Help: Add a manual for compiler feature control.

Link to it from the documentation of related properties, variables
and commands.

Extend the cmake-developer(7) documentation with notes on
extending feature support for compilers.

Help/command/target_compile_features.rst

@@ -28,4 +28,5 @@ an ``IMPORTED`` target.
 Arguments to ``target_compile_features`` may use "generator expressions"
 with the syntax ``$<...>``.
 See the :manual:`cmake-generator-expressions(7)` manual for available
-expressions.
+expressions.  See the :manual:`cmake-compile-features(7)` manual for
+information on compile features.

Help/index.rst

@@ -27,6 +27,7 @@ Reference Manuals
 
    /manual/cmake-buildsystem.7
    /manual/cmake-commands.7
+   /manual/cmake-compile-features.7
    /manual/cmake-developer.7
    /manual/cmake-generator-expressions.7
    /manual/cmake-generators.7

Help/manual/cmake-compile-features.7.rst

@@ -0,0 +1,297 @@
+.. cmake-manual-description: CMake Compile Features Reference
+
+cmake-compile-features(7)
+*************************
+
+.. only:: html or latex
+
+   .. contents::
+
+Introduction
+============
+
+Project source code may depend on, or be conditional on, the availability
+of certain features of the compiler.  There are three use-cases which arise:
+`Compile Feature Requirements`_, `Optional Compile Features`_
+and `Conditional Compilation Options`_.
+
+While features are typically specified in programming language standards,
+CMake provides a primary user interface based on granular handling of
+the features, not the language standard that introduced the feature.
+
+The :prop_gbl:`CMAKE_C_KNOWN_FEATURES` and
+:prop_gbl:`CMAKE_CXX_KNOWN_FEATURES` global properties contain all the
+features known to CMake, regardless of compiler support for the feature.
+The :variable:`CMAKE_C_COMPILE_FEATURES` and
+:variable:`CMAKE_CXX_COMPILE_FEATURES` variables contain all features
+CMake knows are known to the compiler, regardless of language standard
+or compile flags needed to use them.
+
+Features known to CMake are named mostly following the same convention
+as the Clang feature test macros.  The are some execptions, such as
+CMake using ``cxx_final`` and ``cxx_override`` instead of the single
+``cxx_override_control`` used by Clang.
+
+Compile Feature Requirements
+============================
+
+Compile feature requirements may be specified with the
+:command:`target_compile_features` command.  For example, if a target must
+be compiled with compiler support for the
+:prop_gbl:`cxx_constexpr <CMAKE_CXX_KNOWN_FEATURES>` feature:
+
+.. code-block:: cmake
+
+  add_library(mylib requires_constexpr.cpp)
+  target_compile_features(mylib PRIVATE cxx_constexpr)
+
+In processing the requirement for the ``cxx_constexpr`` feature,
+:manual:`cmake(1)` will ensure that the in-use C++ compiler is capable
+of the feature, and will add any necessary flags such as ``-std=c++11``
+to the compile lines of C++ files in the ``mylib`` target.  A
+``FATAL_ERROR`` is issued if the compiler is not capable of the
+feature.
+
+The exact compile flags and language standard are deliberately not part
+of the user interface for this use-case.  CMake will compute the
+appropriate compile flags to use by considering the features specified
+for each target.
+
+Such compile flags are added even if the compiler supports the
+particular feature without the flag. For example, the GNU compiler
+supports variadic templates (with a warning) even if ``-std=c++98`` is
+used.  CMake adds the ``-std=c++11`` flag if ``cxx_variadic_templates``
+is specified as a requirement.
+
+In the above example, ``mylib`` requires ``cxx_constexpr`` when it
+is built itself, but consumers of ``mylib`` are not required to use a
+compiler which supports ``cxx_constexpr``.  If the interface of
+``mylib`` does require the ``cxx_constexpr`` feature (or any other
