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

Help/command/add_library.rst

@@ -118,8 +118,35 @@ target using the commands:
 and then it is used as an argument to :command:`target_link_libraries`
 like any other target.
 
-An interface library has no source files itself and is not included
+An interface library created with the above signature has no source files
-as a target in the generated buildsystem.
+itself and is not included as a target in the generated buildsystem.
+
+Since CMake 3.19, an interface library target may be created with
+source files:
+
+.. code-block:: cmake
+
+  add_library(<name> INTERFACE [<source>...] [EXCLUDE_FROM_ALL])
+
+Source files may be listed directly in the ``add_library`` call or added
+later by calls to :command:`target_sources` with the ``PRIVATE`` or
+``PUBLIC`` keywords.
+
+If an interface library has source files (i.e. the :prop_tgt:`SOURCES`
+target property is set), it will appear in the generated buildsystem
+as a build target much like a target defined by the
+:command:`add_custom_target` command.  It does not compile any sources,
+but does contain build rules for custom commands created by the
+:command:`add_custom_command` command.
+
+.. note::
+  In most command signatures where the ``INTERFACE`` keyword appears,
+  the items listed after it only become part of that target's usage
+  requirements and are not part of the target's own settings.  However,
+  in this signature of ``add_library``, the ``INTERFACE`` keyword refers
+  to the library type only.  Sources listed after it in the ``add_library``
+  call are ``PRIVATE`` to the interface library and do not appear in its
+  :prop_tgt:`INTERFACE_SOURCES` target property.
 
 Imported Libraries
 ^^^^^^^^^^^^^^^^^^

Help/manual/cmake-buildsystem.7.rst

@@ -922,8 +922,8 @@ property from it:
 Interface Libraries
 -------------------
 
-An ``INTERFACE`` target has no :prop_tgt:`LOCATION` and is mutable, but is
+An ``INTERFACE`` library target does not compile sources and does not
-otherwise similar to an :prop_tgt:`IMPORTED` target.
+produce a library artifact on disk, so it has no :prop_tgt:`LOCATION`.
 
 It may specify usage requirements such as
 :prop_tgt:`INTERFACE_INCLUDE_DIRECTORIES`,
@@ -937,11 +937,22 @@ Only the ``INTERFACE`` modes of the :command:`target_include_directories`,
 :command:`target_sources`, and :command:`target_link_libraries` commands
 may be used with ``INTERFACE`` libraries.
 
+Since CMake 3.19, an ``INTERFACE`` library target may optionally contain
+source files.  An interface library that contains source files will be
+included as a build target in the generated buildsystem.  It does not
+compile sources, but may contain custom commands to generate other sources.
+Additionally, IDEs will show the source files as part of the target for
+interactive reading and editing.
+
 A primary use-case for ``INTERFACE`` libraries is header-only libraries.
 
 .. code-block:: cmake
 
-  add_library(Eigen INTERFACE)
+  add_library(Eigen INTERFACE
+    src/eigen.h
+    src/vector.h
+    src/matrix.h
+    )
   target_include_directories(Eigen INTERFACE
     $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/src>
     $<INSTALL_INTERFACE:include/Eigen>
@@ -980,7 +991,12 @@ to must be installed separately:
 
 .. code-block:: cmake
 
-  add_library(Eigen INTERFACE)
+  set(Eigen_headers
+    src/eigen.h
+    src/vector.h
+    src/matrix.h
+    )
+  add_library(Eigen INTERFACE ${Eigen_headers})
   target_include_directories(Eigen INTERFACE
     $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/src>
     $<INSTALL_INTERFACE:include/Eigen>
@@ -990,9 +1006,6 @@ to must be installed separately:
   install(EXPORT eigenExport NAMESPACE Upstream::
     DESTINATION lib/cmake/Eigen
   )
-  install(FILES
+  install(FILES ${Eigen_headers}
-      ${CMAKE_CURRENT_SOURCE_DIR}/src/eigen.h
-      ${CMAKE_CURRENT_SOURCE_DIR}/src/vector.h
-      ${CMAKE_CURRENT_SOURCE_DIR}/src/matrix.h
     DESTINATION include/Eigen
   )

Help/prop_tgt/EXPORT_PROPERTIES.rst

@@ -14,3 +14,11 @@ Properties starting with ``INTERFACE_`` or ``IMPORTED_`` are not allowed as
 they are reserved for internal CMake use.
 
