libstdc++: add missing return in generator assignment operator [PR118196]

libstdc++-v3/ChangeLog:

	PR libstdc++/118196
	* include/std/generator (generator::operator=(generator)): Add
	missing 'return *this;'.
	* testsuite/24_iterators/range_generators/pr118196.cc: New test.

libstdc++-v3/include/std/generator

@@ -717,6 +717,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       {
 	swap(__other._M_coro, this->_M_coro);
 	swap(__other._M_began, this->_M_began);
+	return *this;
       }
 
       _Iterator

libstdc++-v3/testsuite/24_iterators/range_generators/pr118196.cc

+// { dg-do compile { target c++23 } }
+#include "<generator>"
+
+std::generator<int>
+generator();
+
+void
+try_reassigning()
+{
+  auto gen = generator();
+  auto gen2 = generator();
+  gen = std::move(gen2);
+}