chore: fix C++ warning (leanprover#10922)

Author: Kha

.github/workflows/ci.yml

@@ -200,8 +200,8 @@ jobs:
                 "test": true,
                 // NOTE: `test-speedcenter` currently seems to be broken on `ubuntu-latest`
                 "test-speedcenter": large && level >= 2,
-                // made explicit until it can be assumed to have propagated to PRs
-                "CMAKE_OPTIONS": "-DUSE_LAKE=ON",
+                // We are not warning-free yet on all platforms, start here
+                "CMAKE_OPTIONS": "-DLEAN_EXTRA_CXX_FLAGS=-Werror",
               },
               {
                 "name": "Linux Reldebug",

src/util/timeit.h

@@ -69,6 +69,7 @@ class xtimeit {
         m_start     = std::move(other.m_start);
         m_fn        = std::move(other.m_fn); // TODO: use `std::exchange(_, nullptr)` in C++14
         other.m_fn = nullptr;
+        return *this;
     }
     ~xtimeit() {
         if (!m_fn) return;