go/types: implemented AcceptContracts flag to enable/disable contracts

AcceptContracts is a global flag (api.go) to enable/disable contracts.
All go/types tests pass with the flag in either setting, i.e., all
go/types tests involving contracts have been rewritten or disabled.
To ensure the existing go2go code runs, AcceptContracts is set to true
for now.

If contracts are disabled, "comparable" is a predeclared interface
(without type parameters) rather than a predeclared contract (with
one type parameter).

Change-Id: I2ddade9ae38d61d9edca6020f3334d2215b84f1d
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go2-dev/+/734984
Reviewed-by: Robert Griesemer <gri@google.com>

src/go/types/api.go

@@ -34,6 +34,9 @@ import (
 	"go/token"
 )
 
+// If AcceptContracts is set, contracts are accepted.
+const AcceptContracts = true
+
 // An Error describes a type-checking error; it implements the error interface.
 // A "soft" error is an error that still permits a valid interpretation of a
 // package (such as "unused variable"); "hard" errors may lead to unpredictable

src/go/types/check_test.go

@@ -104,7 +104,7 @@ var tests = [][]string{
 	{"testdata/typeparams.go2"},
 	{"testdata/typeinst.go2"},
 	{"testdata/typeinst2.go2"},
-	{"testdata/contracts.go2"},
+	// {"testdata/contracts.go2"}, // disabled for now
 	{"testdata/issues.go2"},
 	{"testdata/todos.go2"},
 
@@ -116,7 +116,7 @@ var tests = [][]string{
 	{"testdata/linalg.go2"},
 
 	// Go 2 prototype examples
-	{"examples/contracts.go2"},
+	// {"examples/contracts.go2"}, disabled for now
 	{"examples/functions.go2"},
 	{"examples/methods.go2"},
 	{"examples/types.go2"},

src/go/types/decl.go

@@ -205,6 +205,11 @@ func (check *Checker) objDecl(obj Object, def *Named) {
 		// functions may be recursive - no need to track dependencies
 		check.funcDecl(obj, d)
 	case *Contract:
+		if !AcceptContracts {
+			check.errorf(obj.pos, "contracts are not accepted")
+			obj.typ = Typ[Invalid]
+			break
+		}
 		check.contractDecl(obj, d.cdecl)
 	default:
 		unreachable()

src/go/types/testdata/issues.go2

@@ -16,6 +16,7 @@ import "io"
 // the numbers for anInt and twoInt (which embedds anInt).
 // The fix simply uses the instantiated non-parameterized
 // underlying interface of atInt<K> rather than anInt<K>.
+/*
 contract anInt(T) {
 	T int
 }
@@ -29,8 +30,10 @@ func f(type K, V twoInt)()
 func _ () {
 	f(int, int)()
 }
+*/
 
 // This is the original (simplified) program causing the same issue.
+/*
 contract onecomparable(T) {
 	T int, int8, int16, int32, int64,
 		uint, uint8, uint16, uint32, uint64, uintptr,
@@ -49,6 +52,7 @@ func _() {
 	var m map[int]int
 	Equal(m, nil)
 }
+*/
 
 // Interfaces are always comparable (though the comparison may panic at runtime).
 func eql(type T comparable)(x, y T) bool {
@@ -69,21 +73,23 @@ func _() {
 // the pointer in the implementation of the method lookup because
 // the type bound of T is an interface an pointer to interface types
 // have no methods and then the lookup would fail.
-contract C(T) {
-    T m()
+type C(type T) interface {
+    m()
 }
 
-// using contract C
+// using type bound C
 func _(type T C)(x *T) {
 	x.m()
 }
 
 // using an interface as bound
+// TODO(gri) this is now the same as above (no contracts anymore)
 func _(type T interface{ m() })(x *T) {
 	x.m()
 }
 
 // This is the original (simplified) program causing the same issue.
+/*
 type GraphP(type Node, Edge GP) struct {
 	nodes []*Node
 }
@@ -95,6 +101,7 @@ contract GP(Node, Edge) {
 func (g *GraphP(Node, Edge)) Edges(n *Node) []*Edge {
 	return n.Edges()
 }
+*/
 
 // In a generic function body all method calls will be pointer method calls.
 // If necessary, the function body will insert temporary variables, not seen
@@ -114,6 +121,7 @@ func _() {
 }
 