 Properties containing generator expressions are also not allowed.
+
+.. note::
+
+  Since CMake 3.19, :ref:`Interface Libraries` may have arbitrary
+  target properties.  If a project exports an interface library
+  with custom properties, the resulting package may not work with
+  dependents configured by older versions of CMake that reject the
+  custom properties.

Help/release/dev/build-interface-targets.rst

@@ -0,0 +1,6 @@
+build-interface-targets
+-----------------------
+
+* :ref:`Interface Libraries` may now have source files added via
+  :command:`add_library` or :command:`target_sources`.  Those
+  with sources will be generated as part of the build system.

Source/cmAddLibraryCommand.cxx

@@ -2,8 +2,6 @@
    file Copyright.txt or https://cmake.org/licensing for details.  */
 #include "cmAddLibraryCommand.h"
 
-#include <cmext/algorithm>
-
 #include "cmExecutionStatus.h"
 #include "cmGeneratorExpression.h"
 #include "cmGlobalGenerator.h"
@@ -111,20 +109,10 @@ bool cmAddLibraryCommand(std::vector<std::string> const& args,
           "INTERFACE library specified with conflicting ALIAS type.");
         return false;
       }
-      if (excludeFromAll) {
-        status.SetError(
-          "INTERFACE library may not be used with EXCLUDE_FROM_ALL.");
-        return false;
-      }
       ++s;
       type = cmStateEnums::INTERFACE_LIBRARY;
       haveSpecifiedType = true;
     } else if (*s == "EXCLUDE_FROM_ALL") {
-      if (type == cmStateEnums::INTERFACE_LIBRARY) {
-        status.SetError(
-          "INTERFACE library may not be used with EXCLUDE_FROM_ALL.");
-        return false;
-      }
       ++s;
       excludeFromAll = true;
     } else if (*s == "IMPORTED") {
@@ -143,10 +131,6 @@ bool cmAddLibraryCommand(std::vector<std::string> const& args,
   }
 
   if (type == cmStateEnums::INTERFACE_LIBRARY) {
-    if (s != args.end()) {
-      status.SetError("INTERFACE library requires no source arguments.");
-      return false;
-    }
     if (importGlobal && !importTarget) {
       status.SetError(
         "INTERFACE library specified as GLOBAL, but not as IMPORTED.");
@@ -302,8 +286,6 @@ bool cmAddLibraryCommand(std::vector<std::string> const& args,
     }
   }
 
-  std::vector<std::string> srclists;
-
   if (type == cmStateEnums::INTERFACE_LIBRARY) {
     if (!cmGeneratorExpression::IsValidTargetName(libName) ||
         libName.find("::") != std::string::npos) {
@@ -311,14 +293,10 @@ bool cmAddLibraryCommand(std::vector<std::string> const& args,
         cmStrCat("Invalid name for INTERFACE library target: ", libName));
       return false;
     }
-
-    mf.AddLibrary(libName, type, srclists, excludeFromAll);
-    return true;
   }
 
-  cm::append(srclists, s, args.end());
+  std::vector<std::string> srcs(s, args.end());
-
+  mf.AddLibrary(libName, type, srcs, excludeFromAll);
-  mf.AddLibrary(libName, type, srclists, excludeFromAll);
 
   return true;
 }

Source/cmGeneratorTarget.cxx

@@ -1099,6 +1099,10 @@ bool cmGeneratorTarget::IsInBuildSystem() const
     case cmStateEnums::GLOBAL_TARGET:
       return true;
     case cmStateEnums::INTERFACE_LIBRARY:
+      // An INTERFACE library is in the build system if it has SOURCES.
+      if (!this->SourceEntries.empty()) {
+        return true;
+      }
     case cmStateEnums::UNKNOWN_LIBRARY:
       break;
   }
@@ -1543,7 +1547,6 @@ std::vector<BT<std::string>> cmGeneratorTarget::GetSourceFilePaths(
   std::string const& config) const
 {
   std::vector<BT<std::string>> files;
-  assert(this->GetType() != cmStateEnums::INTERFACE_LIBRARY);
 
