add_custom_{command,target}: Fix WORKING_DIRECTORY leading genex

Since commit v3.13.0-rc1~39^2 (add_custom_{command,target}:
WORKING_DIRECTORY generator expressions, 2018-09-22) the
`WORKING_DIRECTORY` option accepts generator expressions.
Fix support for the case of a leading generator expression
by deferring conversion to an absolute path until after
evaluation of the generator expression.

Fixes: #18543

Source/cmAddCustomCommandCommand.cxx

@@ -318,12 +318,6 @@ bool cmAddCustomCommandCommand::InitialPass(
     return false;
   }
 
-  // Convert working directory to a full path.
-  if (!working.empty()) {
-    const std::string& build_dir = this->Makefile->GetCurrentBinaryDirectory();
-    working = cmSystemTools::CollapseFullPath(working, build_dir);
-  }
-
   // Choose which mode of the command to use.
   bool escapeOldStyle = !verbatim;
   if (source.empty() && output.empty()) {

Source/cmAddCustomTargetCommand.cxx

@@ -181,13 +181,6 @@ bool cmAddCustomTargetCommand::InitialPass(
     }
   }
 
-  // Convert working directory to a full path.
-  if (!working_directory.empty()) {
-    const std::string& build_dir = this->Makefile->GetCurrentBinaryDirectory();
-    working_directory =
-      cmSystemTools::CollapseFullPath(working_directory, build_dir);
-  }
-
   if (commandLines.empty() && !byproducts.empty()) {
     this->Makefile->IssueMessage(
       cmake::FATAL_ERROR,

Source/cmCustomCommandGenerator.cxx

@@ -70,6 +70,12 @@ cmCustomCommandGenerator::cmCustomCommandGenerator(cmCustomCommand const& cc,
     std::unique_ptr<cmCompiledGeneratorExpression> cge =
       this->GE->Parse(workingdirectory);
     this->WorkingDirectory = cge->Evaluate(this->LG, this->Config);
+    // Convert working directory to a full path.
+    if (!this->WorkingDirectory.empty()) {
+      std::string const& build_dir = this->LG->GetCurrentBinaryDirectory();
+      this->WorkingDirectory =
+        cmSystemTools::CollapseFullPath(this->WorkingDirectory, build_dir);
+    }
   }
 }
 

Tests/CustomCommandWorkingDirectory/CMakeLists.txt

@@ -47,7 +47,7 @@ file(MAKE_DIRECTORY ${TestWorkingDir_BINARY_DIR}/genex)
 add_custom_command(
   OUTPUT "${TestWorkingDir_BINARY_DIR}/genex/working.c"
   COMMAND "${CMAKE_COMMAND}" -E copy "${TestWorkingDir_SOURCE_DIR}/working.c.in" "${TestWorkingDir_BINARY_DIR}/genex/working.c"
-  WORKING_DIRECTORY "${TestWorkingDir_BINARY_DIR}/$<1:genex>/"
+  WORKING_DIRECTORY "$<0:not_used/>${TestWorkingDir_BINARY_DIR}/$<1:genex>/"
   COMMENT "custom command"
 )
 
@@ -58,7 +58,7 @@ add_custom_target(
   CustomGenex ALL
   COMMAND "${CMAKE_COMMAND}" -E copy_if_different "${TestWorkingDir_SOURCE_DIR}/customTarget.c" "${TestWorkingDir_BINARY_DIR}/genex/customTarget.c"
   BYPRODUCTS "${TestWorkingDir_BINARY_DIR}/genex/customTarget.c"
-  WORKING_DIRECTORY "${TestWorkingDir_BINARY_DIR}/$<1:genex>/"
+  WORKING_DIRECTORY "$<0:not_used/>${TestWorkingDir_BINARY_DIR}/$<1:genex>/"
 )
 
 add_dependencies(workinggenex CustomGenex)