 // This is the original (simplified) program causing the same issue.
+/*
 func NewP(type Node, Edge GP)(nodes []*Node) *GraphP(Node, Edge) {
 	return &GraphP(Node, Edge){nodes: nodes}
 }
@@ -124,6 +132,7 @@ type E struct{}
 func F() {
 	_ = NewP(N, E)(nil)
 }
+*/
 
 // When a type parameter is used as an argument to instantiate a parameterized
 // type with a type list constraint, all of the type argument's types in its
@@ -136,6 +145,7 @@ func _(type P)() {
 }
 
 // This is the original (simplified) program causing the same issue.
+/*
 contract Unsigned(T) {
 	T uint
 }
@@ -148,14 +158,15 @@ func (u T2(U)) Add1() U {
     return u.s + 1
 }
 
-func NewT2(type U)() T2(U /* ERROR U has no type constraints */ ) {
-    return T2(U /* ERROR U has no type constraints */ ){}
+func NewT2(type U)() T2(U /- ERROR U has no type constraints -/ ) {
+    return T2(U /- ERROR U has no type constraints -/ ){}
 }
 
 func _() {
     u := NewT2(string)()
     _ = u.Add1()
 }
+*/
 
 // When we encounter an instantiated type such as Elem(T) we must
 // not "expand" the instantiation when the type to be instantiated

src/go/types/testdata/linalg.go2

@@ -6,13 +6,13 @@ package linalg
 
 import "math"
 
-// Numeric is a contract that matches any numeric type.
+// Numeric is type bound that matches any numeric type.
 // It would likely be in a contracts package in the standard library.
-contract Numeric(T) {
-	T int; T int8; T int16; T int32; T int64
-	T uint; T uint8; T uint16; T uint32; T uint64; T uintptr
-	T float32; T float64
-	T complex64; T complex128
+type Numeric interface {
+	type int, int8, int16, int32, int64
+	type uint, uint8, uint16, uint32, uint64, uintptr
+	type float32, float64
+	type complex64, complex128
 }
 
 func DotProduct(type T Numeric)(s1, s2 []T) T {
@@ -27,10 +27,10 @@ func DotProduct(type T Numeric)(s1, s2 []T) T {
 }
 
 // NumericAbs matches numeric types with an Abs method.
-contract NumericAbs(T) {
-	Numeric(T)
+type NumericAbs(type T) interface {
+	Numeric
 
-	T Abs() T
+	Abs() T
 }
 
 // AbsDifference computes the absolute value of the difference of
@@ -40,16 +40,16 @@ func AbsDifference(type T NumericAbs)(a, b T) T {
 	return d.Abs()
 }
 
-// OrderedNumeric matches numeric types that support the < operator.
-contract OrderedNumeric(T) {
-	T int; T int8; T int16; T int32; T int64
-	T uint; T uint8; T uint16; T uint32; T uint64; T uintptr
-	T float32; T float64
+// OrderedNumeric is a type bound that matches numeric types that support the < operator.
+type OrderedNumeric interface {
+	type int, int8, int16, int32, int64
+	type uint, uint8, uint16, uint32, uint64, uintptr
+	type float32, float64
 }
 
-// Complex matches the two complex types, which do not have a < operator.
-contract Complex(T) {
-	T complex64; T complex128
+// Complex is a type bound that matches the two complex types, which do not have a < operator.
+type Complex interface {
+	type complex64, complex128
 }
 
 // OrderedAbs is a helper type that defines an Abs method for

src/go/types/testdata/tmp.go2

@@ -1,15 +1,21 @@
 package p
 
-type Adder(type T) interface {
-	Add(T) T
+// These are only ok if AcceptContracts = false.
+// (The comparable contract cannot be embedded in Bound.)
+/*
+func _(type T comparable)(x, y T) bool {
+	return x == y || x != y
 }
 