   if (!this->LocalGenerator->GetGlobalGenerator()->GetConfigureDoneCMP0026()) {
     // At configure-time, this method can be called as part of getting the
@@ -1735,9 +1738,11 @@ void cmGeneratorTarget::ComputeKindedSources(KindedSources& files,
     std::string ext = cmSystemTools::LowerCase(sf->GetExtension());
     if (sf->GetCustomCommand()) {
       kind = SourceKindCustomCommand;
+    } else if (this->Target->GetType() == cmStateEnums::UTILITY ||
+               this->Target->GetType() == cmStateEnums::INTERFACE_LIBRARY
                // XXX(clang-tidy): https://bugs.llvm.org/show_bug.cgi?id=44165
                // NOLINTNEXTLINE(bugprone-branch-clone)
-    } else if (this->Target->GetType() == cmStateEnums::UTILITY) {
+    ) {
       kind = SourceKindExtra;
     } else if (this->IsSourceFilePartOfUnityBatch(sf->ResolveFullPath())) {
       kind = SourceKindUnityBatched;

Source/cmGlobalNinjaGenerator.cxx

@@ -1093,6 +1093,7 @@ void cmGlobalNinjaGenerator::AppendTargetOutputs(
     }
     // FALLTHROUGH
     case cmStateEnums::GLOBAL_TARGET:
+    case cmStateEnums::INTERFACE_LIBRARY:
     case cmStateEnums::UTILITY: {
       std::string path =
         cmStrCat(target->GetLocalGenerator()->GetCurrentBinaryDirectory(), '/',
@@ -1105,7 +1106,6 @@ void cmGlobalNinjaGenerator::AppendTargetOutputs(
       break;
     }
 
-    case cmStateEnums::INTERFACE_LIBRARY:
     case cmStateEnums::UNKNOWN_LIBRARY:
       break;
   }

Source/cmGlobalXCodeGenerator.cxx

@@ -1204,6 +1204,7 @@ bool cmGlobalXCodeGenerator::CreateXCodeTarget(
   }
 
   if (gtgt->GetType() == cmStateEnums::UTILITY ||
+      gtgt->GetType() == cmStateEnums::INTERFACE_LIBRARY ||
       gtgt->GetType() == cmStateEnums::GLOBAL_TARGET) {
     cmXCodeObject* t = this->CreateUtilityTarget(gtgt);
     if (!t) {
@@ -2536,7 +2537,7 @@ cmXCodeObject* cmGlobalXCodeGenerator::CreateUtilityTarget(
   this->XCodeObjectMap[gtgt] = target;
 
   // Add source files without build rules for editing convenience.
-  if (gtgt->GetType() == cmStateEnums::UTILITY &&
+  if (gtgt->GetType() != cmStateEnums::GLOBAL_TARGET &&
       gtgt->GetName() != CMAKE_CHECK_BUILD_SYSTEM_TARGET) {
     std::vector<cmSourceFile*> sources;
     if (!gtgt->GetConfigCommonSourceFiles(sources)) {

Source/cmLocalVisualStudio7Generator.cxx

@@ -629,9 +629,10 @@ void cmLocalVisualStudio7Generator::WriteConfiguration(
       break;
     case cmStateEnums::UTILITY:
     case cmStateEnums::GLOBAL_TARGET:
+    case cmStateEnums::INTERFACE_LIBRARY:
       configType = "10";
       CM_FALLTHROUGH;
-    default:
+    case cmStateEnums::UNKNOWN_LIBRARY:
       targetBuilds = false;
       break;
   }
@@ -1638,7 +1639,8 @@ bool cmLocalVisualStudio7Generator::WriteGroup(
     std::string source = sf->GetFullPath();
 
