go/types: first cut at type-checking type parameters with operators

- implemented for comparisons for now
- can type-check generic min
- also: rebased on top of Go tip

Change-Id: Id35582a59c4cddcb2b4ae9c7d7154ef8e41580ab

src/go/types/predicates.go

@@ -62,8 +62,13 @@ func isUntyped(typ Type) bool {
 }
 
 func isOrdered(typ Type) bool {
-	t, ok := typ.Underlying().(*Basic)
-	return ok && t.info&IsOrdered != 0
+	switch t := typ.Underlying().(type) {
+	case *Basic:
+		return t.info&IsOrdered != 0
+	case *TypeParam:
+		return t.contr.ifaceAt(t.index).is(isOrdered)
+	}
+	return false
 }
 
 func isConstType(typ Type) bool {

src/go/types/testdata/contracts.go2

@@ -116,3 +116,16 @@ var _ T2(int32)
 var _ T2 /* ERROR not satisfied */ (int64)
 var _ T2(int)
 var _ T2(struct{x int})
+
+// Use of contracts in parameterized functions
+
+contract SignedInteger(T) {
+        T int8, int16, int32, int64, int
+}
+
+func min(type T SignedInteger)(x, y T) T {
+        if x < y {
+                return x
+        }
+        return y
+}

src/go/types/type.go

@@ -254,6 +254,16 @@ type Interface struct {
 	types []Type // for contracts
 }
 
+// is reports whether interface t represents types that all satisfy pred.
+func (t *Interface) is(pred func(Type) bool) bool {
+	for _, t := range t.types {
+		if !pred(t) {
+			return false
+		}
+	}
+	return true
+}
+
 // emptyInterface represents the empty (completed) interface
 var emptyInterface = Interface{allMethods: markComplete}