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, 25 insertions, 0 deletions

diff --git a/Help/policy/CMP0070.rst b/Help/policy/CMP0070.rst
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+++ b/Help/policy/CMP0070.rst
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+CMP0070
+-------
+
+Define :command:`file(GENERATE)` behavior for relative paths.
+
+CMake 3.10 and newer define that relative paths given to ``INPUT`` and
+``OUTPUT`` arguments of ``file(GENERATE)`` are interpreted relative to the
+current source and binary directories, respectively.  CMake 3.9 and lower did
+not define any behavior for relative paths but did not diagnose them either
+and accidentally treated them relative to the process working directory.
+Policy ``CMP0070`` provides compatibility with projects that used the old
+undefined behavior.
+
+This policy affects behavior of relative paths given to ``file(GENERATE)``.
+The ``OLD`` behavior for this policy is to treat the paths relative to the
+working directory of CMake.  The ``NEW`` behavior for this policy is to
+interpret relative paths with respect to the current source or binary
+directory of the caller.
+
+This policy was introduced in CMake version 3.10.  CMake version
+|release| warns when the policy is not set and uses ``OLD`` behavior.
+Use the :command:`cmake_policy` command to set it to ``OLD`` or ``NEW``
+explicitly.
+
+.. include:: DEPRECATED.txt