cmake: build dependency downloaded with ExternalProject_Add before hand - c++

Question

I'm trying to get my test program main.cpp to link against a an externally downloaded library (in this case fmt 8.1.1) via cmake's 'ExternalProject_Add' function as follows
CMakeLists.txt
cmake_minimum_required(VERSION 3.23)
project(test)
set(CMAKE_CXX_STANDARD 20)
include(ExternalProject)
ExternalProject_Add(fmt
GIT_REPOSITORY https://github.com/fmtlib/fmt.git
GIT_TAG 8.1.1
CMAKE_ARGS -DCMAKE_BUILD_TYPE=Release -DCMAKE_CXX_STANDARD=20 -DFMT_TEST=OFF -DFMT_DOC=OFF -DFMT_INSTALL=ON -DCMAKE_INSTALL_PREFIX=${CMAKE_BINARY_DIR}/fmt-build
STEP_TARGETS build
)
set(fmt_BINARY_DIR "${CMAKE_BINARY_DIR}/fmt-prefix/src/fmt-build")
set(fmt_SOURCE_DIR "${CMAKE_BINARY_DIR}/fmt-prefix/src/fmt/include")
add_executable(${PROJECT_NAME} main.cpp)
add_dependencies(${PROJECT_NAME} fmt-build) # does not have an effect
target_link_libraries(${PROJECT_NAME} PRIVATE ${fmt_BINARY_DIR}/libfmt.a)
target_include_directories(${PROJECT_NAME} PUBLIC ${fmt_SOURCE_DIR})
main.cpp
#include <chrono>
#include <fmt/core.h>
#include <fmt/chrono.h>
int main() {
const auto now = std::chrono::system_clock::now();
fmt::print("The time is {}", now);
return 0;
}
The test program main.cpp runs OK if I build the target fmt-build before hand, but complains that
ninja: error: 'fmt-prefix/src/fmt-build/libfmt.a', needed by 'test', missing and no known rule to make it
indicating that the dependency fmt-build must be built before hand. While that's doable, how do I get cmake to autmoatically build fmt-build as a dependency for test without haing to run it manually first? I thought the line add_dependencies(${PROJECT_NAME} fmt-build) should take care of that but evidently it does not.

Answer 1

Based on what #Tsyvarev commented, simply adding the line
BUILD_BYPRODUCTS ${CMAKE_BINARY_DIR}/fmt-prefix/src/fmt-build/libfmt.a
to the ExternalProject_Add call does the trick, and one does not have to manually build fmt-build before hand.