Link libraries by full path even in implicit directories

When CMP0003 was first introduced we wanted to link all libraries by
full path.  However, some projects had problems on platforms where
find_library would find /usr/lib/libfoo.so when the project really
wanted to link to /usr/lib/<arch>/libfoo.so and had been working by
accident because pre-CMP0003 behavior used -lfoo to link.

We first tried to address that in commit v2.6.0~440 (Teach find_library
to avoid returning library paths in system directories, 2008-01-23) by
returning just "foo" for libraries in implicit link directories.  This
caused problems for projects expecting find_library to always return a
full path.  We ended up using the solution in commit v2.6.0~366 (...
switch library paths found in implicit link directories to use -l,
2008-01-31).  However, the special case for libraries in implicit link
directories has also proven problematic and confusing.

Introduce policy CMP0060 to switch to linking all libraries by full path
even if they are in implicit link directories.  Explain in the policy
documentation the factors that led to the original approach and now to
this approach.

1 files changed, 63 insertions, 0 deletions

diff --git a/Help/policy/CMP0060.rst b/Help/policy/CMP0060.rst
new file mode 100644
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+CMP0060
+-------
+
+Link libraries by full path even in implicit directories.
+
+Policy :policy:`CMP0003` was introduced with the intention of always
+linking library files by full path when a full path is given to the
+:command:`target_link_libraries` command.  However, on some platforms
+(e.g. HP-UX) the compiler front-end adds alternative library search paths
+for the current architecture (e.g. ``/usr/lib/<arch>`` has alternatives
+to libraries in ``/usr/lib`` for the current architecture).
+On such platforms the :command:`find_library` may find a library such as
+``/usr/lib/libfoo.so`` that does not belong to the current architecture.
+
+Prior to policy :policy:`CMP0003` projects would still build in such
+cases because the incorrect library path would be converted to ``-lfoo``
+on the link line and the linker would find the proper library in the
+arch-specific search path provided by the compiler front-end implicitly.
+At the time we chose to remain compatible with such projects by always
+converting library files found in implicit link directories to ``-lfoo``
+flags to ask the linker to search for them.  This approach allowed existing
+projects to continue to build while still linking to libraries outside
+implicit link directories via full path (such as those in the build tree).
+
+CMake does allow projects to override this behavior by using an
+:ref:`IMPORTED library target <Imported Targets>` with its
+:prop_tgt:`IMPORTED_LOCATION` property set to the desired full path to
+a library file.  In fact, many :ref:`Find Modules` are learning to provide
+:ref:`Imported Targets` instead of just the traditional ``Foo_LIBRARIES``
+variable listing library files.  However, this makes the link line
+generated for a library found by a Find Module depend on whether it
+is linked through an imported target or not, which is inconsistent.
+Furthermore, this behavior has been a source of confusion because the
+generated link line for a library file depends on its location.  It is
+also problematic for projects trying to link statically because flags
+like ``-Wl,-Bstatic -lfoo -Wl,-Bdynamic`` may be used to help the linker
+select ``libfoo.a`` instead of ``libfoo.so`` but then leak dynamic linking
+to following libraries.  (See the :prop_tgt:`LINK_SEARCH_END_STATIC`
+target property for a solution typically used for that problem.)
+
+When the special case for libraries in implicit link directories was first
+introduced the list of implicit link directories was simply hard-coded
+(e.g. ``/lib``, ``/usr/lib``, and a few others).  Since that time, CMake
+has learned to detect the implicit link directories used by the compiler
+front-end.  If necessary, the :command:`find_library` command could be
+taught to use this information to help find libraries of the proper
+architecture.
+
+For these reasons, CMake 3.3 and above prefer to drop the special case
+and link libraries by full path even when they are in implicit link
+directories.  Policy ``CMP0060`` provides compatibility for existing
+projects.
+
+The OLD behavior for this policy is to ask the linker to search for
+libraries whose full paths are known to be in implicit link directories.
+The NEW behavior for this policy is to link libraries by full path even
+if they are in implicit link directories.
+
+This policy was introduced in CMake version 3.3.  Unlike most policies,
+CMake version |release| does *not* warn by default when this policy
+is not set and simply uses OLD behavior.  See documentation of the
+:variable:`CMAKE_POLICY_WARNING_CMP0060 <CMAKE_POLICY_WARNING_CMP<NNNN>>`
+variable to control the warning.