Add INTERFACE libraries to generated buildsystem if they have SOURCES

INTERFACE libraries were created with the intention of collecting usage
requirements for use by other targets via `target_link_libraries`.
Therefore they were not allowed to have SOURCES and were not included in
the generated buildsystem.  In practice, this has become limiting:

* Header-only libraries do have sources, they just do not compile.
  Developers should be able to edit those sources (the header files)
  in their IDE.

* Header-only libraries may need to generate some of their header
  files via custom commands.

Some projects work around these limitations by pairing each interface
library with an `add_custom_target` that makes the header files and
custom commands appear in the generated buildsystem and in IDEs.

Lift such limitations by allowing INTERFACE libraries to have SOURCES.
For those with sources, add a corresponding build target to the
generated buildsystem.

Fixes: #19145

4 files changed, 41 insertions, 3 deletions

diff --git a/Tests/ExportImport/Export/Interface/CMakeLists.txt b/Tests/ExportImport/Export/Interface/CMakeLists.txt
index 43b7217..ba2164b 100644
--- a/Tests/ExportImport/Export/Interface/CMakeLists.txt
+++ b/Tests/ExportImport/Export/Interface/CMakeLists.txt
@@ -1,11 +1,26 @@
-
-add_library(headeronly INTERFACE)
+set(headeronly_headers headeronly/headeronly.h)
+add_library(headeronly INTERFACE ${headeronly_headers})
 set_property(TARGET headeronly PROPERTY INTERFACE_INCLUDE_DIRECTORIES
   "$<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/headeronly>"
   "$<INSTALL_INTERFACE:${CMAKE_INSTALL_PREFIX}/include/headeronly>"
 )
 set_property(TARGET headeronly PROPERTY INTERFACE_COMPILE_DEFINITIONS "HEADERONLY_DEFINE")
 
+add_custom_command(OUTPUT headergen/headergen.h
+  COMMAND ${CMAKE_COMMAND} -E copy
+    ${CMAKE_CURRENT_SOURCE_DIR}/headergen.h.in
+    ${CMAKE_CURRENT_BINARY_DIR}/headergen/headergen.h
+  DEPENDS
+    ${CMAKE_CURRENT_SOURCE_DIR}/headergen.h.in
+  VERBATIM)
+
+add_library(headergen INTERFACE headergen/headergen.h)
+set_property(TARGET headergen PROPERTY INTERFACE_INCLUDE_DIRECTORIES
+  "$<BUILD_INTERFACE:${CMAKE_CURRENT_BINARY_DIR}/headergen>"
+)
+set_property(TARGET headergen PROPERTY PUBLIC_HEADER
+  ${CMAKE_CURRENT_BINARY_DIR}/headergen/headergen.h)
+
 add_library(pch_iface INTERFACE)
 target_precompile_headers(pch_iface INTERFACE
   "$<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/pch/pch.h>"
@@ -54,6 +69,11 @@ install(TARGETS headeronly sharediface use_auto_type use_c_restrict source_targe
                 pch_iface cmakeonly
   EXPORT expInterface
 )
+install(TARGETS headergen
+  EXPORT expInterface
+  PUBLIC_HEADER DESTINATION include/headergen
+  INCLUDES DESTINATION include/headergen
+)
 install(TARGETS sharedlib
   EXPORT expInterface
   RUNTIME DESTINATION bin
@@ -63,7 +83,7 @@ install(TARGETS sharedlib
   BUNDLE DESTINATION Applications
 )
 install(FILES
-  headeronly/headeronly.h
+  ${headeronly_headers}
   DESTINATION include/headeronly
 )
 install(FILES
diff --git a/Tests/ExportImport/Export/Interface/headergen.h.in b/Tests/ExportImport/Export/Interface/headergen.h.in
new file mode 100644
index 0000000..bda2b81
--- /dev/null
+++ b/Tests/ExportImport/Export/Interface/headergen.h.in
@@ -0,0 +1 @@
+#define HEADERGEN_H
diff --git a/Tests/ExportImport/Import/Interface/CMakeLists.txt b/Tests/ExportImport/Import/Interface/CMakeLists.txt
index ef666b1..202c23e 100644
--- a/Tests/ExportImport/Import/Interface/CMakeLists.txt
+++ b/Tests/ExportImport/Import/Interface/CMakeLists.txt
@@ -12,6 +12,9 @@ set_property(TARGET define_iface PROPERTY
 add_executable(headeronlytest_bld headeronlytest.cpp)
 target_link_libraries(headeronlytest_bld bld::headeronly)
 
+add_executable(headergentest_bld headergentest.cpp)
+target_link_libraries(headergentest_bld bld::headergen)
+
 set_property(TARGET bld::sharediface APPEND PROPERTY INTERFACE_LINK_LIBRARIES define_iface)
 
 add_executable(interfacetest_bld interfacetest.cpp)
@@ -93,6 +96,9 @@ target_compile_definitions(source_target_test_exp PRIVATE USE_FROM_INSTALL_DIR)
 add_executable(headeronlytest_exp headeronlytest.cpp)
 target_link_libraries(headeronlytest_exp exp::headeronly)
 
+add_executable(headergentest_exp headergentest.cpp)
+target_link_libraries(headergentest_exp exp::headergen)
+
 set_property(TARGET exp::sharediface APPEND PROPERTY INTERFACE_LINK_LIBRARIES define_iface)
 
 add_executable(interfacetest_exp interfacetest.cpp)
diff --git a/Tests/ExportImport/Import/Interface/headergentest.cpp b/Tests/ExportImport/Import/Interface/headergentest.cpp
new file mode 100644
index 0000000..88ff7f1
--- /dev/null
+++ b/Tests/ExportImport/Import/Interface/headergentest.cpp
@@ -0,0 +1,11 @@
+
+#include "headergen.h"
+
+#ifndef HEADERGEN_H
+#  error Expected HEADERGEN_H
+#endif
+
+int main()
+{
+  return 0;
+}