Semantic: prevent infinite recursion by alias and generics combination (

#5330)

* Semantic: prevent infinite recursion by alias and generics combination

Fixed #5329

* Fix typos

Thank you @jhass and @lachlan.

spec/compiler/semantic/alias_spec.cr

@@ -297,4 +297,17 @@ describe "Semantic: alias" do
       ),
       "undefined constant T"
   end
+
+  it "doesn't crash by infinite recursion against type alias and generics (#5329)" do
+    assert_error %(
+      class Foo(T)
+        def initialize(@foo : T)
+        end
+      end
+
+      alias Bar = Foo(Bar | Int32)
+
+      Foo(Bar).new(Foo.new(1).as(Bar))
+    ), "can't cast Foo(Int32) to Bar"
+  end
 end

src/compiler/crystal/semantic/restrictions.cr

@@ -667,7 +667,10 @@ module Crystal
       elsif context.strict?
         type_var == other_type_var
       else
-        type_var.restrict(other_type_var, context) == type_var
+        # To prevent infinite recursion, it checks equality between
+        # `type_var` and `other_type_var` directly before try to restrict
+        # `type_var` by `other_type_var`.
+        type_var == other_type_var || type_var.restrict(other_type_var, context) == type_var
       end
     end
   end