     if (source != libName || target->GetType() == cmStateEnums::UTILITY ||
-        target->GetType() == cmStateEnums::GLOBAL_TARGET) {
+        target->GetType() == cmStateEnums::GLOBAL_TARGET ||
+        target->GetType() == cmStateEnums::INTERFACE_LIBRARY) {
       // Look up the source kind and configs.
       std::map<cmSourceFile const*, size_t>::const_iterator map_it =
         sources.Index.find(sf);
@@ -1937,6 +1939,7 @@ void cmLocalVisualStudio7Generator::WriteProjectStartFortran(
   const char* keyword = p ? p->c_str() : "Console Application";
   const char* projectType = 0;
   switch (target->GetType()) {
+    case cmStateEnums::OBJECT_LIBRARY:
     case cmStateEnums::STATIC_LIBRARY:
       projectType = "typeStaticLibrary";
       if (keyword) {
@@ -1958,7 +1961,8 @@ void cmLocalVisualStudio7Generator::WriteProjectStartFortran(
       break;
     case cmStateEnums::UTILITY:
     case cmStateEnums::GLOBAL_TARGET:
-    default:
+    case cmStateEnums::INTERFACE_LIBRARY:
+    case cmStateEnums::UNKNOWN_LIBRARY:
       break;
   }
   if (projectType) {

Source/cmMakefileTargetGenerator.cxx

@@ -71,6 +71,7 @@ std::unique_ptr<cmMakefileTargetGenerator> cmMakefileTargetGenerator::New(
     case cmStateEnums::OBJECT_LIBRARY:
       result = cm::make_unique<cmMakefileLibraryTargetGenerator>(tgt);
       break;
+    case cmStateEnums::INTERFACE_LIBRARY:
     case cmStateEnums::UTILITY:
       result = cm::make_unique<cmMakefileUtilityTargetGenerator>(tgt);
       break;

Source/cmNinjaTargetGenerator.cxx

@@ -51,6 +51,7 @@ std::unique_ptr<cmNinjaTargetGenerator> cmNinjaTargetGenerator::New(
       return cm::make_unique<cmNinjaNormalTargetGenerator>(target);
 
     case cmStateEnums::UTILITY:
+    case cmStateEnums::INTERFACE_LIBRARY:
     case cmStateEnums::GLOBAL_TARGET:
       return cm::make_unique<cmNinjaUtilityTargetGenerator>(target);
 

Source/cmTarget.cxx

@@ -401,13 +401,11 @@ cmTarget::cmTarget(std::string const& name, cmStateEnums::TargetType type,
 #endif
   }
 
-  if (this->GetType() != cmStateEnums::INTERFACE_LIBRARY) {
   initProp("FOLDER");
 
   if (this->GetGlobalGenerator()->IsXcode()) {
     initProp("XCODE_GENERATE_SCHEME");
   }
-  }
 
   // Setup per-configuration property default values.
   if (this->GetType() != cmStateEnums::UTILITY &&
@@ -521,8 +519,6 @@ cmTarget::cmTarget(std::string const& name, cmStateEnums::TargetType type,
     initProp("DOTNET_TARGET_FRAMEWORK_VERSION");
   }
 
-  if (this->GetType() != cmStateEnums::INTERFACE_LIBRARY) {
-
   // check for "CMAKE_VS_GLOBALS" variable and set up target properties
   // if any
   const char* globals = mf->GetDefinition("CMAKE_VS_GLOBALS");
@@ -542,7 +538,6 @@ cmTarget::cmTarget(std::string const& name, cmStateEnums::TargetType type,
       }
     }
   }
-  }
 }
 
 cmTarget::cmTarget(cmTarget&&) noexcept = default;

Source/cmTargetPropCommandBase.cxx

@@ -123,7 +123,7 @@ bool cmTargetPropCommandBase::ProcessContentArgs(
   }
   if (!content.empty()) {
     if (this->Target->GetType() == cmStateEnums::INTERFACE_LIBRARY &&
-        scope != "INTERFACE") {
+        scope != "INTERFACE" && this->Property != "SOURCES") {
       this->SetError("may only set INTERFACE properties on INTERFACE targets");
       return false;
     }

