1 files changed, 280 insertions, 0 deletions
diff --git a/src/go/types/interface.go b/src/go/types/interface.go
new file mode 100644
index 0000000000..d8f9671857
--- /dev/null
+++ b/src/go/types/interface.go
@@ -0,0 +1,280 @@
+// Copyright 2021 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 types
+
+import (
+	"go/ast"
+	"go/internal/typeparams"
+	"go/token"
+)
+
+// ----------------------------------------------------------------------------
+// API
+
+// An Interface represents an interface type.
+type Interface struct {
+	obj       *TypeName    // type name object defining this interface; or nil (for better error messages)
+	methods   []*Func      // ordered list of explicitly declared methods
+	embeddeds []Type       // ordered list of explicitly embedded elements
+	embedPos  *[]token.Pos // positions of embedded elements; or nil (for error messages) - use pointer to save space
+	complete  bool         // indicates that obj, methods, and embeddeds are set and type set can be computed
+
+	tset *_TypeSet // type set described by this interface, computed lazily
+}
+
+// typeSet returns the type set for interface t.
+func (t *Interface) typeSet() *_TypeSet { return computeTypeSet(nil, token.NoPos, t) }
+
+// is reports whether interface t represents types that all satisfy f.
+func (t *Interface) is(f func(Type, bool) bool) bool {
+	switch t := t.typeSet().types.(type) {
+	case nil, *top:
+		// TODO(gri) should settle on top or nil to represent this case
+		return false // we must have at least one type! (was bug)
+	case *Union:
+		return t.is(func(t *term) bool { return f(t.typ, t.tilde) })
+	default:
+		return f(t, false)
+	}
+}
+
+// emptyInterface represents the empty (completed) interface
+var emptyInterface = Interface{complete: true, tset: &topTypeSet}
+
+// NewInterface returns a new interface for the given methods and embedded types.
+// NewInterface takes ownership of the provided methods and may modify their types
+// by setting missing receivers.
+//
+// Deprecated: Use NewInterfaceType instead which allows arbitrary embedded types.
+func NewInterface(methods []*Func, embeddeds []*Named) *Interface {
+	tnames := make([]Type, len(embeddeds))
+	for i, t := range embeddeds {
+		tnames[i] = t
+	}
+	return NewInterfaceType(methods, tnames)
+}
+
+// NewInterfaceType returns a new interface for the given methods and embedded types.
+// NewInterfaceType takes ownership of the provided methods and may modify their types
+// by setting missing receivers.
+func NewInterfaceType(methods []*Func, embeddeds []Type) *Interface {
+	if len(methods) == 0 && len(embeddeds) == 0 {
+		return &emptyInterface
+	}
+
+	// set method receivers if necessary
+	typ := new(Interface)
+	for _, m := range methods {
+		if sig := m.typ.(*Signature); sig.recv == nil {
+			sig.recv = NewVar(m.pos, m.pkg, "", typ)
+		}
+	}
+
+	// sort for API stability
+	sortMethods(methods)
+
+	typ.methods = methods
+	typ.embeddeds = embeddeds
+	typ.complete = true
+
+	return typ
+}
+
+// NumExplicitMethods returns the number of explicitly declared methods of interface t.
+func (t *Interface) NumExplicitMethods() int { return len(t.methods) }
+
+// ExplicitMethod returns the i'th explicitly declared method of interface t for 0 <= i < t.NumExplicitMethods().
+// The methods are ordered by their unique Id.
+func (t *Interface) ExplicitMethod(i int) *Func { return t.methods[i] }
+
+// NumEmbeddeds returns the number of embedded types in interface t.
+func (t *Interface) NumEmbeddeds() int { return len(t.embeddeds) }
+
+// Embedded returns the i'th embedded defined (*Named) type of interface t for 0 <= i < t.NumEmbeddeds().
+// The result is nil if the i'th embedded type is not a defined type.
+//
+// Deprecated: Use EmbeddedType which is not restricted to defined (*Named) types.
+func (t *Interface) Embedded(i int) *Named { tname, _ := t.embeddeds[i].(*Named); return tname }
+
+// EmbeddedType returns the i'th embedded type of interface t for 0 <= i < t.NumEmbeddeds().
+func (t *Interface) EmbeddedType(i int) Type { return t.embeddeds[i] }
+
+// NumMethods returns the total number of methods of interface t.
+func (t *Interface) NumMethods() int { return t.typeSet().NumMethods() }
+
+// Method returns the i'th method of interface t for 0 <= i < t.NumMethods().
+// The methods are ordered by their unique Id.
+func (t *Interface) Method(i int) *Func { return t.typeSet().Method(i) }
+
+// Empty reports whether t is the empty interface.
+func (t *Interface) Empty() bool { return t.typeSet().IsTop() }
+
+// IsComparable reports whether each type in interface t's type set is comparable.
+func (t *Interface) IsComparable() bool { return t.typeSet().IsComparable() }
+
+// IsConstraint reports whether interface t is not just a method set.
+func (t *Interface) IsConstraint() bool { return !t.typeSet().IsMethodSet() }
+
+// isSatisfiedBy reports whether interface t's type list is satisfied by the type typ.
+// If the type list is empty (absent), typ trivially satisfies the interface.
+// TODO(gri) This is not a great name. Eventually, we should have a more comprehensive
+//           "implements" predicate.
