Help: Clarify target_sources path conversion w.r.t generator expressions

With some of the content expanded, reorder a few paragraphs
and tweak some of the wording to improve the flow.

Co-Authored-By: Craig Scott <craig.scott@crascit.com>

1 files changed, 31 insertions, 18 deletions

diff --git a/Help/command/target_sources.rst b/Help/command/target_sources.rst
index 2fded5e..2039da5 100644
--- a/Help/command/target_sources.rst
+++ b/Help/command/target_sources.rst
@@ -15,37 +15,50 @@ Specifies sources to use when building a target and/or its dependents.
 The named ``<target>`` must have been created by a command such as
 :command:`add_executable` or :command:`add_library` or
 :command:`add_custom_target` and must not be an
-:ref:`ALIAS target <Alias Targets>`.
-
-.. versionchanged:: 3.13
-  Relative source file paths are interpreted as being relative to the current
-  source directory (i.e. :variable:`CMAKE_CURRENT_SOURCE_DIR`).
-  See policy :policy:`CMP0076`.
+:ref:`ALIAS target <Alias Targets>`.  The ``<items>`` may use
+:manual:`generator expressions <cmake-generator-expressions(7)>`.
 
 .. versionadded:: 3.20
   ``<target>`` can be a custom target.
 
 The ``INTERFACE``, ``PUBLIC`` and ``PRIVATE`` keywords are required to
-specify the scope of the items following them.  ``PRIVATE`` and ``PUBLIC``
-items will populate the :prop_tgt:`SOURCES` property of
-``<target>``, which are used when building the target itself.
+specify the scope of the source file paths (``<items>``) that follow
+them.  ``PRIVATE`` and ``PUBLIC`` items will populate the :prop_tgt:`SOURCES`
+property of ``<target>``, which are used when building the target itself.
 ``PUBLIC`` and ``INTERFACE`` items will populate the
 :prop_tgt:`INTERFACE_SOURCES` property of ``<target>``, which are used
-when building dependents.
-The following arguments specify sources.  Repeated calls for the same
-``<target>`` append items in the order called. The targets created by
-:command:`add_custom_target` can only have ``PRIVATE`` scope.
+when building dependents.  A target created by :command:`add_custom_target`
+can only have ``PRIVATE`` scope.
+
+Repeated calls for the same ``<target>`` append items in the order called.
 
 .. versionadded:: 3.3
   Allow exporting targets with :prop_tgt:`INTERFACE_SOURCES`.
 
 .. versionadded:: 3.11
-  Allow setting ``INTERFACE`` items on :ref:`IMPORTED targets <Imported Targets>`.
+  Allow setting ``INTERFACE`` items on
+  :ref:`IMPORTED targets <Imported Targets>`.
+
+.. versionchanged:: 3.13
+  Relative source file paths are interpreted as being relative to the current
+  source directory (i.e. :variable:`CMAKE_CURRENT_SOURCE_DIR`).
+  See policy :policy:`CMP0076`.
+
+A path that begins with a generator expression is left unmodified.
+When a target's :prop_tgt:`SOURCE_DIR` property differs from
+:variable:`CMAKE_CURRENT_SOURCE_DIR`, use absolute paths in generator
+expressions to ensure the sources are correctly assigned to the target.
+
+.. code-block:: cmake
+
+  # WRONG: starts with generator expression, but relative path used
+  target_sources(MyTarget "$<$<CONFIG:Debug>:dbgsrc.cpp>")
+
+  # CORRECT: absolute path used inside the generator expression
+  target_sources(MyTarget "$<$<CONFIG:Debug>:${CMAKE_CURRENT_SOURCE_DIR}/dbgsrc.cpp>")
 
-Arguments to ``target_sources`` may use "generator expressions"
-with the syntax ``$<...>``. See the :manual:`cmake-generator-expressions(7)`
-manual for available expressions.  See the :manual:`cmake-buildsystem(7)`
-manual for more on defining buildsystem properties.
+See the :manual:`cmake-buildsystem(7)` manual for more on defining
+buildsystem properties.
 
 .. code-block:: cmake