src/cmd/compile/internal/types2/named.go

// Copyright 2011 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package types2

import (
	"cmd/compile/internal/syntax"
	"sync"
)

// A Named represents a named (defined) type.
type Named struct {
	check      *Checker
	info       typeInfo       // for cycle detection
	obj        *TypeName      // corresponding declared object for declared types; placeholder for instantiated types
	orig       *Named         // original, uninstantiated type
	fromRHS    Type           // type (on RHS of declaration) this *Named type is derived from (for cycle reporting)
	underlying Type           // possibly a *Named during setup; never a *Named once set up completely
	instPos    *syntax.Pos    // position information for lazy instantiation, or nil
	tparams    *TypeParamList // type parameters, or nil
	targs      *TypeList      // type arguments (after instantiation), or nil
	methods    []*Func        // methods declared for this type (not the method set of this type); signatures are type-checked lazily

	resolve func(*Named) ([]*TypeParam, Type, []*Func)
	once    sync.Once
}

// NewNamed returns a new named type for the given type name, underlying type, and associated methods.
// If the given type name obj doesn't have a type yet, its type is set to the returned named type.
// The underlying type must not be a *Named.
func NewNamed(obj *TypeName, underlying Type, methods []*Func) *Named {
	if _, ok := underlying.(*Named); ok {
		panic("underlying type must not be *Named")
	}
	return (*Checker)(nil).newNamed(obj, nil, underlying, nil, methods)
}

func (t *Named) load() *Named {
	// If t is an instantiated type, it derives its methods and tparams from its
	// base type. Since we expect type parameters and methods to be set after a
	// call to load, we must load the base and copy here.
	//
	// underlying is set when t is expanded.
	//
	// By convention, a type instance is loaded iff its tparams are set.
	if t.targs.Len() > 0 && t.tparams == nil {
		t.orig.load()
		t.tparams = t.orig.tparams
		t.methods = t.orig.methods
	}
	if t.resolve == nil {
		return t
	}

	t.once.Do(func() {
		// TODO(mdempsky): Since we're passing t to resolve anyway
		// (necessary because types2 expects the receiver type for methods
		// on defined interface types to be the Named rather than the
		// underlying Interface), maybe it should just handle calling
		// SetTypeParams, SetUnderlying, and AddMethod instead?  Those
		// methods would need to support reentrant calls though.  It would
		// also make the API more future-proof towards further extensions
		// (like SetTypeParams).

		tparams, underlying, methods := t.resolve(t)

		switch underlying.(type) {
		case nil, *Named:
			panic("invalid underlying type")
		}

		t.tparams = bindTParams(tparams)
		t.underlying = underlying
		t.methods = methods
	})
	return t
}

// newNamed is like NewNamed but with a *Checker receiver and additional orig argument.
func (check *Checker) newNamed(obj *TypeName, orig *Named, underlying Type, tparams *TypeParamList, methods []*Func) *Named {
	typ := &Named{check: check, obj: obj, orig: orig, fromRHS: underlying, underlying: underlying, tparams: tparams, methods: methods}
	if typ.orig == nil {
		typ.orig = typ
	}
	if obj.typ == nil {
		obj.typ = typ
	}
	// Ensure that typ is always expanded, at which point the check field can be
	// nilled out.
	//
	// Note that currently we cannot nil out check inside typ.under(), because
	// it's possible that typ is expanded multiple times.
	//
	// TODO(gri): clean this up so that under is the only function mutating
	//            named types.
	if check != nil {
		check.later(func() {
			switch typ.under().(type) {
			case *Named:
				panic("unexpanded underlying type")
			}
			typ.check = nil
		})
	}
	return typ
}

// Obj returns the type name for the declaration defining the named type t. For
// instantiated types, this is the type name of the base type.
func (t *Named) Obj() *TypeName {
	return t.orig.obj // for non-instances this is the same as t.obj
}

// Orig returns the original generic type an instantiated type is derived from.
// If t is not an instantiated type, the result is t.
func (t *Named) Orig() *Named { return t.orig }

// TODO(gri) Come up with a better representation and API to distinguish
//           between parameterized instantiated and non-instantiated types.