+known feature), that may be specified with the ``PUBLIC`` or
+``INTERFACE`` signatures of :command:`target_compile_features`:
+
+.. code-block:: cmake
+
+  add_library(mylib requires_constexpr.cpp)
+  # cxx_constexpr is a usage-requirement
+  target_compile_features(mylib PUBLIC cxx_constexpr)
+
+  # main.cpp will be compiled with -std=c++11 on GNU for cxx_constexpr.
+  add_executable(myexe main.cpp)
+  target_link_libraries(myexe mylib)
+
+Feature requirements are evaluated transitively by consuming the link
+implementation.  See :manual:`cmake-buildsystem(7)` for more on
+transitive behavior of build properties and usage requirements.
+
+Note that new use of compile feature requirements may expose
+cross-platform bugs in user code.  For example, the GNU compiler uses the
+``gnu++98`` language by default as of GCC version 4.8.  User code may
+be relying on that by expecting the ``typeof`` GNU extension to work.
+However, if the :command:`target_compile_features` command is used to
+specify the requirement for ``cxx_constexpr``, a ``-std=c++11`` flag may
+be added, and the ``typeof`` extension would no longer be available.  The
+solution is to specify that compiler extensions are relied upon by setting
+the :prop_tgt:`CXX_EXTENSIONS` target property to ``ON`` when starting to
+use the :command:`target_compile_features` command.
+
+Optional Compile Features
+=========================
+
+Compile features may be preferred if available, without creating a hard
+requirement.  For example, a library may provides alternative
+implementations depending on whether the ``cxx_variadic_templates``
+feature is available:
+
+.. code-block:: c++
+
+  #if Foo_COMPILER_CXX_VARIADIC_TEMPLATES
+  template<int I, int... Is>
+  struct Interface;
+
+  template<int I>
+  struct Interface<I>
+  {
+    static int accumulate()
+    {
+      return I;
+    }
+  };
+
+  template<int I, int... Is>
+  struct Interface
+  {
+    static int accumulate()
+    {
+      return I + Interface<Is...>::accumulate();
+    }
+  };
+  #else
+  template<int I1, int I2 = 0, int I3 = 0, int I4 = 0>
+  struct Interface
+  {
+    static int accumulate() { return I1 + I2 + I3 + I4; }
+  };
+  #endif
+
+Such an interface depends on using the correct preprocessor defines for the
+compiler features.  CMake can generate a header file containing such
+defines using the :module:`WriteCompilerDetectionHeader` module.  The
+module contains the ``write_compiler_detection_header`` function which
+accepts parameters to control the content of the generated header file:
+
+.. code-block:: cmake
+
+  write_compiler_detection_header(
+    FILE "${CMAKE_CURRENT_BINARY_DIR}/foo_compiler_detection.h"
+    PREFIX Foo
+    COMPILERS GNU
+    FEATURES
+      cxx_variadic_templates
+  )
+
+Such a header file may be used internally in the source code of a project,
+and it may be installed and used in the interface of library code.
+
+For each feature listed in ``FEATURES``, a preprocessor definition
+is created in the header file, and defined to either ``1`` or ``0``.
+
+Additionally, some features call for additional defines, such as the
+``cxx_final`` and ``cxx_override`` features. Rather than being used in
+``#ifdef`` code, the ``final`` keyword is abstracted by a symbol
+which is defined to either ``final``, a compiler-specific equivalent, or
+to empty.  That way, C++ code can be written to unconditionally use the
+symbol, and compiler support determines what it is expanded to:
+
+.. code-block:: c++
+
+  struct Interface {
+    virtual void Execute() = 0;
+  };
+
+  struct Concrete Foo_CXX_FINAL {
+    void Execute() Foo_CXX_OVERRIDE;
+  };
+
+In this case, ``Foo_CXX_FINAL`` will expand to ``final`` if the
+compiler supports the keyword, or to empty otherwise.
+
+In this use-case, the CMake code will wish to enable a particular language
