add_custom_command: Clarify error when TARGET is out of scope (#15681)

The add_custom_command(TARGET) signature only works for targets defined
in the current directory.  Clarify this in the error message when the
target exists but was defined elsewhere.
Inspired-by: Bartosz Kosiorek <gang65@poczta.onet.pl>

Source/cmMakefile.cxx

@@ -791,7 +791,24 @@ cmMakefile::AddCustomCommandToTarget(const std::string& target,
 
     if(issueMessage)
       {
-      e << "The target name \"" << target << "\" is unknown in this context.";
+      if (cmTarget const* t = this->FindTargetToUse(target))
+        {
+        if (t->IsImported())
+          {
+          e << "TARGET '" << target
+            << "' is IMPORTED and does not build here.";
+          }
+        else
+          {
+          e << "TARGET '" << target
+            << "' was not created in this directory.";
+          }
+        }
+      else
+        {
+        e << "No TARGET '" << target
+          << "' has been created in this directory.";
+        }
       IssueMessage(messageType, e.str());
       }
 

Tests/RunCMake/CMP0040/CMP0040-NEW-missing-target-stderr.txt

 CMake Error at CMP0040-NEW-missing-target.cmake:3 \(add_custom_command\):
-  The target name "foobar" is unknown in this context.
+  No TARGET 'foobar' has been created in this directory.
 Call Stack \(most recent call first\):
   CMakeLists.txt:3 \(include\)

Tests/RunCMake/CMP0040/CMP0040-WARN-missing-target-stderr.txt

@@ -4,7 +4,7 @@ CMake Warning \(dev\) at CMP0040-WARN-missing-target.cmake:2 \(add_custom_comman
   policy details.  Use the cmake_policy command to set the policy and
   suppress this warning.
 +
-  The target name "foobar" is unknown in this context.
+  No TARGET 'foobar' has been created in this directory.
 Call Stack \(most recent call first\):
   CMakeLists.txt:3 \(include\)
 This warning is for project developers.  Use -Wno-dev to suppress it.

Tests/RunCMake/add_custom_command/RunCMakeTest.cmake

@@ -8,3 +8,5 @@ run_cmake(NoOutputOrTarget)
 run_cmake(OutputAndTarget)
 run_cmake(SourceByproducts)
 run_cmake(SourceUsesTerminal)
+run_cmake(TargetImported)
+run_cmake(TargetNotInDir)

Tests/RunCMake/add_custom_command/TargetImported-result.txt

+1

Tests/RunCMake/add_custom_command/TargetImported-stderr.txt

+^CMake Error at TargetImported.cmake:2 \(add_custom_command\):
+  TARGET 'TargetImported' is IMPORTED and does not build here.
+Call Stack \(most recent call first\):
+  CMakeLists.txt:3 \(include\)$

Tests/RunCMake/add_custom_command/TargetImported.cmake

+add_library(TargetImported UNKNOWN IMPORTED)
+add_custom_command(TARGET TargetImported COMMAND ${CMAKE_COMMAND} -E echo tada)

Tests/RunCMake/add_custom_command/TargetNotInDir-result.txt

+1

Tests/RunCMake/add_custom_command/TargetNotInDir-stderr.txt

+^CMake Error at TargetNotInDir.cmake:2 \(add_custom_command\):
+  TARGET 'TargetNotInDir' was not created in this directory.
+Call Stack \(most recent call first\):
+  CMakeLists.txt:3 \(include\)$

Tests/RunCMake/add_custom_command/TargetNotInDir.cmake

+add_subdirectory(TargetNotInDir)
+add_custom_command(TARGET TargetNotInDir COMMAND ${CMAKE_COMMAND} -E echo tada)

Tests/RunCMake/add_custom_command/TargetNotInDir/CMakeLists.txt

+add_custom_target(TargetNotInDir)