False negative with `defer`

[maratori]

The same issue exists in gostaticanalysis/nilerr#19.

The following test fails:

func TestFails() error {
	defer func() {}() // the test passes if you remove defer
	err := func() error { return nil }()
	if err != nil {
		return err
	}
	err2 := func() error { return nil }()
	if err2 != nil {
		return err // want `return a nil value error after check error`
	}
	return nil
}

[alingse]

 🤔 I will check it 🤔

[alingse]

I use another function and print the blocks and instrs

func Call18() error {
	defer func() {}() // the test passes if you remove defer

	err := Do()
	if err != nil {
		return err
	}
	err2 := Do2()
	if err2 != nil {
		return err // want `return a nil value error after check error`
	}
	return nil
}

blocks[0].Instrs[0]     *ssa.Alloc      local error ()
blocks[0].Instrs[1]     *ssa.Defer      defer Call18$1()
blocks[0].Instrs[2]     *ssa.Call       Do()
blocks[0].Instrs[3]     *ssa.BinOp      t1 != nil:error
blocks[0].Instrs[4]     *ssa.If if t2 goto 2 else 3

blocks[1].Instrs[0]     *ssa.UnOp       *t0
blocks[1].Instrs[1]     *ssa.Return     return t3

blocks[2].Instrs[0]     *ssa.Store      *t0 = t1
blocks[2].Instrs[1]     *ssa.RunDefers  rundefers
blocks[2].Instrs[2]     *ssa.UnOp       *t0
blocks[2].Instrs[3]     *ssa.Return     return t4

blocks[3].Instrs[0]     *ssa.Call       Do2()
blocks[3].Instrs[1]     *ssa.BinOp      t5 != nil:error
blocks[3].Instrs[2]     *ssa.If if t6 goto 4 else 5

blocks[4].Instrs[0]     *ssa.Store      *t0 = t1
blocks[4].Instrs[1]     *ssa.RunDefers  rundefers
blocks[4].Instrs[2]     *ssa.UnOp       *t0
blocks[4].Instrs[3]     *ssa.Return     return t7

blocks[5].Instrs[0]     *ssa.Store      *t0 = nil:error
blocks[5].Instrs[1]     *ssa.RunDefers  rundefers
blocks[5].Instrs[2]     *ssa.UnOp       *t0
blocks[5].Instrs[3]     *ssa.Return     return t8

the return err was convert to *ssa.Store *t0 = nil:error a store instr, this is not support by nilness, I will try to support it if I can.

[thaJeztah]

Ah! Was scratching my head after enabling this linter (which looks very useful!) and my first attempt to make it trip failing; I also deducted it to be related to defers (initially thought its was named output variables); here was my reproducer, which looks quite similar, except in my case the defer was defined after the checks (so effectively, no defer yet in sight)

package main

import "fmt"

func main() {
	_ = example()
}

func example() error {
	_, err := fmt.Println()
	if err != nil {
		return err
	}
	_, err2 := fmt.Println()
	if err2 != nil {
		return err // which should return err2 after check `err2 != nil`, but return a nil value error
	}
	defer func() {}()
	return nil
}