Fix covariance detection on sugared types

iwyu_ast_util.cc

@@ -747,7 +747,8 @@ bool HasCovariantReturnType(const CXXMethodDecl* method_decl) {
     // of where return type differs is when they're actually covariant.
     // That is, if Clang can already compile this code without errors, and
     // return types differ, it can only be due to covariance.
-    if ((*it)->getReturnType() != derived_return_type)
+    if ((*it)->getReturnType().getCanonicalType() !=
+        derived_return_type.getCanonicalType())
       return true;
   }
 

tests/cxx/cvr.cc

@@ -14,8 +14,13 @@
 
 class Base;
 
+namespace ns {
+using ::Base;
+}
+
 class ReturnsBase {
   virtual Base* non_covariant() = 0;
+  virtual ns::Base* non_covariant_sugared() = 0;
 
   virtual Base* covariant_derived() = 0;
   virtual const Class* covariant_cv_qual() = 0;
@@ -28,6 +33,12 @@ class ReturnsDerived : public ReturnsBase {
     return 0;
   }
 
+  static Base b;
+
+  virtual decltype(b)* non_covariant_sugared() {
+    return 0;
+  }
+
   // C++ [class.virtual]p7, second bullet
   // Trigger covariant return types:
   //  Base is an unambiguous and accessible direct or indirect base class of