CMake "CMAKE_CXX_COMPILER not set" in CI

CMake could not determine a C++ compiler when the project enabled the CXX language. The compiler is not installed, not on PATH, or a toolchain file did not set it.

What this error means

Configure aborts complaining CMAKE_CXX_COMPILER is not set after EnableLanguage, usually on a minimal image lacking g++/clang++.

CMake Error at CMakeLists.txt:2 (project):
  No CMAKE_CXX_COMPILER could be found.
  CMAKE_CXX_COMPILER not set, after EnableLanguage

Common causes

How to fix it

Install a C++ compiler

1. Install build-essential or g++/clang++ before configuring.
2. Or point CMake at a specific compiler with -DCMAKE_CXX_COMPILER.

apt-get install -y build-essential
cmake -S . -B build -DCMAKE_CXX_COMPILER=g++

How to prevent it

• Bake a C++ toolchain into the runner image and verify g++/clang++ is on PATH before running CMake.

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