Repository: go
Issue: proposal: spec: infer argument types when a type set only represents its core type
Labels: Proposal,Proposal-Hold,generics,TypeInference

### What version of Go are you using (`go version`)?

<pre>
$ go version
1.18
</pre>

### Does this issue reproduce with the latest release?

Yes

### What operating system and processor architecture are you using (`go env`)?

<details><summary><code>go env</code> Output</summary><br><pre>
$ go env
GO111MODULE="auto"
GOARCH="arm64"
GOBIN=""
GOCACHE="/Users/changkun/Library/Caches/go-build"
GOENV="/Users/changkun/Library/Application Support/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="arm64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/changkun/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/changkun/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/Users/changkun/goes/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/Users/changkun/goes/go/pkg/tool/darwin_arm64"
GOVCS=""
GOVERSION="go1.18"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/changkun/dev/poly.red/polyred/go.mod"
GOWORK=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -arch arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/g2/6fmr1qzx0ns3shq74zrp6bd40000gn/T/go-build1836918707=/tmp/go-build -gno-record-gcc-switches -fno-common"
</pre></details>

### What did you do?

https://go.dev/play/p/bW8xjgG0x6Z

In this example, When we use `Option1[T]` (constrainted by `A` or `B`) as
an argument of `NewA[T]`, there is only one possibility to infer the concrete
type of `Option1[T]`, because `NewA` only accept `A` as type parameter.

```go
package main

import "fmt"

type A struct{ val int }
type B struct{ val int }

type Option[T A | B] func(v *T)

func NewA[T A](opts ...Option[T]) *T {
	a := new(T)
	for _, opt := range opts {
		opt(a)
	}

	return a
}

func NewB[T B](opts ...Option[T]) *T {
	b := new(T)
	for _, opt := range opts {
		opt(b)
	}

	return b
}

func Option1[T A | B](val int) Option[T] {
	return func(v *T) {
		switch vv := any(v).(type) {
		case *A:
			vv.val = val
		case *B:
			vv.val = val
		}
	}
}

func main() {
	u := NewA(Option1[A](42)) // OK
	v := NewB(Option1[B](42)) // OK

	// NewA(Option1(42))    // ERROR: cannot infer T
	// NewB(Option1(42))    // ERROR: cannot infer T

	fmt.Println(u, v) // &{42} &{42}
}
```


### What did you expect to see?

It is possible to write code without specifying type parameter in this case:

```
NewA(Option1(42))
```

### What did you see instead?

Compile error: cannot infer T


### More reasons

This is a quite simplification when the options are distributed in a different package, say `pkgname`:

```go
package pkgname

import "fmt"

type A struct{ val int }
type B struct{ val int }

type Option[T A | B] func(v *T)

func NewA[T A](opts ...Option[T]) *T {
	a := new(T)
	for _, opt := range opts {
		opt(a)
	}

	return a
}

func NewB[T B](opts ...Option[T]) *T {
	b := new(T)
	for _, opt := range opts {
		opt(b)
	}

	return b
}

func Option1[T A | B](val int) Option[T] {
	return func(v *T) {
		switch vv := any(v).(type) {
		case *A:
			vv.val = val
		case *B:
			vv.val = val
		}
	}
}

---

package main

func main() {
	pkgname.NewA(pkgname.Option1[pkgname.A](42)) // Feels stutter
	pkgname.NewB(pkgname.Option1[pkgname.B](42))

	// pkgname.NewA(pkgname.Option1(42)) // Would be much simpler in terms of use and readability
	// pkgname.NewB(pkgname.Option1(42))
}
````