go/types, types2: type inference fails to infer type argument even though correct type is present in argument list

[griesemer]

package main

type A struct{}
type B struct{}

var a A
var b B
var c struct{}

func main() {
	f(a, b, c) // ERROR "type B of b does not match inferred type A for P"
	f[struct{}](a, b, c)
}

func f[P any](...P) {}

fails to infer the type argument struct{} even though it appears in the function argument list (playground).

This bug existed since Go 1.18.
It partly interferes with a fix for #62157 (we can't fully fix #62157 without addressing this issue as well).

cc: @findleyr

[gopherbot]

Change https://go.dev/cl/523579 mentions this issue: go/types, types2: use handle struct instead of *Type to track inferred types

[griesemer]

Per internal discussion: we're not going to change behavior here. If f(a, c) infers A, f(b, c) infers B, and f(a, b) fails, then f(a, b, c) is ok to fail; i.e., the type argument C must be provided explicitly.

Closing as working as intended.