include: Ignore empty string as file name (#13388)

Previously CMake silently accepted the empty string and added a bogus
dependency on the current directory.  Instead warn about the empty file
name and ignore it.  We cannot make this an error because there may be
existing projects that accidentally depend on the old behavior.

Add a RunCMake.include test to cover this case.

Source/cmIncludeCommand.cxx

@@ -70,6 +70,13 @@ bool cmIncludeCommand
         }
     }
 
+  if(fname.empty())
+    {
+    this->Makefile->IssueMessage(cmake::AUTHOR_WARNING,
+                                 "include() given empty file name (ignored).");
+    return true;
+    }
+
   if(!cmSystemTools::FileIsFullPath(fname.c_str()))
     {
     // Not a path. Maybe module.

Tests/RunCMake/CMakeLists.txt

@@ -50,6 +50,7 @@ add_RunCMake_test(ObjectLibrary)
 
 add_RunCMake_test(build_command)
 add_RunCMake_test(find_package)
+add_RunCMake_test(include)
 add_RunCMake_test(list)
 
 if("${CMAKE_TEST_GENERATOR}" MATCHES "Visual Studio [^6]")

Tests/RunCMake/include/CMakeLists.txt

+cmake_minimum_required(VERSION 2.8)
+project(${RunCMake_TEST} NONE)
+include(${RunCMake_TEST}.cmake)

Tests/RunCMake/include/EmptyString-stderr.txt

+CMake Warning \(dev\) at EmptyString.cmake:1 \(include\):
+  include\(\) given empty file name \(ignored\).
+Call Stack \(most recent call first\):
+  CMakeLists.txt:3 \(include\)
+This warning is for project developers.  Use -Wno-dev to suppress it.

Tests/RunCMake/include/EmptyString.cmake

+include("")

Tests/RunCMake/include/EmptyStringOptional-stderr.txt

+CMake Warning \(dev\) at EmptyStringOptional.cmake:1 \(include\):
+  include\(\) given empty file name \(ignored\).
+Call Stack \(most recent call first\):
+  CMakeLists.txt:3 \(include\)
+This warning is for project developers.  Use -Wno-dev to suppress it.

Tests/RunCMake/include/EmptyStringOptional.cmake

+include("" OPTIONAL)

Tests/RunCMake/include/RunCMakeTest.cmake

+include(RunCMake)
+
+run_cmake(EmptyString)
+run_cmake(EmptyStringOptional)