-// We don't need to explicitly instantiate the Adder bound
-// if we have exactly one type parameter.
-func Sum(type T Adder)(list []T) T {
-	var sum T
-	for _, x := range list {
-		sum = sum.Add(x)
-	}
-	return sum
+type Bound interface {
+	comparable
 }
+
+func _(type T Bound)(x, y T) bool {
+	return x == y || x != y
+}
+
+func _(type A, B Bound)(a1, a2 A, b1, b2 B) bool {
+	return a1 == a2 || b1 != b2
+}
+*/

src/go/types/testdata/todos.go2

@@ -9,9 +9,12 @@
 package p
 
 // Pointer designation for type parameters is not yet supported.
+// TODO(gri) Do we need to do something about this now that we have only interfaces?
+/*
 contract _C(T) {
-	* /* ERROR not yet supported */ T m()
+	*T m()
 }
+*/
 
 // Indexing on generic types containing type parameters in their type list
 // is not yet supported.

src/go/types/testdata/typeinst2.go2

@@ -22,7 +22,7 @@ type (
 func _() {
 	var x1 T1(int)
 	var x2 T2(int, float32)
-	
+
 	x1.f1.f2 = 0
 	x1.f1 = x2
 }
@@ -81,8 +81,8 @@ func (it Iterator(K)) Next() K {
 
 // A more complex test case testing type bounds (extracted from linalg.go2 and reduced to essence)
 
-contract NumericAbs(T) {
-	T Abs() T
+type NumericAbs(type T) interface {
+	Abs() T
 }
 
 func AbsDifference(type T NumericAbs)(x T)

src/go/types/testdata/typeparams.go2

@@ -55,8 +55,8 @@ func _(type T interface{type int, float32, bool})(x, y T) bool { return x /* ERR
 func _(type T C1)(x, y T) bool { return x /* ERROR cannot compare */ < y }
 func _(type T C2)(x, y T) bool { return x < y }
 
-contract C1(T) {}
-contract C2(T) { T int, float32 }
+type C1(type T) interface{}
+type C2(type T) interface{ type int, float32 }
 
 func new(type T)() *T {
         var x T
@@ -365,10 +365,10 @@ func _(type T I3)(x I3, p T) {
 }
 
 // error messages related to type bounds mention those bounds
-contract C(P) {}
+type C(type P) interface{}
 
 func _(type P C) (x P) {
-	x.m /* ERROR contract C has no method m */ ()
+	x.m /* ERROR x.m undefined */ ()
 }
 
 type I interface {}

src/go/types/universe.go

@@ -188,7 +188,7 @@ func DefPredeclaredTestFuncs() {
 	def(newBuiltin(_Trace))
 }
 
-func defPredeclaredContracts() {
+func defPredeclaredComparableContract() {
 	// The "comparable" contract can be envisioned as defined like
 	//
 	// contract comparable(T) {
@@ -233,6 +233,33 @@ func defPredeclaredContracts() {
 	def(obj)
 }
 
+func defPredeclaredComparableInterface() {
+	// The "comparable" interface can be envisioned as defined like
+	//
+	// type comparable interface {
+	//         == () untyped bool
+	//         != () untyped bool
+	// }
+	//
+	// == and != cannot be user-declared but we can declare
+	// a magic method == and check for its presence when needed.
+
+	// Define interface { == () }. We don't care about the signature
+	// for == so leave it empty except for the receiver, which is
+	// set up later to match the usual interface method assumptions.
+	sig := new(Signature)
+	eql := NewFunc(token.NoPos, nil, "==", sig)
+	iface := NewInterfaceType([]*Func{eql}, nil).Complete()
+
+	// set up the defined type for the interface
+	obj := NewTypeName(token.NoPos, nil, "comparable", nil)
+	named := NewNamed(obj, iface, nil)
+	obj.color_ = black
+	sig.recv = NewVar(token.NoPos, nil, "", named) // complete == signature
+
+	def(obj)
+}
+
 func init() {
 	Universe = NewScope(nil, token.NoPos, token.NoPos, "universe")
 	Unsafe = NewPackage("unsafe", "unsafe")
@@ -242,7 +269,11 @@ func init() {
 	defPredeclaredConsts()
 	defPredeclaredNil()
 	defPredeclaredFuncs()
-	defPredeclaredContracts()
+	if AcceptContracts {
+		defPredeclaredComparableContract()
+	} else {
+		defPredeclaredComparableInterface()
+	}
 
 	universeIota = Universe.Lookup("iota").(*Const)
 	universeByte = Universe.Lookup("byte").(*TypeName).typ.(*Basic)