if check != nil {
		check.needsCleanup(a)
	}

	return a
}

// newAliasInstance creates a new alias instance for the given origin and type
// arguments, recording pos as the position of its synthetic object (for error
// reporting).
func (check *Checker) newAliasInstance(pos syntax.Pos, orig *Alias, targs []Type, ctxt *Context) *Alias {
	assert(len(targs) > 0)

	if check != nil {
		check.needsCleanup(a)
	}

	return a
}

// newAliasInstance creates a new alias instance for the given origin and type
// arguments, recording pos as the position of its synthetic object (for error
// reporting).
func (check *Checker) newAliasInstance(pos token.Pos, orig *Alias, targs []Type, ctxt *Context) *Alias {
	assert(len(targs) > 0)

		// to be substituted; i.e., if it is or contains a type parameter
		// that has a type argument for it.
		targs, updated := subst.typeList(t.TypeArgs().list())
		if updated {
			return subst.check.newAliasInstance(subst.pos, t.orig, targs, subst.ctxt)
		}

	case *Array:
		elem := subst.typOrNil(t.elem)
		if elem != t.elem {
			return &Array{len: t.len, elem: elem}
		}

	case *Slice:

		// to be substituted; i.e., if it is or contains a type parameter
		// that has a type argument for it.
		targs, updated := subst.typeList(t.TypeArgs().list())
		if updated {
			return subst.check.newAliasInstance(subst.pos, t.orig, targs, subst.ctxt)
		}

	case *Array:
		elem := subst.typOrNil(t.elem)
		if elem != t.elem {
			return &Array{len: t.len, elem: elem}
		}

	case *Slice:

		if !check.validateTArgLen(pos, orig.String(), tparams.Len(), len(targs)) {
			return Typ[Invalid]
		}
		if tparams.Len() == 0 {
			return orig // nothing to do (minor optimization)
		}

		return check.newAliasInstance(pos, orig, targs, ctxt)

	case *Signature:
		assert(expanding == nil) // function instances cannot be reached from Named types

		tparams := orig.TypeParams()

		if !check.validateTArgLen(pos, orig.String(), tparams.Len(), len(targs)) {
			return Typ[Invalid]
		}
		if tparams.Len() == 0 {
			return orig // nothing to do (minor optimization)
		}

		return check.newAliasInstance(pos, orig, targs, ctxt)

	case *Signature:
		assert(expanding == nil) // function instances cannot be reached from Named types

		tparams := orig.TypeParams()