[libcxx] [test] Add return values after assert(false) in some experimental tests

[mstorsjo]

Summary

When building in MSVC mode (in release mode), the assert(false) don't
make the end of the function unreachable, so add return statements to
silence compiler warnings (treated as errors).

[curdeius]

LGTM.

[• Quuxplusone]

Ship it! (Bonus points if you replace virtual by override in all cases; these tests are C++11-and-later-only.)

[mstorsjo]

Rerun CI after changing virtual into override

libcxx/test/std/experimental/memory/memory.resource.global/new_delete_resource.pass.cpp

 
 #include "test_macros.h"
 
 namespace ex = std::experimental::pmr;
 
 struct assert_on_compare : public ex::memory_resource
 {
 protected:
-    virtual void * do_allocate(size_t, size_t)
-    { assert(false); }
+    void * do_allocate(size_t, size_t) override
+    { assert(false); return nullptr; }
 
-    virtual void do_deallocate(void *, size_t, size_t)
+    void do_deallocate(void *, size_t, size_t) override
     { assert(false); }
 
-    virtual bool do_is_equal(ex::memory_resource const &) const noexcept
-    { assert(false); }
+    bool do_is_equal(ex::memory_resource const &) const noexcept override
+    { assert(false); return false; }
 };
 
 void test_return()
 {
     {
         static_assert(std::is_same<
             decltype(ex::new_delete_resource()), ex::memory_resource*
           >::value, "");

libcxx/test/std/experimental/memory/memory.resource.global/null_memory_resource.pass.cpp

 #include "test_macros.h"
 #include "count_new.h"
 
 namespace ex = std::experimental::pmr;
 
 struct assert_on_compare : public ex::memory_resource
 {
 protected:
-    virtual void * do_allocate(size_t, size_t)
-    { assert(false); }
+    void * do_allocate(size_t, size_t) override
+    { assert(false); return nullptr; }
 
-    virtual void do_deallocate(void *, size_t, size_t)
+    void do_deallocate(void *, size_t, size_t) override
     { assert(false); }
 
-    virtual bool do_is_equal(ex::memory_resource const &) const noexcept
-    { assert(false); }
+    bool do_is_equal(ex::memory_resource const &) const noexcept override
+    { assert(false); return false; }
 };
 
 void test_return()
 {
     {
         static_assert(std::is_same<
             decltype(ex::null_memory_resource()), ex::memory_resource*
           >::value, "");