Why Golang allows circular reference in global scope but not in function scope

As I'm writing DB migrations using gormigrate, I need to define a many-to-many relationship between two structs in a function scope. But in golang 1.19 or 1.18 the following won't compile

package main

import "fmt"

func main() {
	type Student struct {
		Courses []*Course
		// [Error] ./prog.go:7:14: undefined: Course
	}
	type Course struct {
		Students []*Student
	}
	fmt.Printf("This won't compile")
}

However moving the definitions outside of the function will just work

package main

import "fmt"

type Student struct {
	Courses []*Course
}
type Course struct {
	Students []*Student
}

func main() {
	fmt.Printf("This works")
}

Can try this yourself at https://go.dev/play/p/GI53hhlUTbk

Why is this the case? And how can I make it work in a function scope?

Is there a similar syntax to typedef in C++, so we can declare a struct first and then define it later?

Thanks!

Circular type references are possible in the package block, but not inside a function. The section on Declarations and Scopes in the specification says:

> 2. The scope of an identifier denoting a constant, type, variable, or function (but not method) declared at top level (outside any function) is the package block.
>
> ⋮
>
>8. The scope of a type identifier declared inside a function begins at the identifier in the TypeSpec and ends at the end of the innermost containing block.

Circular references work at package-level because types declared at package-level are scoped to the entire package block.

The scope of a type declared in a function begins at the declaration, not at the beginning of the containing block. A type cannot reference types declared later in the function because those types are not in scope.
It follows that circular type references are not allowed for types declared in a function.

There is not a way to declare the name of a type and later define the type.