reflect: add Type.Comparable

Like most of the Type methods, the definition of Comparable
is what the Go spec says it is.

Fixes #7911.

LGTM=gri
R=gri, r
CC=golang-codereviews
https://golang.org/cl/144020043

src/reflect/all_test.go

diff --git a/src/reflect/all_test.go b/src/reflect/all_test.go
index 688b5d3..4be0e35 100644
--- a/src/reflect/all_test.go
+++ b/src/reflect/all_test.go
@@ -3185,6 +3185,44 @@
 	}
 }
 
+type ComparableStruct struct {
+	X int
+}
+
+type NonComparableStruct struct {
+	X int
+	Y map[string]int
+}
+
+var comparableTests = []struct {
+	typ Type
+	ok  bool
+}{
+	{TypeOf(1), true},
+	{TypeOf("hello"), true},
+	{TypeOf(new(byte)), true},
+	{TypeOf((func())(nil)), false},
+	{TypeOf([]byte{}), false},
+	{TypeOf(map[string]int{}), false},
+	{TypeOf(make(chan int)), true},
+	{TypeOf(1.5), true},
+	{TypeOf(false), true},
+	{TypeOf(1i), true},
+	{TypeOf(ComparableStruct{}), true},
+	{TypeOf(NonComparableStruct{}), false},
+	{TypeOf([10]map[string]int{}), false},
+	{TypeOf([10]string{}), true},
+	{TypeOf(new(interface{})).Elem(), true},
+}
+
+func TestComparable(t *testing.T) {
+	for _, tt := range comparableTests {
+		if ok := tt.typ.Comparable(); ok != tt.ok {
+			t.Errorf("TypeOf(%v).Comparable() = %v, want %v", tt.typ, ok, tt.ok)
+		}
+	}
+}
+
 func TestOverflow(t *testing.T) {
 	if ovf := V(float64(0)).OverflowFloat(1e300); ovf {
 		t.Errorf("%v wrongly overflows float64", 1e300)

src/reflect/type.go

diff --git a/src/reflect/type.go b/src/reflect/type.go
index 67818f7..f099546 100644
--- a/src/reflect/type.go
+++ b/src/reflect/type.go
@@ -96,6 +96,9 @@
 	// ConvertibleTo returns true if a value of the type is convertible to type u.
 	ConvertibleTo(u Type) bool
 
+	// Comparable returns true if values of this type are comparable.
+	Comparable() bool
+
 	// Methods applicable only to some types, depending on Kind.
 	// The methods allowed for each kind are:
 	//
@@ -248,7 +251,7 @@
 	align         uint8             // alignment of variable with this type
 	fieldAlign    uint8             // alignment of struct field with this type
 	kind          uint8             // enumeration for C
-	alg           *uintptr          // algorithm table (../runtime/runtime.h:/Alg)
+	alg           *typeAlg          // algorithm table (../runtime/runtime.h:/Alg)
 	gc            [2]unsafe.Pointer // garbage collection data
 	string        *string           // string form; unnecessary but undeniably useful
 	*uncommonType                   // (relatively) uncommon fields
@@ -256,6 +259,15 @@
 	zero          unsafe.Pointer    // pointer to zero value
 }
 
+type typeAlg struct {
+	// function for hashing objects of this type
+	// (ptr to object, size, seed) -> hash
+	hash func(unsafe.Pointer, uintptr, uintptr) uintptr
+	// function for comparing objects of this type
+	// (ptr to object A, ptr to object B, size) -> ==?
+	equal func(unsafe.Pointer, unsafe.Pointer, uintptr) bool
+}
+
 // Method on non-interface type
 type method struct {
 	name    *string        // name of method
@@ -1096,6 +1108,10 @@
 	return convertOp(uu, t) != nil
 }
 
+func (t *rtype) Comparable() bool {
+	return t.alg != nil && t.alg.equal != nil
+}
+
 // implements returns true if the type V implements the interface type T.
 func implements(T, V *rtype) bool {
 	if T.Kind() != Interface {