Source/cmVisualStudio10TargetGenerator.cxx

@@ -315,8 +315,7 @@ std::ostream& cmVisualStudio10TargetGenerator::Elem::WriteString(
 void cmVisualStudio10TargetGenerator::Generate()
 {
   // do not generate external ms projects
-  if (this->GeneratorTarget->GetType() == cmStateEnums::INTERFACE_LIBRARY ||
+  if (this->GeneratorTarget->GetProperty("EXTERNAL_MSPROJECT")) {
-      this->GeneratorTarget->GetProperty("EXTERNAL_MSPROJECT")) {
     return;
   }
   const std::string ProjectFileExtension =
@@ -437,7 +436,7 @@ void cmVisualStudio10TargetGenerator::Generate()
       e1.Element("ProjectGuid", "{" + this->GUID + "}");
 
       if ((this->MSTools || this->Android) &&
-          this->GeneratorTarget->GetType() <= cmStateEnums::GLOBAL_TARGET) {
+          this->GeneratorTarget->IsInBuildSystem()) {
         this->WriteApplicationTypeSettings(e1);
         this->VerifyNecessaryFiles();
       }
@@ -605,11 +604,11 @@ void cmVisualStudio10TargetGenerator::Generate()
             }
             break;
           case cmStateEnums::UTILITY:
+          case cmStateEnums::INTERFACE_LIBRARY:
           case cmStateEnums::GLOBAL_TARGET:
             outputType = "Utility";
             break;
           case cmStateEnums::UNKNOWN_LIBRARY:
-          case cmStateEnums::INTERFACE_LIBRARY:
             break;
         }
         e1.Element("OutputType", outputType);
@@ -1157,6 +1156,7 @@ void cmVisualStudio10TargetGenerator::WriteProjectConfigurationValues(Elem& e0)
             }
             break;
           case cmStateEnums::UTILITY:
+          case cmStateEnums::INTERFACE_LIBRARY:
           case cmStateEnums::GLOBAL_TARGET:
             if (this->NsightTegra) {
               // Tegra-Android platform does not understand "Utility".
@@ -1166,7 +1166,6 @@ void cmVisualStudio10TargetGenerator::WriteProjectConfigurationValues(Elem& e0)
             }
             break;
           case cmStateEnums::UNKNOWN_LIBRARY:
-          case cmStateEnums::INTERFACE_LIBRARY:
             break;
         }
       }
@@ -2152,7 +2151,7 @@ void cmVisualStudio10TargetGenerator::WriteSource(Elem& e2,
 
 void cmVisualStudio10TargetGenerator::WriteAllSources(Elem& e0)
 {
-  if (this->GeneratorTarget->GetType() > cmStateEnums::UTILITY) {
+  if (this->GeneratorTarget->GetType() == cmStateEnums::GLOBAL_TARGET) {
     return;
   }
 

Tests/ExportImport/Export/Interface/CMakeLists.txt

@@ -1,11 +1,26 @@
-
+set(headeronly_headers headeronly/headeronly.h)
-add_library(headeronly INTERFACE)
+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

Tests/ExportImport/Export/Interface/headergen.h.in

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

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)

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;
+}

Tests/InterfaceLibrary/CMakeLists.txt

@@ -44,6 +44,7 @@ add_executable(InterfaceLibrary definetestexe.cpp)
 target_link_libraries(InterfaceLibrary
   iface_nodepends
   headeriface
+  iface_genheader
   subiface
   intermediate
 

Tests/InterfaceLibrary/definetestexe.cpp

@@ -15,6 +15,12 @@
 #  error Expected IFACE_HEADER_BUILDDIR
 #endif
 
+#include "iface_genheader.h"
+
+#ifndef IFACE_GENHEADER
+#  error Expected IFACE_GENHEADER
+#endif
+
 extern int obj();
 extern int sub();
 extern int item();

Tests/InterfaceLibrary/headerdir/CMakeLists.txt

@@ -11,3 +11,12 @@ add_custom_target(headeriface_gen
   VERBATIM
   )
 add_dependencies(headeriface headeriface_gen)
+
+add_custom_command(OUTPUT iface_genheader.h
+  COMMAND ${CMAKE_COMMAND} -E copy
+    ${CMAKE_CURRENT_SOURCE_DIR}/iface_genheader.h.in
+    ${CMAKE_CURRENT_BINARY_DIR}/iface_genheader.h
+  DEPENDS
+    ${CMAKE_CURRENT_SOURCE_DIR}/iface_genheader.h.in
+  VERBATIM)
+add_library(iface_genheader INTERFACE iface_genheader.h)

