exec go run  main1.go
cp stdout result1
exec go run main2.go
cp stdout result2
grep '1 allocs/op' result1
grep '1 allocs/op' result2

cmp stdout want-stdout
-- go.mod --
module m
-- main1.go --

// This version uses an extra layer of iteration.
package main

import (
	"fmt"
	"testing"
)

func main() {
	r := testing.Benchmark(func(b *testing.B) {
		err := errorT{"a"}
		for range b.N {
			if !AsType[errorT](err) {
				panic("not ok")
			}
		}
	})
	fmt.Printf("BenchmarkAsType1: %v %v\n", r, r.MemString())
}

func AsType[T any](err error) bool {
	for _ = range AllAs[T](err) {
		return true
	}
	return false
}

func AllAs[T any](err error) func(func(T) bool) {
	return func(yield func(T) bool) {
		for err := range All(err) {
			if x, ok := err.(T); ok {
				if !yield(x) {
					return
				}
			}
		}
	}
}

func All(err error) func(func(error) bool) {
	return func(yield func(error) bool) {
		yield(err)
	}
}

type errorT struct{ s string }

func (e errorT) Error() string { return fmt.Sprintf("errorT(%s)", e.s) }


-- main2.go --

// This version avoids the intermediate iterator.
package main

import (
	"fmt"
	"testing"
)

func main() {
	r := testing.Benchmark(func(b *testing.B) {
		err := errorT{"a"}
		for range b.N {
			if !AsType[errorT](err) {
				panic("not ok")
			}
		}
	})
	fmt.Printf("BenchmarkAsType2: %v %v\n", r, r.MemString())
}

func AsType[T any](err error) bool {
	for err := range All(err) {
		if _, ok := err.(T); ok {
			return true
		}
	}
	return false
}

func All(err error) func(func(error) bool) {
	return func(yield func(error) bool) {
		yield(err)
	}
}

type errorT struct{ s string }

func (e errorT) Error() string { return fmt.Sprintf("errorT(%s)", e.s) }

> exec go run  main1.go
[stdout]
BenchmarkAsType1:  5617491	       199.9 ns/op      168 B/op	       9 allocs/op
> cp stdout result1
> exec go run main2.go
[stdout]
BenchmarkAsType2: 31012682	        32.98 ns/op       16 B/op	       1 allocs/op
> cp stdout result2
> grep '1 allocs/op' result1
[result1]
BenchmarkAsType1:  5617491	       199.9 ns/op      168 B/op	       9 allocs/op

FAIL: /tmp/testscript4012864748/x.txtar/script.txtar:5: no match for `1 allocs/op` found in result1
error running /tmp/x.txtar in /tmp/testscript4012864748/x.txtar