Index: libcxx/test/libcxx/experimental/memory/memory.resource.adaptor/memory.resource.adaptor.mem/db_deallocate.pass.cpp
===================================================================
--- libcxx/test/libcxx/experimental/memory/memory.resource.adaptor/memory.resource.adaptor.mem/db_deallocate.pass.cpp
+++ libcxx/test/libcxx/experimental/memory/memory.resource.adaptor/memory.resource.adaptor.mem/db_deallocate.pass.cpp
@@ -14,8 +14,6 @@
 
 // T* polymorphic_allocator<T>::deallocate(T*, size_t size)
 
-// XFAIL: LIBCXX-WINDOWS-FIXME
-
 int AssertCount = 0;
 
 #define _LIBCPP_ASSERT(x, m) ((x) ? (void)0 : (void)::AssertCount++)
@@ -38,13 +36,8 @@
     ex::resource_adaptor<Alloc> r(Alloc{P});
     ex::memory_resource & m1 = r;
 
-#ifdef __STDCPP_DEFAULT_NEW_ALIGNMENT__
-    std::size_t maxSize = std::numeric_limits<std::size_t>::max()
-                            - __STDCPP_DEFAULT_NEW_ALIGNMENT__;
-#else
     std::size_t maxSize = std::numeric_limits<std::size_t>::max()
                             - alignof(std::max_align_t);
-#endif
 
     m1.deallocate(nullptr, maxSize);
     assert(AssertCount == 0);
Index: libcxx/test/libcxx/memory/trivial_abi/weak_ptr_ret.pass.cpp
===================================================================
--- libcxx/test/libcxx/memory/trivial_abi/weak_ptr_ret.pass.cpp
+++ libcxx/test/libcxx/memory/trivial_abi/weak_ptr_ret.pass.cpp
@@ -15,8 +15,6 @@
 // There were assertion failures in both parse and codegen, which are fixed in clang 11.
 // UNSUPPORTED: gcc, clang-4, clang-5, clang-6, clang-7, clang-8, clang-9, clang-10
 
-// XFAIL: LIBCXX-WINDOWS-FIXME
-
 #include <memory>
 #include <cassert>
 
@@ -51,9 +49,10 @@
   //
   // With trivial_abi, local_addr is the address of a local variable in
   // make_val, and hence different from &ret.
-#if !defined(__i386__) && !defined(__arm__)
+#if !defined(__i386__) && !defined(__arm__) && !defined(_WIN32)
   // On X86, structs are never returned in registers.
   // On ARM32, structs larger than 4 bytes cannot be returned in registers.
+  // On Windows, structs with a destructor are always returned indirectly.
   // Thus, weak_ptr will be passed indirectly even if it is trivial.
   assert((void*)&ret != local_addr);
 #endif
Index: libcxx/test/std/language.support/support.dynamic/new.delete/new.delete.array/delete_align_val_t_replace.pass.cpp
===================================================================
--- libcxx/test/std/language.support/support.dynamic/new.delete/new.delete.array/delete_align_val_t_replace.pass.cpp
+++ libcxx/test/std/language.support/support.dynamic/new.delete/new.delete.array/delete_align_val_t_replace.pass.cpp
@@ -20,11 +20,6 @@
 // UNSUPPORTED: clang-5, clang-6, clang-7
 // XFAIL: use_system_cxx_lib && target={{.+}}-apple-macosx10.{{9|10|11|12|13}}
 
-// On Windows libc++ doesn't provide its own definitions for new/delete
-// but instead depends on the ones in VCRuntime. However VCRuntime does not
-// yet provide aligned new/delete definitions so this test fails to compile/link.
-// XFAIL: LIBCXX-WINDOWS-FIXME
-
 // Libcxx when built for z/OS doesn't contain the aligned allocation functions,
 // nor does the dynamic library shipped with z/OS.
 // UNSUPPORTED: target={{.+}}-zos{{.*}}
@@ -48,13 +43,21 @@
     aligned_delete_called = 0;
 }
 
-void operator delete(void* p) TEST_NOEXCEPT
+// Override aligned operator new, to make sure that the returned pointer
+// is freeable with std::free(). (__libcpp_aligned_alloc and
+// __libcpp_aligned_free may be using functions incompatible with std::free().)
+void* operator new [] (std::size_t s, std::align_val_t) TEST_THROW_SPEC(std::bad_alloc)
+{
+    return std::malloc(s);
+}
+
+void operator delete [] (void* p) TEST_NOEXCEPT
 {
     ++unsized_delete_called;
     std::free(p);
 }
 
-void operator delete(void* p, const std::nothrow_t&) TEST_NOEXCEPT
+void operator delete [] (void* p, const std::nothrow_t&) TEST_NOEXCEPT
 {
     ++unsized_delete_nothrow_called;
     std::free(p);
Index: libcxx/test/std/language.support/support.dynamic/new.delete/new.delete.single/delete_align_val_t_replace.pass.cpp
===================================================================
--- libcxx/test/std/language.support/support.dynamic/new.delete/new.delete.single/delete_align_val_t_replace.pass.cpp
+++ libcxx/test/std/language.support/support.dynamic/new.delete/new.delete.single/delete_align_val_t_replace.pass.cpp
@@ -20,11 +20,6 @@
 // UNSUPPORTED: clang-5, clang-6, clang-7
 // XFAIL: use_system_cxx_lib && target={{.+}}-apple-macosx10.{{9|10|11|12|13}}
 
-// On Windows libc++ doesn't provide its own definitions for new/delete
-// but instead depends on the ones in VCRuntime. However VCRuntime does not
-// yet provide aligned new/delete definitions so this test fails to compile/link.
-// XFAIL: LIBCXX-WINDOWS-FIXME
-
 // Libcxx when built for z/OS doesn't contain the aligned allocation functions,
 // nor does the dynamic library shipped with z/OS.
 // UNSUPPORTED: target={{.+}}-zos{{.*}}
@@ -48,6 +43,14 @@
     aligned_delete_called = 0;
 }
 
+// Override aligned operator new, to make sure that the returned pointer
+// is freeable with std::free(). (__libcpp_aligned_alloc and
+// __libcpp_aligned_free may be using functions incompatible with std::free().)
+void* operator new (std::size_t s, std::align_val_t) TEST_THROW_SPEC(std::bad_alloc)
+{
+    return std::malloc(s);
+}
+
 void operator delete(void* p) TEST_NOEXCEPT
 {
     ++unsized_delete_called;