Tests/InterfaceLibrary/headerdir/iface_genheader.h.in

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

Tests/RunCMake/InterfaceLibrary/ConfigSources.cmake

@@ -0,0 +1,2 @@
+# Test an interface library added to the build system by a per-config source.
+add_library(iface INTERFACE $<$<CONFIG:NotAConfig>:${CMAKE_CURRENT_SOURCE_DIR}/iface.c>)

Tests/RunCMake/InterfaceLibrary/EmptySources-build2-result.txt

@@ -0,0 +1 @@
+[^0]

Tests/RunCMake/InterfaceLibrary/EmptySources-build2-stdout.txt

@@ -0,0 +1 @@
+iface2|Invalid project

Tests/RunCMake/InterfaceLibrary/EmptySources.cmake

@@ -0,0 +1,8 @@
+# Test an interface library added to the build system by empty SOURCES.
+add_library(iface INTERFACE)
+set_property(TARGET iface PROPERTY SOURCES "")
+
+# ...but not added by unset SOURCES.
+add_library(iface2 INTERFACE)
+set_property(TARGET iface2 PROPERTY SOURCES "")
+set_property(TARGET iface2 PROPERTY SOURCES)

Tests/RunCMake/InterfaceLibrary/ExcludeFromAll-build1-check.cmake

@@ -0,0 +1,4 @@
+if(EXISTS "${RunCMake_TEST_BINARY_DIR}/iface.txt")
+  set(RunCMake_TEST_FAILED "iface target built as part of 'all'")
+  return()
+endif()

Tests/RunCMake/InterfaceLibrary/ExcludeFromAll-build2-check.cmake

@@ -0,0 +1,4 @@
+if(NOT EXISTS "${RunCMake_TEST_BINARY_DIR}/iface.txt")
+  set(RunCMake_TEST_FAILED "iface target not built")
+  return()
+endif()

Tests/RunCMake/InterfaceLibrary/ExcludeFromAll-build3-result.txt

@@ -0,0 +1 @@
+[^0]

Tests/RunCMake/InterfaceLibrary/ExcludeFromAll-build3-stdout.txt

@@ -0,0 +1 @@
+iface2|Invalid project

Tests/RunCMake/InterfaceLibrary/ExcludeFromAll.cmake

@@ -0,0 +1,7 @@
+# Test an interface library with a custom command, but excluded from all.
+add_custom_command(OUTPUT iface.txt COMMAND ${CMAKE_COMMAND} -E touch iface.txt)
+add_library(iface INTERFACE EXCLUDE_FROM_ALL iface.txt)
+
+# Test that EXCLUDE_FROM_ALL is allowed even if the interface library has
+# no sources, and does not cause it to appear in the build system.
+add_library(iface2 INTERFACE EXCLUDE_FROM_ALL)

Tests/RunCMake/InterfaceLibrary/PublicSources-build3-result.txt

@@ -0,0 +1 @@
+[^0]

Tests/RunCMake/InterfaceLibrary/PublicSources-build3-stdout.txt

@@ -0,0 +1 @@
+iface2|Invalid project

Tests/RunCMake/InterfaceLibrary/PublicSources.cmake

@@ -0,0 +1,20 @@
+cmake_policy(SET CMP0076 NEW)
+enable_language(C)
+
+# Test that an interface library can have PUBLIC sources.
+# This causes the target to appear in the build system
+# *and* causes consumers to use the source.
+add_library(iface INTERFACE)
+target_sources(iface
+  PUBLIC iface.c
+  # Private sources do not compile here or propagate.
+  PRIVATE iface_broken.c
+  )
+
+# Test that an intermediate interface library does not get the
+# sources and does not appear in the build system.
+add_library(iface2 INTERFACE)
+target_link_libraries(iface2 INTERFACE iface)
+
+add_executable(use_iface use_iface.c)
+target_link_libraries(use_iface PRIVATE iface2)

