x/tools/gopls: objectpath: panic due to nil *types.Signature

[adonovan]

#!stacks
"sigpanic" && "(*Signature).RecvTypeParams" && "objectpath.(*finder).find:+22"

Issue created by stacks.

Possible type checker bug: the parameter of an interface method is a alias to a nil signature. Invalid code that uses interface methods and aliases have been a fertile source of bugs due to cycles and ordering subtleties. Also possibly memory corruption.

This stack olW9_w was reported by telemetry:

• crash/crash
• runtime.gopanic:+69,+0x153
• runtime.panicmem:=262,+0x2ff
• runtime.sigpanic:+19,+0x2cc
• go/types.(*Signature).RecvTypeParams:=90,+0x3a4
• golang.org/x/tools/go/types/objectpath.(*finder).find:+22,+0x3a8
• golang.org/x/tools/go/types/objectpath.(*finder).find:+3,+0x2db
• golang.org/x/tools/go/types/objectpath.(*finder).find:+51,+0xb3f
• golang.org/x/tools/go/types/objectpath.(*finder).find:+28,+0x457
• golang.org/x/tools/go/types/objectpath.(*finder).find:+70,+0x977
• golang.org/x/tools/go/types/objectpath.find:=458,+0x847
• golang.org/x/tools/go/types/objectpath.(*Encoder).For:+128,+0x7ac
• golang.org/x/tools/go/types/objectpath.(*Encoder).For-fm:+0,+0x3b
• golang.org/x/tools/gopls/internal/cache/xrefs.Index-range1:+20,+0x20b
• golang.org/x/tools/go/ast/inspector.Cursor.Preorder.func1:+6,+0xbf
• golang.org/x/tools/gopls/internal/cache/xrefs.Index:+21,+0x1f7
• golang.org/x/tools/gopls/internal/cache.(*syntaxPackage).xrefs.func1:+1,+0x37
• sync.(*Once).doSlow:+5,+0xef
• sync.(*Once).Do:=69,+0x47
• golang.org/x/tools/gopls/internal/cache.(*syntaxPackage).xrefs:+1,+0x18
• golang.org/x/tools/gopls/internal/cache.storePackageResults:+2,+0x87

golang.org/x/tools/gopls@v0.20.0 go1.24.5 darwin/arm64 neovim (1)

[findleyr]

I suspect a type checker bug here, rather than memory corruption. Without a reproducer, it may be impossible to fix.