x/tools/gopls: out-of-bounds index in "stub methods" code action

[adonovan]

Another crash w2Y-6g, H0UbWA reported by telemetry:

golang.org/x/tools/gopls/internal/lsp/analysis/stubmethods fromReturnStmt:18
golang.org/x/tools/gopls/internal/lsp/analysis/stubmethods GetStubInfo:10
golang.org/x/tools/gopls/internal/lsp/analysis/stubmethods DiagnosticForError:6
golang.org/x/tools/gopls/internal/lsp (*Server).codeAction
golang.org/x/tools/gopls/internal/lsp (*Server).codeAction:210
golang.org/x/tools/gopls/internal/lsp (*Server).CodeAction:1
- golang.org/x/tools/gopls@v0.14.1 go1.19.1 darwin/amd64 (1)

func fromReturnStmt(fset *token.FileSet, ti *types.Info, pos token.Pos, path []ast.Node, rs *ast.ReturnStmt) (*StubInfo, error) {
	returnIdx := -1
	for i, r := range rs.Results {
		if pos >= r.Pos() && pos <= r.End() {
			returnIdx = i
		}
	}
	if returnIdx == -1 {
		return nil, fmt.Errorf("pos %d not within return statement bounds: [%d-%d]", pos, rs.Pos(), rs.End())
	}
	concObj, pointer := concreteType(rs.Results[returnIdx], ti)
	if concObj == nil || concObj.Obj().Pkg() == nil {
		return nil, nil
	}
	ef := enclosingFunction(path, ti)
	if ef == nil {
		return nil, fmt.Errorf("could not find the enclosing function of the return statement")
	}
	iface := ifaceType(ef.Results.List[returnIdx].Type, ti) /// <--- this line fails, presumably because the index is OOB
	if iface == nil {
		return nil, nil
	}
...

[adonovan]

This program reproduces the panic. The int("") triggers a "cannot convert" error, which causes GetStubInfo to assume the problem is because of a mismatched return operand and result type, but in fact there is no corresponding result type.

package p

func f() error {
	return nil, myerror{int("")}
}

type myerror struct{}

[gopherbot]

Change https://go.dev/cl/543018 mentions this issue: gopls/internal/lsp/source: stubmethods: fix out-of-bounds index