Tests/RunCMake/InterfaceLibrary/RunCMakeTest.cmake

@@ -10,3 +10,27 @@ run_cmake(add_custom_command-TARGET)
 run_cmake(IMPORTED_LIBNAME-bad-value)
 run_cmake(IMPORTED_LIBNAME-non-iface)
 run_cmake(IMPORTED_LIBNAME-non-imported)
+
+function(run_WithSources CASE)
+  if(NOT RunCMake_GENERATOR_IS_MULTI_CONFIG)
+    set(RunCMake_TEST_OPTIONS -DCMAKE_BUILD_TYPE=Debug)
+  endif()
+  set(RunCMake_TEST_BINARY_DIR ${RunCMake_BINARY_DIR}/${CASE}-build)
+  run_cmake(${CASE})
+  set(RunCMake_TEST_NO_CLEAN 1)
+  foreach(build IN LISTS ARGN)
+    if(build MATCHES "^([^:]+)$")
+      run_cmake_command(${CASE}-${CMAKE_MATCH_1} ${CMAKE_COMMAND} --build . --config Debug)
+    elseif(build MATCHES "^([^:]+):([^:,]+)(,merge)?$")
+      if(CMAKE_MATCH_3 STREQUAL ",merge")
+        set(RunCMake_TEST_OUTPUT_MERGE 1)
+      endif()
+      run_cmake_command(${CASE}-${CMAKE_MATCH_1} ${CMAKE_COMMAND} --build . --config Debug --target ${CMAKE_MATCH_2})
+    endif()
+  endforeach()
+endfunction()
+
+run_WithSources(ConfigSources "build1:iface")
+run_WithSources(EmptySources "build1:iface" "build2:iface2,merge")
+run_WithSources(ExcludeFromAll "build1" "build2:iface" "build3:iface2,merge")
+run_WithSources(PublicSources "build1" "build2:iface" "build3:iface2,merge")

Tests/RunCMake/InterfaceLibrary/iface.c

@@ -0,0 +1,4 @@
+int iface(void)
+{
+  return 0;
+}

Tests/RunCMake/InterfaceLibrary/iface_broken.c

@@ -0,0 +1 @@
+#error This file should not compile

Tests/RunCMake/InterfaceLibrary/invalid_signature-stderr.txt

@@ -1,8 +1,3 @@
-CMake Error at invalid_signature.cmake:2 \(add_library\):
-  add_library INTERFACE library requires no source arguments.
-Call Stack \(most recent call first\):
-  CMakeLists.txt:3 \(include\)
-+
 CMake Error at invalid_signature.cmake:3 \(add_library\):
   add_library INTERFACE library specified with conflicting/multiple types.
 Call Stack \(most recent call first\):
@@ -73,16 +68,6 @@ CMake Error at invalid_signature.cmake:16 \(add_library\):
 Call Stack \(most recent call first\):
   CMakeLists.txt:3 \(include\)
 +
-CMake Error at invalid_signature.cmake:17 \(add_library\):
-  add_library INTERFACE library may not be used with EXCLUDE_FROM_ALL.
-Call Stack \(most recent call first\):
-  CMakeLists.txt:3 \(include\)
-+
-CMake Error at invalid_signature.cmake:18 \(add_library\):
-  add_library INTERFACE library may not be used with EXCLUDE_FROM_ALL.
-Call Stack \(most recent call first\):
-  CMakeLists.txt:3 \(include\)
-+
 CMake Error at invalid_signature.cmake:20 \(add_library\):
   add_library GLOBAL option may only be used with IMPORTED libraries.
 Call Stack \(most recent call first\):

Tests/RunCMake/InterfaceLibrary/invalid_signature.cmake

@@ -1,5 +1,5 @@
 
-add_library(iface1 INTERFACE empty.cpp)
+
 add_library(iface3 STATIC INTERFACE)
 add_library(iface4 STATIC INTERFACE empty.cpp)
 add_library(iface5 SHARED INTERFACE)
@@ -14,7 +14,7 @@ add_library(iface13 INTERFACE UNKNOWN)
 add_library(iface14 INTERFACE ALIAS)
 add_library(iface15 ALIAS INTERFACE)
 add_library(iface16 INTERFACE INTERFACE)
-add_library(iface17 INTERFACE EXCLUDE_FROM_ALL)
+
-add_library(iface18 EXCLUDE_FROM_ALL INTERFACE)
+
 # add_library(iface19 GLOBAL INTERFACE) Tested separately
 add_library(iface20 INTERFACE GLOBAL)

