go/types: report error when recognizing issue #18395.

The fix (CL 79575) for #18395 is too risky at this stage of the Go 1.10
release process.

Since issue #18395 is easily recognized (but not easily fixed), report
an error instead of silently continuing. This avoids inscrutable follow
on errors.

Also, make sure all empty interfaces are "completed", and adjust
printing code to report incomplete interfaces.

For #18395.

Change-Id: I7fa5f97ff31ac9775c9a6d318fce9f526b0350cd
Reviewed-on: https://go-review.googlesource.com/80455
Reviewed-by: Alan Donovan <adonovan@google.com>

src/go/internal/gccgoimporter/parser_test.go

@@ -45,6 +45,11 @@ func TestTypeParser(t *testing.T) {
 			t.Errorf("expected full parse, stopped at %q", p.lit)
 		}
 
+		// interfaces must be explicitly completed
+		if ityp, _ := typ.(*types.Interface); ityp != nil {
+			ityp.Complete()
+		}
+
 		got := typ.String()
 		if got != test.want {
 			t.Errorf("got type %q, expected %q", got, test.want)

src/go/types/builtins.go

@@ -470,15 +470,14 @@ func (check *Checker) builtin(x *operand, call *ast.CallExpr, id builtinId) (_ b
 
 	case _Panic:
 		// panic(x)
-		T := new(Interface)
+		check.assignment(x, &emptyInterface, "argument to panic")
-		check.assignment(x, T, "argument to panic")
 		if x.mode == invalid {
 			return
 		}
 
 		x.mode = novalue
 		if check.Types != nil {
-			check.recordBuiltinType(call.Fun, makeSig(nil, T))
+			check.recordBuiltinType(call.Fun, makeSig(nil, &emptyInterface))
 		}
 
 	case _Print, _Println:
@@ -508,7 +507,7 @@ func (check *Checker) builtin(x *operand, call *ast.CallExpr, id builtinId) (_ b
 	case _Recover:
 		// recover() interface{}
 		x.mode = value
-		x.typ = new(Interface)
+		x.typ = &emptyInterface
 		if check.Types != nil {
 			check.recordBuiltinType(call.Fun, makeSig(x.typ))
 		}

src/go/types/object_test.go

@@ -32,7 +32,7 @@ func TestIsAlias(t *testing.T) {
 		{NewTypeName(0, nil, "t0", nil), false}, // no type yet
 		{NewTypeName(0, pkg, "t0", nil), false}, // no type yet
 		{t1, false},                             // type name refers to named type and vice versa
-		{NewTypeName(0, nil, "t2", new(Interface)), true}, // type name refers to unnamed type
+		{NewTypeName(0, nil, "t2", &emptyInterface), true}, // type name refers to unnamed type
 		{NewTypeName(0, pkg, "t3", n1), true},              // type name refers to named type with different type name
 		{NewTypeName(0, nil, "t4", Typ[Int32]), true},      // type name refers to basic type with different name
 		{NewTypeName(0, nil, "int32", Typ[Int32]), false},  // type name refers to basic type with same name

src/go/types/testdata/cycles4.src

@@ -108,3 +108,15 @@ type Element interface {
 type Event interface {
 	Target() Element
 }
+
+// Recognize issue #13895.
+
+type (
+	_ interface{ m(B1) }
+	A1 interface{ a(D1) }
+	B1 interface{ A1 }
+	C1 interface{ B1 /* ERROR issue #18395 */ }
+	D1 interface{ C1 }
+)
+
+var _ A1 = C1 /* ERROR cannot use C1 */ (nil)

src/go/types/type.go

@@ -246,10 +246,22 @@ type Interface struct {
 	allMethods []*Func // ordered list of methods declared with or embedded in this interface (TODO(gri): replace with mset)
 }
 
