Fix class member completion for classes with base types (#19179)

src/System.Management.Automation/engine/parser/TypeInferenceVisitor.cs

@@ -322,7 +322,18 @@ namespace System.Management.Automation
                 }
 
                 var baseTypeDefinitionAst = baseTypeName._typeDefinitionAst;
-                results.AddRange(GetMembersByInferredType(new PSTypeName(baseTypeDefinitionAst), isStatic, filterToCall));
+                if (baseTypeDefinitionAst is null)
+                {
+                    var baseReflectionType = baseTypeName.GetReflectionType();
+                    if (baseReflectionType is not null)
+                    {
+                        results.AddRange(GetMembersByInferredType(new PSTypeName(baseReflectionType), isStatic, filterToCall));
+                    }
+                }
+                else
+                {
+                    results.AddRange(GetMembersByInferredType(new PSTypeName(baseTypeDefinitionAst), isStatic, filterToCall));
+                }
             }
 
             // Add stuff from our base class System.Object.

test/powershell/Host/TabCompletion/TabCompletion.Tests.ps1

@@ -843,6 +843,17 @@ Verb-Noun -Param1 Hello ^
         $res.CompletionMatches[0].CompletionText | Should -Be "Attributes"
     }
 
+    it 'Should complete base class members of types without type definition AST' {
+        $res = TabExpansion2 -inputScript @'
+class InheritedClassTest : System.Attribute
+{
+    [void] TestMethod()
+    {
+        $this.
+'@
+        $res.CompletionMatches.CompletionText | Should -Contain 'TypeId'
+    }
+
     Context "Script name completion" {
         BeforeAll {
             Setup -f 'install-powershell.ps1' -Content ""