cmd/compile: error in Go 1.23.0 with generics, type aliases and indexing

[phynes-sensiblecode]

Go version

go1.23.0

Output of go env in your module/workspace:

Can be seen in Go Playground

What did you do?

package main

import "fmt"

type A = map[string]interface{}

func m[T ~A](a interface{}) {
	fmt.Println(a.(T)["key"])
}

func main() {
	b := A{"key": "value"}
	m[A](b)
}

See: https://go.dev/play/p/cFkeJ6ppEBk

What did you see happen?

This compiler error is seen on Go 1.23:

./prog.go:8:19: invalid operation: cannot index a.(T) (comma, ok expression of type T constrained by ~A)

The program compiles and runs as expected on Go 1.22.

If we move the type definition of A to a different package then the code runs fine in Go 1.23:
https://go.dev/play/p/l_x4F190M3y

What did you expect to see?

There should be no compilation error.

[gabyhelp]

Related Issues and Documentation

• cmd/compile/internal/types2: review implementation of indexing of generic types #49275 (closed)
• map with interface{} key does not accept concretely-typed key expressions #778 (closed)
• cmd/compile: internal compiler error: exporter: unexpected type alias in inlined function body #24173 (closed)
• cmd/compile: importing generic alias types is not supported in 1.23 #68526
• cmd/compile: cannot index pointer to generic array #49447 (closed)
• generics: indexing operations don't work on pointers to constraints derived from arrays #52802 (closed)
• cmd/go2go: type parameter used in map key causes compile-time error #41154 (closed)
• gccgo: internal compiler error for (invalid) program using interface aliases #25302 (closed)
• generics: Inconsistent ability to access struct fields in functions with type parameter of struct underlying types #51977 (closed)

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[ianlancetaylor]

CC @griesemer @findleyr @timothy-king

[ianlancetaylor]

@gopherbot Please open a backport to the 1.23 release.

This appears to be a regression in the compiler.

[gopherbot]

Backport issue(s) opened: #68905 (for 1.23).

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

[timothy-king]

Thank you for the report.

This looks like a bug in (go/types._TypeSet).underIs (and types2). This internal comment // x == under(x) for ~x terms is not true for Aliases. Needs more investigation.

You can get the 1.22 behavior by overriding the 1.23 default GODEBUG=gotypesalias=0. https://go.dev/doc/godebug#default has instructions for different ways to change the default value.

[gopherbot]

Change https://go.dev/cl/606075 mentions this issue: go/types: unalias tilde terms in underIs

[gopherbot]

Change https://go.dev/cl/607635 mentions this issue: [release-branch.go1.23] go/types, types2: unalias tilde terms in underIs