cmd/compile: type assertion on generic type fails incorrectly

[heschi]

The following program panics for no reason I can discern:

package workflow

import (
	"fmt"
	"testing"
)

type TaskInput interface {
	deps() []*taskDefinition
}

type Value[T any] interface {
	metaValue
}

type metaValue interface {
	TaskInput
}

type taskDefinition struct {
}

type taskResult struct {
	task *taskDefinition
}

func (tr *taskResult) deps() []*taskDefinition {
	return nil
}

func TestTrivial(t *testing.T) {
	tr := &taskResult{&taskDefinition{}}
	use(Value[string](tr))
}


func use[T any](v Value[T]) {
	_, ok := v.(*taskResult)
	if !ok {
		panic(fmt.Errorf("output must be a task result, is %T", v))
	}
}

cc @golang/runtime

[prattmic]

I notice that the caller at use(Value[string](tr)) is using:

go.itab.*example%2ecom.taskResult,example%2ecom.Value[string], but use itself is checking against
go.itab.*example%2ecom.taskResult,example%2ecom.Value[go.shape.string_0].

Based on the register flow, these pointers are compared in use for the type assertion, so the difference here is the immediate cause of the failure.

[gopherbot]

Change https://go.dev/cl/411617 mentions this issue: cmd/compile: fix missing dict pass for type assertions

[cuonglm]

@randall77 @mdempsky should we backport this?

[randall77]

@gopherbot please open a backport issue to 1.18.

[gopherbot]

Backport issue(s) opened: #53357 (for 1.18).

Remember to create the cherry-pick CL(s) as soon as the patch is submitted to master, according to https://go.dev/wiki/MinorReleases.