Cannot configure CMake to look for Homebrew installed version of Bison - cmake

Question

I'm running macOS 10.14 and I installed bison version 3.2 with brew, but it refuses to link:
$ brew link bison --force
Warning: Refusing to link macOS-provided software: bison
If you need to have bison first in your PATH run:
echo 'export PATH="/usr/local/opt/bison/bin:$PATH"' >> ~/.bash_profile
For compilers to find bison you may need to set:
export LDFLAGS="-L/usr/local/opt/bison/lib"
My CMakeLists.txt has the following lines:
find_package(BISON 3.2 REQUIRED)
...
message(STATUS "Found bison ${BISON_VERSION} at: ${BISON_EXECUTABLE}")
cmake fails with the following output:
Could NOT find BISON: Found unsuitable version "2.3", but required is at
least "3.2" (found /usr/bin/bison)
The system-installed bison is at version 2.3, and I know that the Homebrew-installed version is at version 3.2.
The FindBISON module apparently uses the find_program CMake directive, and I've tried adding /usr/local/opt/bison/bin to CMAKE_PROGRAM_PATH, but /usr/bin/bison is always selected first no matter what. How can I force CMake to favor the Homebrew installed version of the Bison executable?

Answer 1

Since some things in macos depend on the Apple-provided versions of flex and bison, homebrew installs them outside of the user's normal $PATH as a "keg". A manual solution is to make sure you (and your users) add the program's brew prefix to their path (can be found by running brew --prefix bison etc.).
An automatic solution is to put this logic in your CMake files. A couple open-source projects have worked around it like this. The earliest example of a fix for this that I could find is from this commit to STP (essential part copied below).
Be sure to clear your CMake cache when messing around with all this, as BISON_EXECUTABLE (etc.) is cached.
# On macOS, search Homebrew for keg-only versions of Bison and Flex. Xcode does
# not provide new enough versions for us to use.
if (CMAKE_HOST_SYSTEM_NAME MATCHES "Darwin")
execute_process(
COMMAND brew --prefix bison
RESULT_VARIABLE BREW_BISON
OUTPUT_VARIABLE BREW_BISON_PREFIX
OUTPUT_STRIP_TRAILING_WHITESPACE
)
if (BREW_BISON EQUAL 0 AND EXISTS "${BREW_BISON_PREFIX}")
message(STATUS "Found Bison keg installed by Homebrew at ${BREW_BISON_PREFIX}")
set(BISON_EXECUTABLE "${BREW_BISON_PREFIX}/bin/bison")
endif()
execute_process(
COMMAND brew --prefix flex
RESULT_VARIABLE BREW_FLEX
OUTPUT_VARIABLE BREW_FLEX_PREFIX
OUTPUT_STRIP_TRAILING_WHITESPACE
)
if (BREW_FLEX EQUAL 0 AND EXISTS "${BREW_FLEX_PREFIX}")
message(STATUS "Found Flex keg installed by Homebrew at ${BREW_FLEX_PREFIX}")
set(FLEX_EXECUTABLE "${BREW_FLEX_PREFIX}/bin/flex")
endif()
endif()
find_package(bison REQUIRED)
find_package(flex REQUIRED)