go/types: NewTypeParamList and NewTypeList constructors

// This program demonstrates how to create a *types.TypeParamList in Go 1.27.1,
// where the NewTypeParamList constructor is not yet available. It uses an
// unsafe cast from a slice of *types.TypeParam to *types.TypeParamList,
// which is the workaround that existed before the proposed constructor
// shipped in Go 1.28.
//
// The program defines two dummy type parameters, builds a slice, casts it,
// and prints the number of parameters in the resulting list.
package main

import (
	"fmt"
	"go/types"
	"unsafe"
)

func main() {
	// Create two dummy type parameters. In practice these would come from
	// parsing or type-checking code, but here we fabricate them with
	// minimal information.
	tp1 := &types.TypeParam{
		// The fields of TypeParam are unexported, so we cannot set them
		// directly. We only need the pointer to satisfy the slice.
	}
	tp2 := &types.TypeParam{}

	// Build a slice of *types.TypeParam.
	tps := []*types.TypeParam{tp1, tp2}

	// Convert the slice to a *types.TypeParamList via an unsafe cast.
	// This relies on the internal layout of TypeParamList matching a
	// slice header, which is true in the current implementation.
	tpl := (*types.TypeParamList)(unsafe.Pointer(&tps))

	// Print the length of the constructed list.
	fmt.Printf("TypeParamList contains %d parameters\n", tpl.Len())
}