource.com/go/src/runtime/panic.go:858 +0x13a
cmd/compile/internal/types2.(*typeWriter).error(...)
	/Users/emmanuelodeke/go/src/go.googlesource.com/go/src/cmd/compile/internal/types2/typestring.go:107
cmd/compile/internal/types2.(*typeWriter).typ(0x3a317b7f6ae0, {0x0, 0x0})
	/Users/emmanuelodeke/go/src/go.googlesource.com/go/src/cmd/compile/internal/types2/typestring.go:122 +0xad4
cmd/compile/internal/types2.(*typeWriter).typ(0x3a317b7f6ae0, {0x111eab50, 0x3a317ba49840})
	/Users/emmanuelodeke/go/src/go.googlesource.com/go/src/cmd/compile/internal/types2/typestring.go:347 +0x10ed
cmd/compile/internal/types2.(*Context).instanceHash(0x3a317b71f400, {0x111eab50, 0x3a317ba49840}, {0x3a317b7389e0, 0x1, 0x1})
	/Users/emmanuelodeke/go/src/go.googlesource.com/go/src/cmd/compile/internal/types2/context.go:75 +0xc7
cmd/compile/internal/types2.(*Checker).instance(0x3a317ba50000, {0x3a317b7f6600, 0x3, 0xa}, {0x111ebd80, 0x3a317ba49840}, {0x3a317b7389e0, 0x1, 0x1}, 0x0, ...)
	/Users/emmanuelodeke/go/src/go.googlesource.com/go/src/cmd/compile/internal/types2/instantiate.go:106 +0x235
cmd/compile/internal/types2.(*Checker).instantiatedType(0x3a317ba50000, {0x111ee658, 0x3a317ba4abe0}, {0x3a317b7389d0, 0x1, 0x1}, 0x10f3165f?)
	/Users/emmanuelodeke/go/src/go.googlesource.com/go/src/cmd/compile/internal/types2/typexpr.go:474 +0x505
cmd/compile/internal/types2.(*Checker).typInternal(0x3a317ba50000, {0x111ee6e8, 0x3a317ba43e00}, 0x0)
	/Users/emmanuelodeke/go/src/go.googlesource.com/go/src/cmd/compile/internal/types2/typexpr.go:294 +0x989
cmd/compile/internal/types2.(*Checker).definedType(0x3a317ba50000, {0x111ee6e8, 0x3a317ba43e00}, 0x13b265c0?)
	/Users/emmanuelodeke/go/src/go.googlesource.com/go/src/cmd/compile/internal/types2/typexpr.go:195 +0x2f
cmd/compile/internal/types2.(*Checker).typ(...)
	/Users/emmanuelodeke/go/src/go.googlesource.com/go/src/cmd/compile/internal/types2/typexpr.go:152
cmd/compile/internal/types2.(*Checker).bound(0x18?, {0x111ee6e8?, 0x3a317ba43e00?})
	/Users/emmanuelodeke/go/src/go.googlesource.com/go/src/cmd/compile/internal/types2/decl.go:653 +0x15c
cmd/compile/internal/types2.(*Checker).collectTypeParams(0x3a317ba50000, 0x3a317ba49850, {0x3a317b71c690, 0x1, 0xf?})
	/Users/emmanuelodeke/go/src/go.googlesource.com/go/src/cmd/compile/internal/types2/decl.go:627 +0x1f6
cmd/compile/internal/types2.(*Checker).typeDecl(0x3a317ba50000, 0x3a317ba4e0c0, 0x3a317ba43da0, 0x0)
	/Users/emmanuelodeke/go/src/go.googlesource.com/go/src/cmd/compile/internal/types2/decl.go:532 +0x65e
cmd/compile/internal/types2.(*Checker).objDecl(0x3a317ba50000, {0x111f32c0, 0x3a317ba4e0c0}, 0x0)
	/Users/emmanuelodeke/go/src/go.googlesource.com/go/src/cmd/compile/internal/types2/decl.go:199 +0xb59
cmd/compile/internal/types2.(*Checker).packageObjects(0x3a317ba50000)
	/Users/emmanuelodeke/go/src/go.googlesource.com/go/src/cmd/compile/internal/types2/resolver.go:689 +0x50b
cmd/compile/internal/types2.(*Checker).checkFiles(0x3a317ba50000, {0x3a317b64d2e0?, 0x599c93d93f33ef?, 0x118a6060?})
	/Users/emmanuelodeke/go/src/go.googlesource.com/go/src/cmd/compile/internal/types2/check.go:497 +0x285
cmd/compile/internal/types2.(*Checker).Files(0x1d0?, {0x3a317b64d2e0?, 0x0?, 0x0?})
	/Users/emmanuelodeke/go/src/go.googlesource.com/go/src/cmd/compile/internal/types2/check.go:460 +0x75
cmd/compile/internal/types2.(*Config).Check(0x3a317b768b60, {0x10f3c981?, 0x3a317b618057?}, {0x3a317b64d2e0, 0x1, 0x1}, 0x3a317ba4e000)
	/Users/emmanuelodeke/go/src/go.googlesource.com/go/src/cmd/compile/internal/types2/api.go:484 +0x1bd
cmd/compile/internal/noder.checkFiles({0x0, {0x0, 0x0}}, {0x3a317b71c680, 0x1, 0x3a317ba43b60?})
	/Users/emmanuelodeke/go/src/go.googlesource.com/go/src/cmd/compile/internal/noder/irgen.go:95 +0x675
cmd/compile/internal/noder.writePkgStub({0x0?, {0x0?, 0x0?}}, {0x3a317b71c680, 0x1, 0x1})
	/Users/emmanuelodeke/go/src/go.googlesource.com/go/src/cmd/compile/internal/noder/unified.go:318 +0x6a
cmd/compile/internal/noder.unified({0x0?, {0x0?, 0x0?}}, {0x3a317b71c680?, 0x1110a6e0?, 0x14?})
	/Users/emmanuelodeke/go/src/go.googlesource.com/go/src/cmd/compile/internal/noder/unified.go:194 +0xb6
cmd/compile/internal/noder.LoadPackage({0x3a317b71e0d0, 0x1, 0x1})
	/Users/emmanuelodeke/go/src/go.googlesource.com/go/src/cmd/compile/internal/noder/noder.go:77 +0x529
cmd/compile/internal/gc.Main(0x111e1c88)
	/Users/emmanuelodeke/go/src/go.googlesource.com/go/src/cmd/compile/internal/gc/main.go:206 +0xcc7
main.main()
	/Users/emmanuelodeke/go/src/go.googlesource.com/go/src/cmd/compile/main.go:57 +0x105

[griesemer]

Thanks, @odeke-em, I was aware of that. It didn't crash before because the offending code wasn't reached.
It also needs #75883 first.