moc: fix GCC -Wuseless-cast warnings

When being queried for QMetaObject::IndexOfMethod, qt_static_metacall
would compare the argument with each of the class' methods'
addresses. Taking the address of an overloaded function is ambiguous
unless you cast to the right type, which the moc-generated code did
using explicit static_cast<>s. If the function is not overloaded, GCC
would warn about the static_cast<> being "useless", which isn't wrong.

Fix by using an implicit cast to a local variable of the correct type
instead of an explicit cast. Since there's no explicit cast anymore,
GCC doesn't warn.

Code before the change:

  using _t = void(Counter::*)(int );
  if (*reinterpret_cast<_t*>(_a[1]) == static_cast<_t>(&Counter::valueChanged)) {
                                       ^--------------------------------------
				              -Wuseless-cast
      *result = 0;
      return;
  }

After:

  using _t = void(Counter::*)(int );
  if (_t _q_method = &Counter::valueChanged; *reinterpret_cast<_t*>(_a[1]) == _q_method) {
      *result = 0;
      return;
  }

Since we're using a C++17 construct, we can't pick to 5.15.

Pick-to: 6.4 6.3 6.2
Fixes: QTBUG-71938
Change-Id: If6ba4bf17b3bf7f64e9662ba9d085273882fb460
Reviewed-by: Mate Barany <mate.barany@qt.io>
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/src/tools/moc/generator.cpp b/src/tools/moc/generator.cpp
index 49cdf4a0ad..f030bc8918 100644
--- a/src/tools/moc/generator.cpp
+++ b/src/tools/moc/generator.cpp
@@ -1186,7 +1186,7 @@ void Generator::generateStaticMetacall()
                 fprintf(out, ") const;\n");
             else
                 fprintf(out, ");\n");
-            fprintf(out, "            if (*reinterpret_cast<_t *>(_a[1]) == static_cast<_t>(&%s::%s)) {\n",
+            fprintf(out, "            if (_t _q_method = &%s::%s; *reinterpret_cast<_t *>(_a[1]) == _q_method) {\n",
                     cdef->classname.constData(), f.name.constData());
             fprintf(out, "                *result = %d;\n", methodindex);
             fprintf(out, "                return;\n");