-// NewInterface returns a new interface for the given methods and embedded types.
+// emptyInterface represents the empty (completed) interface
+var emptyInterface = Interface{allMethods: markComplete}
+
+// markComplete is used to mark an empty interface as completely
+// set up by setting the allMethods field to a non-nil empty slice.
+var markComplete = make([]*Func, 0)
+
+// NewInterface returns a new (incomplete) interface for the given methods and embedded types.
+// To compute the method set of the interface, Complete must be called.
 func NewInterface(methods []*Func, embeddeds []*Named) *Interface {
 	typ := new(Interface)
 
+	if len(methods) == 0 && len(embeddeds) == 0 {
+		return typ
+	}
+
 	var mset objset
 	for _, m := range methods {
 		if mset.insert(m) != nil {

src/go/types/typestring.go

@@ -156,6 +156,7 @@ func writeType(buf *bytes.Buffer, typ Type, qf Qualifier, visited []Type) {
 		//     }
 		//
 		buf.WriteString("interface{")
+		empty := true
 		if gcCompatibilityMode {
 			// print flattened interface
 			// (useful to compare against gc-generated interfaces)
@@ -165,6 +166,7 @@ func writeType(buf *bytes.Buffer, typ Type, qf Qualifier, visited []Type) {
 				}
 				buf.WriteString(m.name)
 				writeSignature(buf, m.typ.(*Signature), qf, visited)
+				empty = false
 			}
 		} else {
 			// print explicit interface methods and embedded types
@@ -174,14 +176,22 @@ func writeType(buf *bytes.Buffer, typ Type, qf Qualifier, visited []Type) {
 				}
 				buf.WriteString(m.name)
 				writeSignature(buf, m.typ.(*Signature), qf, visited)
+				empty = false
 			}
 			for i, typ := range t.embeddeds {
 				if i > 0 || len(t.methods) > 0 {
 					buf.WriteString("; ")
 				}
 				writeType(buf, typ, qf, visited)
+				empty = false
 			}
 		}
+		if t.allMethods == nil || len(t.methods) > len(t.allMethods) {
+			if !empty {
+				buf.WriteByte(' ')
+			}
+			buf.WriteString("/* incomplete */")
+		}
 		buf.WriteByte('}')
 
 	case *Map:

src/go/types/typestring_test.go

@@ -138,6 +138,26 @@ func TestTypeString(t *testing.T) {
 	}
 }
 
+func TestIncompleteInterfaces(t *testing.T) {
+	sig := NewSignature(nil, nil, nil, false)
+	for _, test := range []struct {
+		typ  *Interface
+		want string
+	}{
+		{new(Interface), "interface{/* incomplete */}"},
+		{new(Interface).Complete(), "interface{}"},
+		{NewInterface(nil, nil), "interface{/* incomplete */}"},
+		{NewInterface(nil, nil).Complete(), "interface{}"},
+		{NewInterface([]*Func{NewFunc(token.NoPos, nil, "m", sig)}, nil), "interface{m() /* incomplete */}"},
+		{NewInterface([]*Func{NewFunc(token.NoPos, nil, "m", sig)}, nil).Complete(), "interface{m()}"},
+	} {
+		got := test.typ.String()
+		if got != test.want {
+			t.Errorf("got: %s, want: %s", got, test.want)
+		}
+	}
+}
+
 func TestQualifiedTypeString(t *testing.T) {
 	p, _ := pkgFor("p.go", "package p; type T int", nil)
 	q, _ := pkgFor("q.go", "package q", nil)

src/go/types/typexpr.go

@@ -540,9 +540,8 @@ func (check *Checker) interfaceType(iface *Interface, ityp *ast.InterfaceType, d
 		}
 		iface.embeddeds = append(iface.embeddeds, named)
 		// collect embedded methods
-		if debug && embed.allMethods == nil {
+		if embed.allMethods == nil {
-			check.dump("%s: incomplete embedded interface %s", pos, named)
+			check.errorf(pos, "internal error: incomplete embedded interface %s (issue #18395)", named)
-			unreachable()
 		}
 		for _, m := range embed.allMethods {
 			if check.declareInSet(&mset, pos, m) {