Implement basic constraints unit tests. (#13210)

This adds some basic constraints tests with the intent of adding more complex 
ones for variadic generics later.

Co-authored-by: Nikita Sobolev <[email protected]>

Author: jhance

mypy/constraints.py

@@ -54,6 +54,14 @@ def __repr__(self) -> str:
             op_str = ':>'
         return f'{self.type_var} {op_str} {self.target}'
 
+    def __hash__(self) -> int:
+        return hash((self.type_var, self.op, self.target))
+
+    def __eq__(self, other: object) -> bool:
+        if not isinstance(other, Constraint):
+            return False
+        return (self.type_var, self.op, self.target) == (other.type_var, other.op, other.target)
+
 
 def infer_constraints_for_callable(
         callee: CallableType,

mypy/test/testconstraints.py

@@ -0,0 +1,22 @@
+from mypy.test.helpers import Suite
+from mypy.test.typefixture import TypeFixture
+from mypy.constraints import infer_constraints, SUBTYPE_OF, SUPERTYPE_OF, Constraint
+
+
+class ConstraintsSuite(Suite):
+    def setUp(self) -> None:
+        self.fx = TypeFixture()
+
+    def test_no_type_variables(self) -> None:
+        assert not infer_constraints(self.fx.o, self.fx.o, SUBTYPE_OF)
+
+    def test_basic_type_variable(self) -> None:
+        fx = self.fx
+        for direction in [SUBTYPE_OF, SUPERTYPE_OF]:
+            assert infer_constraints(fx.gt, fx.ga, direction) == [
+                Constraint(
+                    type_var=fx.t.id,
+                    op=direction,
+                    target=fx.a,
+                )
+            ]