gopls/internal/golang/stubmethods: refine crash into bug report

This CL refine an as-yet unreproducible crash into a bug report. It
seems clear that the code intended to check that sig != nil, yet instead
checked that sig.Results != nil. With that said, I don't think it's
possible to have a return statement without an enclosing signature. For
now, turn this into a bug report.

For golang/go#70666

Change-Id: I79031d437d643f6d70360e3abde86166176647b3
Reviewed-on: https://go-review.googlesource.com/c/tools/+/633375
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

gopls/internal/golang/stubmethods/stubmethods.go

@@ -13,9 +13,11 @@ import (
 	"go/ast"
 	"go/token"
 	"go/types"
-	"golang.org/x/tools/internal/typesinternal"
 	"strings"
 
+	"golang.org/x/tools/internal/typesinternal"
+
+	"golang.org/x/tools/gopls/internal/util/bug"
 	"golang.org/x/tools/gopls/internal/util/typesutil"
 )
 
@@ -282,8 +284,9 @@ func fromReturnStmt(fset *token.FileSet, info *types.Info, pos token.Pos, path [
 	}
 
 	sig := typesutil.EnclosingSignature(path, info)
-	if sig.Results() == nil {
-		return nil, fmt.Errorf("could not find the enclosing function of the return statement")
+	if sig == nil {
+		// golang/go#70666: this bug may be reached in practice.
+		return nil, bug.Errorf("could not find the enclosing function of the return statement")
 	}
 	rets := sig.Results()
 	// The return operands and function results must match.