go/types: alias TypeName has nil typ

[adonovan]

#!stacks
"gopanic" && "go/types.(*typeWriter).error:=110"

Issue created by stacks.

This looks like an alias bug in the type checker: the crash indicates that a TypeName's obj.typ field is nil.

func (w *typeWriter) typ(typ Type) {
...	switch t := typ.(type) {
	case nil:
		w.error("nil") // panics

Called from:

func (w *typeWriter) typ(typ Type) {
...
	case *Alias:
		w.typeName(t.obj) // dereferences t.obj.{pkg,name}
	...
		w.typ(Unalias(t.obj.typ)) // w.typ arg was nil => Unalias returned nil => t.obj.typ is nil

It's also possibly just another instance of memory corruption.

This stack UAPFHA was reported by telemetry:

• crash/crash
• runtime.gopanic:+70,+0x16e
• go/types.(*Checker).handleBailout:+25,+0x90
• go/types.(*Checker).Files.deferwrap1:+0,+0x1a
• runtime.gopanic:+51,+0x139
• go/types.(*Checker).objDecl.func1:+3,+0x5b
• runtime.gopanic:+51,+0x139
• go/types.(*typeWriter).error:=110,+0xab8
• go/types.(*typeWriter).typ:+10,+0x98b
• go/types.(*typeWriter).typ:+235,+0xd04
• go/types.(*Context).instanceHash:+10,+0xc7
• go/types.(*Checker).instance:+19,+0x23c
• go/types.(*Checker).instantiatedType:+35,+0x439
• go/types.(*Checker).typInternal:+57,+0x133e
• go/types.(*Checker).declaredType:+1,+0x2e
• go/types.(*Checker).varType:+1,+0x24
• go/types.(*Checker).typInternal:+121,+0xc54
• go/types.(*Checker).declaredType:+1,+0x2e

golang.org/x/tools/gopls@v0.21.0 go1.26rc2 linux/amd64 gopls.client=neovim (1)

Use this command to reproduce the executable:
(HOME=$(mktemp -d); GOOS=linux GOARCH=amd64 GOTOOLCHAIN=go1.26rc2 go install golang.org/x/tools/gopls@v0.21.0 && find $HOME/go/bin -type f)
To disassemble: go tool objdump exe | less
Dups: 1VpMXw 0R0w6A HwXxCg Uwn_Cw A1V8tg

[gabyhelp]

Related Issues

• x/tools/gopls: SEGV in (*types.*Checker).Ident due to nil or invalid *ast.Ident in syntax tree #77078
• x/tools/gopls: impossible panic reading rtype.hash in type assertion #76328
• x/tools/gopls: impossible crash in (*types.Checker).basicLit #76329
• x/tools/gopls: impossible panic in BlockStmt.End (type checking IfStmt with nil Body) #76327
• go/types: failed genericType assertion in Checker.instantiatedType #71198 (closed)
• x/tools/gopls: SEGV loading m.used field from non-nil map pointer (in types.Scope.elems map) #76979
• cmd/compile: unreachable panic with GODEBUG=gotypesalias=1 #65778 (closed)
• x/tools/gopls: objectpath: panic due to nil *types.Signature #75575
• x/tools/gopls: Hover: nil deref in typeDeclContent #74056 (closed)
• x/tools/gopls: SEGV in go/types due to invalid pointer in BinaryExpr.Y #76238

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[mrkfrmn]

Just based on the stack, it looks like it has to do with instantiation. I sifted through the program generator I wrote and found this which panics at the same place:

type t0 t3
type t2[p any] t2[func(func() t3)]
type t3 = t2[t0]

Interestingly, if that alias is changed to a defined type, it reports a cycle rather than a panic. Will dig a bit more.

[mrkfrmn]

Another reproducer — seems this is an older issue around declaration ordering. If you remove the first line (t0), which seems like it should have no impact, there's no panic.

type t0 t2
type t1[p any] t1[t2]
type t2 = t1[int]

[adonovan]

Found new stacks for predicate:

#!stacks
"gopanic" && "go/types.(*typeWriter).error:=110"

This stack 1VpMXw was reported by telemetry:

• crash/crash
• runtime.gopanic:+70,+0x16e
• go/types.(*Checker).handleBailout:+25,+0x90
• go/types.(*Checker).Files.deferwrap1:+0,+0x1a
• runtime.gopanic:+51,+0x139
• go/types.(*Checker).objDecl.func1:+3,+0x5b
• runtime.gopanic:+51,+0x139
• go/types.(*typeWriter).error:=110,+0xab8
• go/types.(*typeWriter).typ:+10,+0x98b
• go/types.(*typeWriter).typ:+235,+0xd04
• go/types.(*Context).instanceHash:+10,+0xc4
• go/types.(*Checker).instance:+19,+0x23c
• go/types.(*Checker).instantiatedType:+35,+0x42d
• go/types.(*Checker).typInternal:+57,+0x128b
• go/types.(*Checker).declaredType:+1,+0x2e
• go/types.(*Checker).varType:+1,+0x24
• go/types.(*Checker).typInternal:+98,+0x453
• go/types.(*Checker).declaredType:+1,+0x2e

golang.org/x/tools/gopls@v0.21.1 go1.26.1 linux/amd64 gopls.client=neovim,other,vscode (1)

