tests: Work around gcc12 bug with c++20 edge cases

There are multiple tests that, with gcc 12, get an ambiguous overload
error, with the basic problem being multiple inheritance paths.
Somehow, c++20 requires that this work, and gcc 13 accepts the code.
(clang 15 also rejects this code, and clang 16 is ok).

Because the point is to test qgis, not compiler conformance, and
because this overload pattern does not appear in the codebase (it
would error if it did), downcast one side of the comparison to the
base class to avoid the error.  Analysis and fix suggestion are due to
Even Rouault.

credit to @gdt for original commit

Author: nyalldawson

tests/src/core/geometry/testqgsgeometrycollection.cpp

@@ -195,8 +195,8 @@ void TestQgsGeometryCollection::geometryCollection()
   QVERIFY( !( emptyCollection == c11 ) );
   QVERIFY( emptyCollection != c11 );
   QgsPoint notCollection;
-  QVERIFY( !( emptyCollection == notCollection ) );
-  QVERIFY( emptyCollection != notCollection );
+  QVERIFY( !( *static_cast< QgsAbstractGeometry * >( &emptyCollection ) == notCollection ) );
+  QVERIFY( *static_cast< QgsAbstractGeometry * >( &emptyCollection ) != notCollection );
   QgsMultiPoint mp;
   QgsMultiLineString ml;
   QVERIFY( mp != ml );

tests/src/core/geometry/testqgslinestring.cpp

@@ -953,8 +953,8 @@ void TestQgsLineString::equality()
   QVERIFY( ls6 != QgsCircularString() );
 
   QgsPoint p1;
-  QVERIFY( !( ls6 == p1 ) );
-  QVERIFY( ls6 != p1 );
+  QVERIFY( !( *static_cast< QgsAbstractGeometry * >( &ls6 ) == p1 ) );
+  QVERIFY( *static_cast< QgsAbstractGeometry * >( &ls6 ) != p1 );
   QVERIFY( ls6 == ls6 );
 }
 

tests/src/core/geometry/testqgspoint.cpp

@@ -372,9 +372,17 @@ void TestQgsPoint::equality()
 
   QVERIFY( QgsPoint( Qgis::WkbType::Point, 2 / 3.0, 1 / 3.0 ) != QgsPoint( Qgis::WkbType::PointZ, 2 / 3.0, 1 / 3.0 ) );
 
+  /*
+   * gcc12 throws an "ambiguous overload" error when there are
+   * multiple possible choices due to inheritance.  Downcast pt1 to
+   * reduce the search space.  This problem does not occur in actual
+   * code -- only in tests.  Accept a workaround because the point is
+   * to test the code, not verify that the compiler is pedantically
+   * correct.
+   */
   QgsLineString ls;
-  QVERIFY( pt1 != ls );
-  QVERIFY( !( pt1 == ls ) );
+  QVERIFY( *static_cast< QgsAbstractGeometry * >( &pt1 ) != ls );
+  QVERIFY( !( *static_cast< QgsAbstractGeometry * >( &pt1 ) == ls ) );
 }
 
 void TestQgsPoint::operators()

tests/src/core/geometry/testqgspolygon.cpp

@@ -267,8 +267,8 @@ void TestQgsPolygon::equality()
   QVERIFY( !( pl1 != pl2 ) );
 
   QgsLineString ls;
-  QVERIFY( pl1 != ls );
-  QVERIFY( !( pl1 == ls ) );
+  QVERIFY( *static_cast< QgsAbstractGeometry * >( &pl1 ) != ls );
+  QVERIFY( !( *static_cast< QgsAbstractGeometry * >( &pl1 ) == ls ) );
 }
 
 void TestQgsPolygon::clone()