Tests/RunCMake/InterfaceLibrary/use_iface.c

@@ -0,0 +1,6 @@
+extern int iface(void);
+
+int main(void)
+{
+  return iface();
+}

Tests/RunCMake/VS10Project/InterfaceLibSources-check.cmake

@@ -0,0 +1,25 @@
+set(vcProjectFile "${RunCMake_TEST_BINARY_DIR}/iface.vcxproj")
+if(NOT EXISTS "${vcProjectFile}")
+  set(RunCMake_TEST_FAILED "Project file ${vcProjectFile} does not exist.")
+  return()
+endif()
+
+set(found_iface_h 0)
+file(STRINGS "${vcProjectFile}" lines)
+foreach(line IN LISTS lines)
+  if(line MATCHES "<([A-Za-z0-9_]+) +Include=.*iface\\.h")
+    set(rule "${CMAKE_MATCH_1}")
+    if(NOT rule STREQUAL "None")
+      set(RunCMake_TEST_FAILED "iface.h referenced as ${rule} instead of None in\n  ${vcProjectFile}")
+      return()
+    endif()
+    if(found_iface_h)
+      set(RunCMake_TEST_FAILED "iface.h referenced multiple times in\n  ${vcProjectFile}")
+      return()
+    endif()
+    set(found_iface_h 1)
+  endif()
+endforeach()
+if(NOT found_iface_h)
+  set(RunCMake_TEST_FAILED "iface.h not referenced in\n  ${vcProjectFile}")
+endif()

Tests/RunCMake/VS10Project/InterfaceLibSources.cmake

@@ -0,0 +1 @@
+add_library(iface INTERFACE iface.h)

Tests/RunCMake/VS10Project/RunCMakeTest.cmake

@@ -6,6 +6,7 @@ cmake_policy(SET CMP0054 NEW)
 run_cmake(VsCsharpSourceGroup)
 run_cmake(VsCSharpCompilerOpts)
 run_cmake(ExplicitCMakeLists)
+run_cmake(InterfaceLibSources)
 run_cmake(RuntimeLibrary)
 run_cmake(SourceGroupCMakeLists)
 run_cmake(SourceGroupTreeCMakeLists)

Tests/RunCMake/XcodeProject/InterfaceLibSources-check.cmake

@@ -0,0 +1,16 @@
+set(xcProjectFile "${RunCMake_TEST_BINARY_DIR}/InterfaceLibSources.xcodeproj/project.pbxproj")
+if(NOT EXISTS "${xcProjectFile}")
+  set(RunCMake_TEST_FAILED "Project file ${xcProjectFile} does not exist.")
+  return()
+endif()
+
+set(found_iface_h 0)
+file(STRINGS "${xcProjectFile}" lines)
+foreach(line IN LISTS lines)
+  if(line MATCHES "PBXFileReference.*explicitFileType.*sourcecode\\.c\\.h.*iface\\.h")
+    set(found_iface_h 1)
+  endif()
+endforeach()
+if(NOT found_iface_h)
+  set(RunCMake_TEST_FAILED "iface.h not referenced in\n  ${xcProjectFile}")
+endif()

Tests/RunCMake/XcodeProject/InterfaceLibSources.cmake

@@ -0,0 +1 @@
+add_library(iface INTERFACE iface.h)

Tests/RunCMake/XcodeProject/RunCMakeTest.cmake

@@ -2,6 +2,7 @@ include(RunCMake)
 
 run_cmake(ExplicitCMakeLists)
 run_cmake(ImplicitCMakeLists)
+run_cmake(InterfaceLibSources)
 
 run_cmake(XcodeFileType)
 run_cmake(XcodeAttributeLocation)