don't diagnose -Wunused-parameter in virtual method or method that overrides base class method

[danielmarjamaki]

Summary

Don't diagnose -Wunused-parameter in methods that override other methods because the overridden methods might use the parameter

Don't diagnose -Wunused-parameter in virtual methods because these might be overriden by other methods that use the parameter.

Such diagnostics could be more accurately written if they are based on whole-program-analysis that establish if such parameter is unused in all methods.

[thakis]

Can't you just change your signature to

virtual void a(int /* x */) {}

in these cases?

[danielmarjamaki]

In D11940#221718, @thakis wrote:

Can't you just change your signature to

virtual void a(int /* x */) {}

in these cases?

hmm.. yes I agree.

You are right, so I change my mind about this patch.

lib/Sema/SemaDecl.cpp

if (FD) {
 
     // MSVC permits the use of pure specifier (=0) on function definition,
     // defined at class scope, warn about this non-standard construct.
     if (getLangOpts().MicrosoftExt && FD->isPure() && FD->isCanonicalDecl())
       Diag(FD->getLocation(), diag::ext_pure_function_definition);
 
     if (!FD->isInvalidDecl()) {
       // Don't diagnose unused parameters of defaulted or deleted functions.
-      if (!FD->isDeleted() && !FD->isDefaulted())
+      if (!FD->isDeleted() && !FD->isDefaulted()) {
+        // Don't diagnose unused parameters in virtual methods or
+        // in methods that override base class methods.
+        const auto MD = dyn_cast<CXXMethodDecl>(FD);
+        if (!MD || (MD->size_overridden_methods() == 0U && !MD->isVirtual()))
           DiagnoseUnusedParameters(FD->param_begin(), FD->param_end());
+      }
       DiagnoseSizeOfParametersAndReturnValue(FD->param_begin(), FD->param_end(),
                                              FD->getReturnType(), FD);
 
       // If this is a structor, we need a vtable.
       if (CXXConstructorDecl *Constructor = dyn_cast<CXXConstructorDecl>(FD))
         MarkVTableUsed(FD->getLocation(), Constructor->getParent());
       else if (CXXDestructorDecl *Destructor = dyn_cast<CXXDestructorDecl>(FD))
         MarkVTableUsed(FD->getLocation(), Destructor->getParent());

test/SemaCXX/warn-unused-parameters.cpp

 
 // Make sure both parameters aren't considered unused.
 template <typename... T>
 static int test_pack(T... t, T... s)
 {
   auto l = [&t...]() { return sizeof...(s); };
   return l();
 }
+
+// Don't diagnose virtual methods or methods that override base class
+// methods.
+class Base {
+public:
+  virtual void f(int x);
+};
+
+class Derived : public Base {
+public:
+  // Don't warn in overridden methods.
+  virtual void f(int x) {}
+
+  // Don't warn in virtual methods.
+  virtual void a(int x) {}
+};
+