+standard if available from the compiler. The :prop_tgt:`CXX_STANDARD`
+target property variable may be set to the desired language standard
+for a particular target, and the :variable:`CMAKE_CXX_STANDARD` may be
+set to influence all following targets:
+
+.. code-block:: cmake
+
+  write_compiler_detection_header(
+    FILE "${CMAKE_CURRENT_BINARY_DIR}/foo_compiler_detection.h"
+    PREFIX Foo
+    COMPILERS GNU
+    FEATURES
+      cxx_final cxx_override
+  )
+
+  # Includes foo_compiler_detection.h and uses the Foo_DECL_CXX_FINAL symbol
+  # which will expand to 'final' if the compiler supports the requested
+  # CXX_STANDARD.
+  add_library(foo foo.cpp)
+  set_property(TARGET foo PROPERTY CXX_STANDARD 11)
+
+  # Includes foo_compiler_detection.h and uses the Foo_DECL_CXX_FINAL symbol
+  # which will expand to 'final' if the compiler supports the feature,
+  # even though CXX_STANDARD is not set explicitly.  The requirement of
+  # cxx_constexpr causes CMake to set CXX_STANDARD internally, which
+  # affects the compile flags.
+  add_library(foo_impl foo_impl.cpp)
+  target_compile_features(foo_impl PRIVATE cxx_constexpr)
+
+The ``write_compiler_detection_header`` function also creates compatibility
+code for other features which have standard equivalents.  For example, the
+``cxx_static_assert`` feature is emulated with a template and abstracted
+via the ``<PREFIX>_STATIC_ASSERT`` and ``<PREFIX>_STATIC_ASSERT_MSG``
+function-macros.
+
+Conditional Compilation Options
+===============================
+
+Libraries may provide entirely different header files depending on
+requested compiler features.
+
+For example, a header at ``with_variadics/interface.h`` may contain:
+
+.. code-block:: c++
+
+  template<int I, int... Is>
+  struct Interface;
+
+  template<int I>
+  struct Interface<I>
+  {
+    static int accumulate()
+    {
+      return I;
+    }
+  };
+
+  template<int I, int... Is>
+  struct Interface
+  {
+    static int accumulate()
+    {
+      return I + Interface<Is...>::accumulate();
+    }
+  };
+
+while a header at ``no_variadics/interface.h`` may contain:
+
+.. code-block:: c++
+
+  template<int I1, int I2 = 0, int I3 = 0, int I4 = 0>
+  struct Interface
+  {
+    static int accumulate() { return I1 + I2 + I3 + I4; }
+  };
+
+It would be possible to write a abstraction ``interface.h`` header
+containing something like:
+
+.. code-block:: c++
+
+  #include "foo_compiler_detection.h"
+  #if Foo_COMPILER_CXX_VARIADIC_TEMPLATES
+  #include "with_variadics/interface.h"
+  #else
+  #include "no_variadics/interface.h"
+  #endif
+
+However this could be unmaintainable if there are many files to
+abstract. What is needed is to use alternative include directories
+depending on the compiler capabilities.
+
+CMake provides a ``COMPILE_FEATURES``
+:manual:`generator expression <cmake-generator-expressions(7)>` to implement
+such conditions.  This may be used with the build-property commands such as
+:command:`target_include_directories` and :command:`target_link_libraries`
+to set the appropriate :manual:`buildsystem <cmake-buildsystem(7)>`
+properties:
+
+.. code-block:: cmake
+
+  add_library(foo INTERFACE)
+  target_link_libraries(foo
+    INTERFACE
+      "$<$<COMPILE_FEATURES:cxx_variadic_templates>:${CMAKE_CURRENT_SOURCE_DIR}/with_variadics>"
+      "$<$<NOT:$<COMPILE_FEATURES:cxx_variadic_templates>>:${CMAKE_CURRENT_SOURCE_DIR}/no_variadics>")
+
+Consuming code then simply links to the ``foo`` target as usual and uses
+the feature-appropriate include directory
+
+.. code-block:: cmake
+
+  add_executable(consumer_with consumer_with.cpp)
+  target_link_libraries(consumer_with foo)
+  set_property(TARGET consumer_with CXX_STANDARD 11)
+
+  add_executable(consumer_no consumer_no.cpp)
+  target_link_libraries(consumer_no foo)

