Allow add_dependencies() on INTERFACE libraries (#15414)

Revert commit v3.0.0-rc1~175^2~20 (add_dependencies: Disallow use with
INTERFACE_LIBRARY, 2013-12-25).  Teach our dependency analysis to
transitively follow INTERFACE target utility dependencies as was done or
IMPORTED targets in commit v2.8.6~127^2~1 (Allow add_dependencies() on
imported targets, 2010-11-19).  Extend the InterfaceLibrary test with a
case to cover header generation for a header-only INTERFACE library via
a custom target.

Help/command/add_dependencies.rst

@@ -13,8 +13,8 @@ is one created by one of the :command:`add_executable`,
 :command:`add_library`, or :command:`add_custom_target` commands.
 
 Dependencies added to an :ref:`imported target <Imported Targets>`
-are followed transitively in its place since the target itself does
-not build.
+or an :ref:`interface library <Interface Libraries>` are followed
+transitively in its place since the target itself does not build.
 
 See the ``DEPENDS`` option of :command:`add_custom_target` and
 :command:`add_custom_command` commands for adding file-level

Help/release/dev/add_dependencies-INTERFACE-libraries.rst

@@ -0,0 +1,7 @@
+add_dependencies-INTERFACE-libraries
+------------------------------------
+
+* The :command:`add_dependencies` command learned to allow dependencies
+  to be added to :ref:`interface libraries <Interface Libraries>`.
+  Dependencies added to an interface library are followed transitively
+  in its place since the target itself does not build.

Source/cmAddDependenciesCommand.cxx

@@ -33,15 +33,6 @@ bool cmAddDependenciesCommand
     }
   if(cmTarget* target = this->Makefile->FindTargetToUse(target_name))
     {
-    if (target->GetType() == cmTarget::INTERFACE_LIBRARY)
-      {
-      std::ostringstream e;
-      e << "Cannot add target-level dependencies to INTERFACE library "
-        "target \"" << target_name << "\".\n";
-      this->SetError(e.str());
-      return false;
-      }
-
     std::vector<std::string>::const_iterator s = args.begin();
     ++s; // skip over target_name
     for (; s != args.end(); ++s)

Source/cmComputeTargetDepends.cxx

@@ -418,9 +418,11 @@ void cmComputeTargetDepends::AddTargetDepend(int depender_index,
                                              cmTarget const* dependee,
                                              bool linking)
 {
-  if(dependee->IsImported())
+  if(dependee->IsImported() ||
+     dependee->GetType() == cmTarget::INTERFACE_LIBRARY)
     {
-    // Skip imported targets but follow their utility dependencies.
+    // Skip IMPORTED and INTERFACE targets but follow their utility
+    // dependencies.
     std::set<cmLinkItem> const& utils = dependee->GetUtilityItems();
     for(std::set<cmLinkItem>::const_iterator i = utils.begin();
         i != utils.end(); ++i)

Tests/InterfaceLibrary/headerdir/CMakeLists.txt

@@ -3,6 +3,11 @@ set(CMAKE_INCLUDE_CURRENT_DIR_IN_INTERFACE ON)
 
 add_library(headeriface INTERFACE)
 
-file(WRITE "${CMAKE_CURRENT_BINARY_DIR}/iface_header_builddir.h"
-  "#define IFACE_HEADER_BUILDDIR\n"
-)
+add_custom_target(headeriface_gen
+  COMMENT "Generating iface_header_builddir.h"
+  COMMAND ${CMAKE_COMMAND} -E copy_if_different
+    ${CMAKE_CURRENT_SOURCE_DIR}/iface_header_builddir.h.in
+    ${CMAKE_CURRENT_BINARY_DIR}/iface_header_builddir.h
+  VERBATIM
+  )
+add_dependencies(headeriface headeriface_gen)

Tests/InterfaceLibrary/headerdir/iface_header_builddir.h.in

@@ -0,0 +1 @@
+#define IFACE_HEADER_BUILDDIR

Tests/RunCMake/interface_library/RunCMakeTest.cmake

@@ -7,5 +7,4 @@ run_cmake(whitelist)
 run_cmake(invalid_signature)
 run_cmake(global-interface)
 run_cmake(genex_link)
-run_cmake(add_dependencies)
 run_cmake(add_custom_command-TARGET)

Tests/RunCMake/interface_library/add_dependencies-result.txt

@@ -1 +0,0 @@
-1

Tests/RunCMake/interface_library/add_dependencies-stderr.txt

@@ -1,6 +0,0 @@
-CMake Error at add_dependencies.cmake:4 \(add_dependencies\):
-  add_dependencies Cannot add target-level dependencies to INTERFACE library
-  target "iface".
-
-Call Stack \(most recent call first\):
-  CMakeLists.txt:3 \(include\)

Tests/RunCMake/interface_library/add_dependencies.cmake

@@ -1,4 +0,0 @@
-
-add_library(foo empty.cpp)
-add_library(iface INTERFACE)
-add_dependencies(iface foo)