+func (t *Interface) isSatisfiedBy(typ Type) bool {
+	t.Complete()
+	switch t := t.typeSet().types.(type) {
+	case nil:
+		return true // no type restrictions
+	case *Union:
+		r, _ := t.intersect(typ, false)
+		return r != nil
+	default:
+		return Identical(t, typ)
+	}
+}
+
+// Complete computes the interface's type set. It must be called by users of
+// NewInterfaceType and NewInterface after the interface's embedded types are
+// fully defined and before using the interface type in any way other than to
+// form other types. The interface must not contain duplicate methods or a
+// panic occurs. Complete returns the receiver.
+//
+// Deprecated: Type sets are now computed lazily, on demand; this function
+//             is only here for backward-compatibility. It does not have to
+//             be called explicitly anymore.
+func (t *Interface) Complete() *Interface {
+	// Some tests are still depending on the state change
+	// (string representation of an Interface not containing an
+	// /* incomplete */ marker) caused by the explicit Complete
+	// call, so we compute the type set eagerly here.
+	t.complete = true
+	t.typeSet()
+	return t
+}
+
+func (t *Interface) Underlying() Type { return t }
+func (t *Interface) String() string   { return TypeString(t, nil) }
+
+// ----------------------------------------------------------------------------
+// Implementation
+
+func (check *Checker) interfaceType(ityp *Interface, iface *ast.InterfaceType, def *Named) {
+	var tlist []ast.Expr
+	var tname *ast.Ident // "type" name of first entry in a type list declaration
+
+	addEmbedded := func(pos token.Pos, typ Type) {
+		ityp.embeddeds = append(ityp.embeddeds, typ)
+		if ityp.embedPos == nil {
+			ityp.embedPos = new([]token.Pos)
+		}
+		*ityp.embedPos = append(*ityp.embedPos, pos)
+	}
+
+	for _, f := range iface.Methods.List {
+		if len(f.Names) == 0 {
+			// We have an embedded type; possibly a union of types.
+			addEmbedded(f.Type.Pos(), parseUnion(check, flattenUnion(nil, f.Type)))
+			continue
+		}
+
+		// We have a method with name f.Names[0], or a type
+		// of a type list (name.Name == "type").
+		// (The parser ensures that there's only one method
+		// and we don't care if a constructed AST has more.)
+		name := f.Names[0]
+		if name.Name == "_" {
+			check.errorf(name, _BlankIfaceMethod, "invalid method name _")
+			continue // ignore
+		}
+
+		// TODO(rfindley) Remove type list handling once the parser doesn't accept type lists anymore.
+		if name.Name == "type" {
+			// Report an error for the first type list per interface
+			// if we don't allow type lists, but continue.
+			if !allowTypeLists && tlist == nil {
+				check.softErrorf(name, _Todo, "use generalized embedding syntax instead of a type list")
+			}
+			// For now, collect all type list entries as if it
+			// were a single union, where each union element is
+			// of the form ~T.
+			// TODO(rfindley) remove once we disallow type lists
+			op := new(ast.UnaryExpr)
+			op.Op = token.TILDE
+			op.X = f.Type
+			tlist = append(tlist, op)
+			// Report an error if we have multiple type lists in an
+			// interface, but only if they are permitted in the first place.
+			if allowTypeLists && tname != nil && tname != name {
+				check.errorf(name, _Todo, "cannot have multiple type lists in an interface")
+			}
+			tname = name
+			continue
+		}
+
+		typ := check.typ(f.Type)
+		sig, _ := typ.(*Signature)
+		if sig == nil {
+			if typ != Typ[Invalid] {
+				check.invalidAST(f.Type, "%s is not a method signature", typ)
+			}
+			continue // ignore
+		}
+
+		// Always type-check method type parameters but complain if they are not enabled.
+		// (This extra check is needed here because interface method signatures don't have
+		// a receiver specification.)
+		if sig.tparams != nil {
+			var at positioner = f.Type
+			if tparams := typeparams.Get(f.Type); tparams != nil {
+				at = tparams
+			}
+			check.errorf(at, _Todo, "methods cannot have type parameters")
+		}
+
+		// use named receiver type if available (for better error messages)
+		var recvTyp Type = ityp
+		if def != nil {
+			recvTyp = def
+		}
+		sig.recv = NewVar(name.Pos(), check.pkg, "", recvTyp)
+
+		m := NewFunc(name.Pos(), check.pkg, name.Name, sig)
+		check.recordDef(name, m)
+		ityp.methods = append(ityp.methods, m)
+	}
+
+	// type constraints
+	if tlist != nil {
+		// TODO(rfindley): this differs from types2 due to the use of Pos() below,
+		// which should actually be on the ~. Confirm that this position is correct.
+		addEmbedded(tlist[0].Pos(), parseUnion(check, tlist))
+	}
+
+	// All methods and embedded elements for this interface are collected;
+	// i.e., this interface is may be used in a type set computation.
+	ityp.complete = true
+
+	if len(ityp.methods) == 0 && len(ityp.embeddeds) == 0 {
+		// empty interface
+		ityp.tset = &topTypeSet
+		return
+	}
+
+	// sort for API stability
+	sortMethods(ityp.methods)
+	// (don't sort embeddeds: they must correspond to *embedPos entries)
+
+	// Compute type set with a non-nil *Checker as soon as possible
+	// to report any errors. Subsequent uses of type sets will use
+	// this computed type set and won't need to pass in a *Checker.
+	check.later(func() { computeTypeSet(check, iface.Pos(), ityp) })
+}
+
+func flattenUnion(list []ast.Expr, x ast.Expr) []ast.Expr {
+	if o, _ := x.(*ast.BinaryExpr); o != nil && o.Op == token.OR {
+		list = flattenUnion(list, o.X)
+		x = o.Y
+	}
+	return append(list, x)
+}