Use this command to reproduce the executable:
(HOME=$(mktemp -d); GOOS=linux GOARCH=amd64 GOTOOLCHAIN=go1.26.1 go install golang.org/x/tools/gopls@v0.21.1 && find $HOME/go/bin -type f)
To disassemble: go tool objdump exe | less

This stack 0R0w6A was reported by telemetry:

• crash/crash
• runtime.gopanic:+70,+0x16e
• go/types.(*Checker).handleBailout:+25,+0x90
• go/types.(*Checker).Files.deferwrap1:+0,+0x1a
• runtime.gopanic:+51,+0x139
• go/types.(*Checker).objDecl.func1:+3,+0x5b
• runtime.gopanic:+51,+0x139
• go/types.(*typeWriter).error:=110,+0xab8
• go/types.(*typeWriter).typ:+10,+0x98b
• go/types.(*typeWriter).typ:+235,+0xd04
• go/types.(*Context).instanceHash:+10,+0xc4
• go/types.(*Checker).instance:+19,+0x23c
• go/types.(*Checker).instantiatedType:+35,+0x42d
• go/types.(*Checker).typInternal:+57,+0x128b
• go/types.(*Checker).declaredType:+1,+0x2e
• go/types.(*Checker).typ:=151,+0x198
• go/types.(*Checker).bound:+20,+0x199
• go/types.(*Checker).collectTypeParams:+36,+0x35c
• go/types.(*Checker).typeDecl:+48,+0x62e

golang.org/x/tools/gopls@v0.21.1 go1.26.1 linux/amd64 gopls.client=neovim,other,vscode (1)

Use this command to reproduce the executable:
(HOME=$(mktemp -d); GOOS=linux GOARCH=amd64 GOTOOLCHAIN=go1.26.1 go install golang.org/x/tools/gopls@v0.21.1 && find $HOME/go/bin -type f)
To disassemble: go tool objdump exe | less

[adonovan]

Found new stacks for predicate:

#!stacks
"gopanic" && "go/types.(*typeWriter).error:=110"

This stack HwXxCg was reported by telemetry:

• crash/crash
• runtime.gopanic:+70,+0x15b
• go/types.(*Checker).handleBailout:+25,+0x93
• go/types.(*Checker).Files.deferwrap1:+0,+0x1f
• runtime.gopanic:+51,+0x12b
• go/types.(*Checker).objDecl.func1:+3,+0x57
• runtime.gopanic:+51,+0x12b
• go/types.(*Checker).objDecl.func1:+3,+0x57
• runtime.gopanic:+51,+0x12b
• go/types.(*Checker).objDecl.func1:+3,+0x57
• runtime.gopanic:+51,+0x12b
• go/types.(*Checker).objDecl.func1:+3,+0x57
• runtime.gopanic:+51,+0x12b
• go/types.(*typeWriter).error:=110,+0x94b
• go/types.(*typeWriter).typ:+10,+0x834
• go/types.(*typeWriter).typ:+235,+0xb23
• go/types.(*typeWriter).typeList:+6,+0x43
• go/types.(*Context).instanceHash:+14,+0xcf

golang.org/x/tools/gopls@v0.21.1 go1.26.2 darwin/arm64 gopls.client=vscode (2)

Use this command to reproduce the executable:
(HOME=$(mktemp -d); GOOS=darwin GOARCH=arm64 GOTOOLCHAIN=go1.26.2 go install golang.org/x/tools/gopls@v0.21.1 && find $HOME/go/bin -type f)
To disassemble: go tool objdump exe | less

This stack Uwn_Cw was reported by telemetry:

• crash/crash
• runtime.gopanic:+70,+0x15b
• go/types.(*Checker).handleBailout:+25,+0x93
• go/types.(*Checker).Files.deferwrap1:+0,+0x1f
• runtime.gopanic:+51,+0x12b
• go/types.(*Checker).objDecl.func1:+3,+0x57
• runtime.gopanic:+51,+0x12b
• go/types.(*Checker).objDecl.func1:+3,+0x57
• runtime.gopanic:+51,+0x12b
• go/types.(*Checker).objDecl.func1:+3,+0x57
• runtime.gopanic:+51,+0x12b
• go/types.(*Checker).objDecl.func1:+3,+0x57
• runtime.gopanic:+51,+0x12b
• go/types.(*Checker).objDecl.func1:+3,+0x57
• runtime.gopanic:+51,+0x12b
• go/types.(*typeWriter).error:=110,+0x94b
• go/types.(*typeWriter).typ:+10,+0x834
• go/types.(*typeWriter).typ:+235,+0xb23

golang.org/x/tools/gopls@v0.21.1 go1.26.2 darwin/arm64 gopls.client=vscode (28)

Use this command to reproduce the executable:
(HOME=$(mktemp -d); GOOS=darwin GOARCH=arm64 GOTOOLCHAIN=go1.26.2 go install golang.org/x/tools/gopls@v0.21.1 && find $HOME/go/bin -type f)
To disassemble: go tool objdump exe | less