// TypeParams returns the type parameters of the named type t, or nil.
// The result is non-nil for an (originally) parameterized type even if it is instantiated.
func (t *Named) TypeParams() *TypeParamList { return t.load().tparams }

// SetTypeParams sets the type parameters of the named type t.
func (t *Named) SetTypeParams(tparams []*TypeParam) { t.load().tparams = bindTParams(tparams) }

// TypeArgs returns the type arguments used to instantiate the named type t.
func (t *Named) TypeArgs() *TypeList { return t.targs }

// NumMethods returns the number of explicit methods whose receiver is named type t.
func (t *Named) NumMethods() int { return len(t.load().methods) }

// Method returns the i'th method of named type t for 0 <= i < t.NumMethods().
func (t *Named) Method(i int) *Func { return t.load().methods[i] }

// SetUnderlying sets the underlying type and marks t as complete.
func (t *Named) SetUnderlying(underlying Type) {
	if underlying == nil {
		panic("underlying type must not be nil")
	}
	if _, ok := underlying.(*Named); ok {
		panic("underlying type must not be *Named")
	}
	t.load().underlying = underlying
}

// AddMethod adds method m unless it is already in the method list.
func (t *Named) AddMethod(m *Func) {
	t.load()
	if i, _ := lookupMethod(t.methods, m.pkg, m.name); i < 0 {
		t.methods = append(t.methods, m)
	}
}

func (t *Named) Underlying() Type { return t.load().expand(nil).underlying }
func (t *Named) String() string   { return TypeString(t, nil) }

// ----------------------------------------------------------------------------
// Implementation

// under returns the expanded underlying type of n0; possibly by following
// forward chains of named types. If an underlying type is found, resolve
// the chain by setting the underlying type for each defined type in the
// chain before returning it. If no underlying type is found or a cycle
// is detected, the result is Typ[Invalid]. If a cycle is detected and
// n0.check != nil, the cycle is reported.
func (n0 *Named) under() Type {
	u := n0.Underlying()

	// If the underlying type of a defined type is not a defined
	// (incl. instance) type, then that is the desired underlying
	// type.
	var n1 *Named
	switch u1 := u.(type) {
	case nil:
		return Typ[Invalid]
	default:
		// common case
		return u
	case *Named:
		// handled below
		n1 = u1
	}

	if n0.check == nil {
		panic("Named.check == nil but type is incomplete")
	}

	// Invariant: after this point n0 as well as any named types in its
	// underlying chain should be set up when this function exits.
	check := n0.check
	n := n0

	seen := make(map[*Named]int) // types that need their underlying resolved
	var path []Object            // objects encountered, for cycle reporting

loop:
	for {
		seen[n] = len(seen)
		path = append(path, n.obj)
		n = n1
		if i, ok := seen[n]; ok {
			// cycle
			check.cycleError(path[i:])
			u = Typ[Invalid]
			break
		}
		u = n.Underlying()
		switch u1 := u.(type) {
		case nil:
			u = Typ[Invalid]
			break loop
		default:
			break loop
		case *Named:
			// Continue collecting *Named types in the chain.
			n1 = u1
		}
	}

	for n := range seen {
		// We should never have to update the underlying type of an imported type;
		// those underlying types should have been resolved during the import.
		// Also, doing so would lead to a race condition (was issue #31749).
		// Do this check always, not just in debug mode (it's cheap).
		if n.obj.pkg != check.pkg {
			panic("imported type with unresolved underlying type")
		}
		n.underlying = u
	}

	return u
}

func (n *Named) setUnderlying(typ Type) {
	if n != nil {
		n.underlying = typ
	}
}

// expand ensures that the underlying type of n is instantiated.
// The underlying type will be Typ[Invalid] if there was an error.
func (n *Named) expand(env *Environment) *Named {
	if n.instPos != nil {
		// n must be loaded before instantiation, in order to have accurate
		// tparams. This is done implicitly by the call to n.TypeParams, but making
		// it explicit is harmless: load is idempotent.
		n.load()
		var u Type
		if n.check.validateTArgLen(*n.instPos, n.tparams.Len(), n.targs.Len()) {
			// TODO(rfindley): handling an optional Checker and Environment here (and
			// in subst) feels overly complicated. Can we simplify?
			if env == nil {
				if n.check != nil {
					env = n.check.conf.Environment
				} else {
					// If we're instantiating lazily, we might be outside the scope of a
					// type-checking pass. In that case we won't have a pre-existing
					// environment, but don't want to create a duplicate of the current
					// instance in the process of expansion.
					env = NewEnvironment()
				}
				h := env.TypeHash(n.orig, n.targs.list())
				// add the instance to the environment to avoid infinite recursion.
				// addInstance may return a different, existing instance, but we
				// shouldn't return that instance from expand.
				env.typeForHash(h, n)
			}
			u = n.check.subst(*n.instPos, n.orig.underlying, makeSubstMap(n.TypeParams().list(), n.targs.list()), env)
		} else {
			u = Typ[Invalid]
		}
		n.underlying = u
		n.fromRHS = u
		n.instPos = nil
	}
	return n
}

// safeUnderlying returns the underlying of typ without expanding instances, to
// avoid infinite recursion.
//
// TODO(rfindley): eliminate this function or give it a better name.
func safeUnderlying(typ Type) Type {
	if t, _ := typ.(*Named); t != nil {
		return t.load().underlying
	}
	return typ.Underlying()
}