Help/manual/cmake-developer.7.rst

@@ -197,6 +197,51 @@ Templates
 Some template code is permitted, but with some limitations. Member templates
 may not be used, and template friends may not be used.
 
+Adding Compile Features
+=======================
+
+CMake reports an error if a compiler whose features are known does not report
+support for a particular requested feature.  A compiler is considered to have
+known features if it reports support for at least one feature.
+
+When adding a new compile feature to CMake, it is therefore necessary to list
+support for the feature for all CompilerIds which already have one or more
+feature supported, if the new feature is available for any version of the
+compiler.
+
+When adding the first supported feature to a particular CompilerId, it is
+necessary to list support for all features known to cmake (See
+:variable:`CMAKE_C_COMPILE_FEATURES` and
+:variable:`CMAKE_CXX_COMPILE_FEATURES` as appropriate), where available for
+the compiler.
+
+It is sensible to record the features for the most recent version of a
+particular CompilerId first, and then work backwards.  It is sensible to
+try to create a continuous range of versions of feature releases of the
+compiler.  Gaps in the range indicate incorrect features recorded for
+intermediate releases.
+
+Generally, features are made available for a particular version if the
+compiler vendor documents availability of the feature with that
+version.  Note that sometimes partially implemented features appear to
+be functional in previous releases (such as ``cxx_constexpr`` in GNU 4.6,
+though availability is documented in GNU 4.7), and sometimes compiler vendors
+document availability of features, though supporting infrastructure is
+not available (such as ``__has_feature(cxx_generic_lambdas)`` indicating
+non-availability in Clang 3.4, though it is documented as available, and
+fixed in Clang 3.5).  Similar cases for other compilers and versions
+need to be investigated when extending CMake to support them.
+
+When a vendor releases a new version of a known compiler which supports
+a previously unsupported feature, and there are already known features for
+that compiler, the feature should be listed as supported in CMake for
+that version of the compiler as soon as reasonably possible.
+
+Standard-specific/compiler-specific variables such
+``CMAKE_CXX98_COMPILE_FEATURES`` are deliberately not documented.  They
+only exist for the compiler-specific implementation of adding the ``-std``
+compile flag for compilers which need that.
+
 Help
 ====
 

Help/manual/cmake-generator-expressions.7.rst

@@ -88,7 +88,9 @@ otherwise expands to nothing.
   target, and ``0`` otherwise. If this expression is used while evaluating
   the link implementation of a target and if any dependency transitively
   increases the required :prop_tgt:`C_STANDARD` or :prop_tgt:`CXX_STANDARD`
-  for the 'head' target, an error is reported.
+  for the 'head' target, an error is reported.  See the
+  :manual:`cmake-compile-features(7)` manual for information on
+  compile features.
 
 Informational Expressions
 =========================

Help/prop_gbl/CMAKE_CXX_KNOWN_FEATURES.rst

@@ -8,7 +8,9 @@ C++ compiler.  If the feature is available with the C++ compiler, it will
 be listed in the :variable:`CMAKE_CXX_COMPILE_FEATURES` variable.
 
 The features listed here may be used with the :command:`target_compile_features`
-command.
+command.  See the :manual:`cmake-compile-features(7)` manual for information on
+compile features.
+
 
 The features known to this version of CMake are:
 

Help/prop_gbl/CMAKE_C_KNOWN_FEATURES.rst

@@ -8,7 +8,8 @@ C compiler.  If the feature is available with the C compiler, it will
 be listed in the :variable:`CMAKE_C_COMPILE_FEATURES` variable.
 
 The features listed here may be used with the :command:`target_compile_features`
-command.
+command.  See the :manual:`cmake-compile-features(7)` manual for information on
+compile features.
 
 The features known to this version of CMake are:
 

Help/prop_tgt/COMPILE_FEATURES.rst

@@ -8,4 +8,5 @@ in the :variable:`CMAKE_CXX_COMPILE_FEATURES` variable.
 
 Contents of ``COMPILE_FEATURES`` may use "generator expressions" with the
 syntax ``$<...>``.  See the :manual:`cmake-generator-expressions(7)` manual for
-available expressions.
+available expressions.  See the :manual:`cmake-compile-features(7)` manual
+for information on compile features.

Help/prop_tgt/CXX_EXTENSIONS.rst

@@ -7,6 +7,9 @@ This property specifies whether compiler specific extensions should be
 used.  For some compilers, this results in adding a flag such
 as ``-std=gnu++11`` instead of ``-std=c++11`` to the compile line.
 
+See the :manual:`cmake-compile-features(7)` manual for information on
+compile features.
+
 This property is initialized by the value of
 the :variable:`CMAKE_CXX_EXTENSIONS` variable if it is set when a target
 is created.

