Symmetric common_descendent for two generic class types (#10831)

spec/compiler/semantic/is_a_spec.cr

@@ -258,4 +258,15 @@ describe "Semantic: is_a?" do
       end
       CRYSTAL
   end
+
+  it "matches `is_a?(GenericA) && is_a?(GenericB)` when GenericB <= GenericA (#10831)" do
+    run(<<-CRYSTAL).to_b.should be_true
+      class A; end
+      class B(T) < A; end
+      class C(T) < B(T); end
+
+      x = C(Int32).new.as(A)
+      x.is_a?(B) && x.is_a?(C)
+      CRYSTAL
+  end
 end

src/compiler/crystal/semantic/type_intersect.cr

@@ -214,7 +214,13 @@ module Crystal
     def self.common_descendent(type1 : GenericClassType, type2 : GenericClassType)
       return type1 if type1 == type2
 
-      common_descendent_instance_and_generic(type1, type2)
+      # Try both directions: e.g. for `class C(T) < B(T)`, `common_descendent(B, C)`
+      # should resolve to `C` because every C is also a B. Without the symmetric
+      # check, chained type filters like `x.is_a?(B) && x.is_a?(C)` narrow `x`
+      # to `B(T)` and `is_a?(C)` returns `false` even when the runtime value is
+      # actually a C (#10831).
+      common_descendent_instance_and_generic(type1, type2) ||
+        common_descendent_instance_and_generic(type2, type1)
     end
 
     def self.common_descendent(type1 : Type, type2 : AliasType)