Generate CMake export set regardless of FMT_INSTALL (#4850)

export() was scoped inside if (FMT_INSTALL), so projects that pull in
fmt via add_subdirectory()/FetchContent without installing it had no
way to get the exported fmt::* targets. If such a project tries to
export its own targets that depend on fmt, CMake fails with "target
... requires target fmt that is not in any export set".

Hoist the target list/export name and the export() call itself out of
the FMT_INSTALL guard so the build-tree export file is always
generated; the install()-only pieces (config/version files, pkgconfig,
install(EXPORT ...)) stay behind the guard and installed behavior is
unchanged.

Fixes #4806

Co-authored-by: flink <w741069229@gmail.com>

CMakeLists.txt

@@ -441,6 +441,16 @@ add_library(fmt::fmt-c ALIAS fmt-c)
 set_target_properties(fmt-c PROPERTIES PUBLIC_HEADER include/fmt/fmt-c.h)
 
 # Install targets.
+set(targets_export_name fmt-targets)
+set(INSTALL_TARGETS fmt fmt-header-only fmt-c)
+
+if (FMT_MODULE)
+  list(APPEND INSTALL_TARGETS fmt-module)
+  if (FMT_USE_CMAKE_MODULES)
+    set(INSTALL_FILE_SET FILE_SET fmt DESTINATION ${FMT_INC_DIR}/fmt)
+  endif ()
+endif ()
+
 if (FMT_INSTALL)
   include(CMakePackageConfigHelpers)
   set_verbose(
@@ -454,7 +464,6 @@ if (FMT_INSTALL)
   set(version_config ${PROJECT_BINARY_DIR}/fmt-config-version.cmake)
   set(project_config ${PROJECT_BINARY_DIR}/fmt-config.cmake)
   set(pkgconfig ${PROJECT_BINARY_DIR}/fmt.pc)
-  set(targets_export_name fmt-targets)
 
   set_verbose(
     FMT_LIB_DIR ${CMAKE_INSTALL_LIBDIR} CACHE STRING
@@ -485,15 +494,6 @@ if (FMT_INSTALL)
     ${PROJECT_SOURCE_DIR}/support/cmake/fmt-config.cmake.in ${project_config}
     INSTALL_DESTINATION ${FMT_CMAKE_DIR})
 
-  set(INSTALL_TARGETS fmt fmt-header-only fmt-c)
-
-  if (FMT_MODULE)
-    list(APPEND INSTALL_TARGETS fmt-module)
-    if (FMT_USE_CMAKE_MODULES)
-      set(INSTALL_FILE_SET FILE_SET fmt DESTINATION ${FMT_INC_DIR}/fmt)
-    endif ()
-  endif ()
-
   # Install the library and headers.
   install(
     TARGETS ${INSTALL_TARGETS}
@@ -504,13 +504,6 @@ if (FMT_INSTALL)
     PUBLIC_HEADER DESTINATION ${FMT_INC_DIR}/fmt
     RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR} ${INSTALL_FILE_SET})
 
-  # Use a namespace because CMake provides better diagnostics for namespaced
-  # imported targets.
-  export(
-    TARGETS ${INSTALL_TARGETS}
-    NAMESPACE fmt::
-    FILE ${PROJECT_BINARY_DIR}/${targets_export_name}.cmake)
-
   # Install version, config and target files.
   install(
     FILES ${project_config} ${version_config}
@@ -528,6 +521,15 @@ if (FMT_INSTALL)
     COMPONENT fmt_core)
 endif ()
 
+# Generate an export file in the build tree so that fmt can be used by
+# add_subdirectory() or FetchContent without being installed. Use a
+# namespace because CMake provides better diagnostics for namespaced
+# imported targets.
+export(
+  TARGETS ${INSTALL_TARGETS}
+  NAMESPACE fmt::
+  FILE ${PROJECT_BINARY_DIR}/${targets_export_name}.cmake)
+
 function (add_doc_target)
   find_program(DOXYGEN doxygen PATHS "$ENV{ProgramFiles}/doxygen/bin"
                                      "$ENV{ProgramFiles\(x86\)}/doxygen/bin")

test/add-subdirectory-test/CMakeLists.txt

@@ -15,3 +15,12 @@ if (TARGET fmt::fmt-header-only)
   target_compile_options(header-only-test PRIVATE ${PEDANTIC_COMPILE_FLAGS})
   target_link_libraries(header-only-test fmt::fmt-header-only)
 endif ()
+
+# Test that a target publicly depending on fmt can itself be exported even
+# though fmt is added via add_subdirectory and FMT_INSTALL is off (#4806).
+add_library(export-test STATIC export-lib.cc)
+target_link_libraries(export-test PUBLIC fmt::fmt)
+export(
+  TARGETS export-test
+  NAMESPACE test::
+  FILE ${CMAKE_CURRENT_BINARY_DIR}/export-test-targets.cmake)

test/add-subdirectory-test/export-lib.cc

@@ -0,0 +1,3 @@
+#include "fmt/base.h"
+
+void greet(const char* name) { fmt::print("Hello, {}!\n", name); }