file(GENERATE): Add policy CMP0070 to define relative path behavior

Previously `file(GENERATE)` did not define any behavior for relative
paths given to the `OUTPUT` or `INPUT` arguments.  Define behavior
consistent with CMake conventions and add a policy to provide
compatibility for projects that relied on the old accidental behavior.

Fixes: #16786

1 files changed, 5 insertions, 0 deletions

diff --git a/Help/command/file.rst b/Help/command/file.rst
index b2e4eea..7afb715 100644
--- a/Help/command/file.rst
+++ b/Help/command/file.rst
@@ -291,6 +291,8 @@ from the input content to produce the output content.  The options are:
 
 ``INPUT <input-file>``
   Use the content from a given file as input.
+  A relative path is treated with respect to the value of
+  :variable:`CMAKE_CURRENT_SOURCE_DIR`.  See policy :policy:`CMP0070`.
 
 ``OUTPUT <output-file>``
   Specify the output file name to generate.  Use generator expressions
@@ -298,6 +300,9 @@ from the input content to produce the output content.  The options are:
   name.  Multiple configurations may generate the same output file only
   if the generated content is identical.  Otherwise, the ``<output-file>``
   must evaluate to an unique name for each configuration.
+  A relative path (after evaluating generator expressions) is treated
+  with respect to the value of :variable:`CMAKE_CURRENT_BINARY_DIR`.
+  See policy :policy:`CMP0070`.
 
 Exactly one ``CONTENT`` or ``INPUT`` option must be given.  A specific
 ``OUTPUT`` file may be named by at most one invocation of ``file(GENERATE)``.