Make clangd check your asserts even with -DNDEBUG

Andrew Noyes — Fri, 21 Aug 2026 00:00:00 +0000

When you configure a CMake project with -DCMAKE_BUILD_TYPE=Release</code>, CMake adds -DNDEBUG</code> to every compile command, and that flag lands in your compile_commands.json</code>. assert</code> is a no-op when NDEBUG</code> is defined, so clangd treats the body of every assert(...)</code> as dead code: no autocomplete, no type checking, no diagnostics. But when you're editing code, you want asserts typechecked no matter what build type you have configured.</p>

The fix is a .clangd</code> file at the root of your project that strips -DNDEBUG</code> for clangd only, without touching your actual build:</p>

C</span>ompileFlags</span>:</span></span>
  R</span>emove</span>:</span> [</span>-D</span>NDEBUG</span>]</span></span></code></pre>🔗</a>
Toy example</h2>
CMakeLists.txt</code>:</p>
cmake_minimum_required</span>(</span>VERSION</span> 3.20)</span></span>
project</span>(toy CXX)</span></span>
set</span>(CMAKE_CXX_STANDARD 17)</span></span>
set</span>(CMAKE_EXPORT_COMPILE_COMMANDS </span>ON</span>)</span></span>
add_executable</span>(toy main.cpp)</span></span></code></pre>
main.cpp</code> contains a deliberate type error inside</em> an assert:</p>
#</span>include</span> <</span>cassert</span>></span></span>
#</span>include</span> <</span>vector</span>></span></span>
</span>
int</span> main</span>(</span>)</span> {</span></span>
    std</span>::</span>vector</span><</span>int</span>></span> v</span>;</span></span>
    assert</span>(</span>v</span>.</span>size</span>(</span>)</span> ==</span> "</span>oops</span>"</span>)</span>;</span></span>
}</span></span></code></pre>
Configure a Release build so the compile database carries -DNDEBUG</code>:</p>
cmake</span> -</span>S</span> .</span> -</span>B</span> build</span> -</span>G</span> Ninja</span> -</span>DCMAKE_BUILD_TYPE=Release</span></span></code></pre>🔗</a>
Before</h2>
With no .clangd</code> file, clangd picks up -DNDEBUG</code> from the compile database, skips the assert body, and reports zero errors:</p>
$ clangd --check=main.cpp</span></span>
I Compile command from CDB is: ... -O3 -DNDEBUG -std=gnu++17 ... -c main.cpp</span></span>
I All checks completed, 0 errors</span></span></code></pre>🔗</a>
After</h2>
Add the .clangd</code> file above. clangd now drops -DNDEBUG</code> from the compile command, the assert becomes live, and the type error surfaces:</p>
$ clangd --check=main.cpp</span></span>
I Loading config file at .../.clangd</span></span>
I Compile command from CDB is: ... -O3 -std=gnu++17 ... -c main.cpp</span></span>
E [typecheck_comparison_of_pointer_integer] Line 6: comparison between pointer and integer ('size_type' (aka 'unsigned long') and 'const char *')</span></span>
I All checks completed, 1 errors</span></span></code></pre>
The Release build is unchanged; only the editor now sees the code you actually wrote inside asserts.</p>

Weaselab - clangd

Make clangd check your asserts even with -DNDEBUG