Treat CMake Warnings as Errors

Warnings in build systems are often ignored because they don't break the build. But ignoring them can lead to hidden problems and future breakage - especially when CMake deprecates features or when you accidentally rely on unintended behavior. A good practice is to treat warnings as errors, forcing yourself (or your team) to fix them early rather than letting them pile up. This is common with compiler warnings (-Werror option in GCC or Clang), and CMake has a similar mechanism. This tutorial explains how to treat CMake warnings as errors.

Consider the following CMakeLists.txt file:

cmake_minimum_required(VERSION 3.9)
project(myapp)

set(CMAKE_CXX_STANDARD 17)

add_executable(${PROJECT_NAME} main.cpp)

If you generate build files with a modern CMake, you may see this warning:

CMake Deprecation Warning at CMakeLists.txt:1 (cmake_minimum_required):
  Compatibility with CMake < 3.10 will be removed from a future version of
  CMake.

  Update the VERSION argument <min> value.  Or, use the <min>...<max> syntax
  to tell CMake that the project requires at least <min> but has been updated
  to work with policies introduced by <max> or earlier.

Since this is only a warning, the build still succeeds - and it's easy to miss.

You can tell CMake to treat deprecation warnings as errors:

cmake -S . -B build -Werror=deprecated

Now the same project will fail at the configure step with a CMake Deprecation Error:

CMake Deprecation Error at CMakeLists.txt:1 (cmake_minimum_required):
  Compatibility with CMake < 3.10 will be removed from a future version of
  CMake.

  Update the VERSION argument <min> value.  Or, use the <min>...<max> syntax
  to tell CMake that the project requires at least <min> but has been updated
  to work with policies introduced by <max> or earlier.

...
-- Configuring incomplete, errors occurred!

CMake also has developer warnings: these are hints about suspicious usage and questionable patterns. Normally, they're just advisory, but you can make them as error too:

cmake -S . -B build -Werror=dev

When to use these options:

• -Werror=deprecated - ensures the project doesn't rely on features that will disappear in future CMake releases.
• -Werror=dev - keeps the CMake codebase clean by enforcing best practices and avoiding problematic constructs. By default, this will also turn on deprecated warnings as errors.