go/types: SEGV while loading types.Var.name field (lookupScope returns invalid *types.Var)

[adonovan]

#!stacks
"sigpanic" && "types.(*Var).Name:+0" && "go/types.(*Checker).ident:+30"

Issue created by stacks.

We have a non-nil Object interface value (witness dominating control flow), evidently containing a *types.Var (see traceback), whose Name() method call crashes. The only instruction in Var.Name is a 2-word load from the object.name field, so the *types.Var pointer is either nil or invalid.

	scope, obj := check.lookupScope(e.Name)
	switch obj {
	case nil:
		...
		return
	...
	}
	...
	_, gotType := obj.(*TypeName)
	if !gotType && wantType {
	  	check.errorf(e, NotAType, "%s is not a type", obj.Name())  <----- panic loading offset 24 of *types.Var pointer

This stack t8TEng was reported by telemetry:

• crash/crash
• runtime.throw:+9
• runtime.sigpanic:+33
• go/types.(*Var).Name:+0
• go/types.(*Checker).ident:+30
• go/types.(*Checker).exprInternal:+11
• go/types.(*Checker).rawExpr:+10
• go/types.(*Checker).exprOrType:+1
• go/types.(*Checker).selector:+97
• go/types.(*Checker).exprInternal:+308
• go/types.(*Checker).rawExpr:+10
• go/types.(*Checker).exprOrType:+1
• go/types.(*Checker).selector:+97
• go/types.(*Checker).exprInternal:+308
• go/types.(*Checker).rawExpr:+10
• go/types.(*Checker).exprOrType:+1
• go/types.(*Checker).callExpr:+14

golang.org/x/tools/gopls@v0.18.1 go1.23.6 linux/amd64 vscode (1)

@griesemer