Also consider accessed type when checking for classwide type

When checking if types match (in an assign context), also consider
accessed type when checking for classwide type.

ada/ast.py

@@ -7104,7 +7104,8 @@ def matching_assign_type(expected_type=T.BaseTypeDecl.entity):
             Entity.matching_type(expected_type),
 
             And(
-                expected_type.is_classwide,
+                expected_type.is_classwide
+                | expected_type.accessed_type._.is_classwide,
                 actual_type.matching_formal_prim_type(expected_type)
             ),
 

testsuite/tests/name_resolution/matching_assign_type/test.adb

@@ -0,0 +1,20 @@
+procedure Test is
+
+   type MDI is tagged null record;
+   type MDI_Child is access all MDI'Class;
+
+   type Local_T is new MDI with null record;
+
+   function Set return access Local_T'Class;
+
+   function Set return access Local_T'Class is
+   begin
+      return null;
+   end Set;
+
+   Child : MDI_Child;
+
+begin
+    Child := Set;
+    pragma Test_Statement;
+end Test;

testsuite/tests/name_resolution/matching_assign_type/test.out

@@ -0,0 +1,17 @@
+Analyzing test.adb
+##################
+
+Resolving xrefs for node <AssignStmt test.adb:18:5-18:18>
+*********************************************************
+
+Expr: <Id "Child" test.adb:18:5-18:10>
+  references:    <DefiningName "Child" test.adb:15:4-15:9>
+  type:          <ConcreteTypeDecl ["MDI_Child"] test.adb:4:4-4:43>
+  expected type: None
+Expr: <Id "Set" test.adb:18:14-18:17>
+  references:    <DefiningName "Set" test.adb:10:13-10:16>
+  type:          <AnonymousTypeDecl ["None"] test.adb:10:24-10:44>
+  expected type: <ConcreteTypeDecl ["MDI_Child"] test.adb:4:4-4:43>
+
+
+Done.

testsuite/tests/name_resolution/matching_assign_type/test.yaml

@@ -0,0 +1,2 @@
+driver: name-resolution
+input_sources: [test.adb]