Help/prop_tgt/CXX_STANDARD.rst

@@ -22,6 +22,9 @@ flag will not result in an error or warning, but will instead add the
 ``-std=c++98`` flag if supported.  This "decay" behavior may be controlled
 with the :prop_tgt:`CXX_STANDARD_REQUIRED` target property.
 
+See the :manual:`cmake-compile-features(7)` manual for information on
+compile features.
+
 This property is initialized by the value of
 the :variable:`CMAKE_CXX_STANDARD` variable if it is set when a target
 is created.

Help/prop_tgt/CXX_STANDARD_REQUIRED.rst

@@ -9,6 +9,9 @@ property is ``OFF`` or unset, the :prop_tgt:`CXX_STANDARD` target property is
 treated as optional and may "decay" to a previous standard if the requested is
 not available.
 
+See the :manual:`cmake-compile-features(7)` manual for information on
+compile features.
+
 This property is initialized by the value of
 the :variable:`CMAKE_CXX_STANDARD_REQUIRED` variable if it is set when a
 target is created.

Help/prop_tgt/C_EXTENSIONS.rst

@@ -7,6 +7,9 @@ This property specifies whether compiler specific extensions should be
 used.  For some compilers, this results in adding a flag such
 as ``-std=gnu11`` instead of ``-std=c11`` to the compile line.
 
+See the :manual:`cmake-compile-features(7)` manual for information on
+compile features.
+
 This property is initialized by the value of
 the :variable:`CMAKE_C_EXTENSIONS` variable if it is set when a target
 is created.

Help/prop_tgt/C_STANDARD.rst

@@ -22,6 +22,9 @@ flag will not result in an error or warning, but will instead add the
 ``-std=c99`` or ``-std=c90`` flag if supported.  This "decay" behavior may
 be controlled with the :prop_tgt:`C_STANDARD_REQUIRED` target property.
 
+See the :manual:`cmake-compile-features(7)` manual for information on
+compile features.
+
 This property is initialized by the value of
 the :variable:`CMAKE_C_STANDARD` variable if it is set when a target
 is created.

Help/prop_tgt/C_STANDARD_REQUIRED.rst

@@ -9,6 +9,9 @@ property is ``OFF`` or unset, the :prop_tgt:`C_STANDARD` target property is
 treated as optional and may "decay" to a previous standard if the requested is
 not available.
 
+See the :manual:`cmake-compile-features(7)` manual for information on
+compile features.
+
 This property is initialized by the value of
 the :variable:`CMAKE_C_STANDARD_REQUIRED` variable if it is set when a
 target is created.

Help/prop_tgt/INTERFACE_COMPILE_FEATURES.rst

@@ -11,4 +11,6 @@ to require the features specified in the interface of ``foo``.
 
 Contents of ``INTERFACE_COMPILE_FEATURES`` may use "generator expressions"
 with the syntax ``$<...>``.  See the :manual:`cmake-generator-expressions(7)`
-manual for available expressions.
+manual for available expressions.  See the
+:manual:`cmake-compile-features(7)` manual for information on compile
+features.

Help/variable/CMAKE_CXX_COMPILE_FEATURES.rst

@@ -6,3 +6,6 @@ List of features known to the C++ compiler
 These features are known to be available for use with the C++ compiler. This
 list is a subset of the features listed in the :prop_gbl:`CMAKE_CXX_KNOWN_FEATURES`
 global property.
+
+See the :manual:`cmake-compile-features(7)` manual for information on
+compile features.

Help/variable/CMAKE_CXX_EXTENSIONS.rst

@@ -6,3 +6,6 @@ Default value for ``CXX_EXTENSIONS`` property of targets.
 This variable is used to initialize the :prop_tgt:`CXX_EXTENSIONS`
 property on all targets.  See that target property for additional
 information.
+
+See the :manual:`cmake-compile-features(7)` manual for information on
+compile features.

Help/variable/CMAKE_CXX_STANDARD.rst

@@ -6,3 +6,6 @@ Default value for ``CXX_STANDARD`` property of targets.
 This variable is used to initialize the :prop_tgt:`CXX_STANDARD`
 property on all targets.  See that target property for additional
 information.
+
+See the :manual:`cmake-compile-features(7)` manual for information on
+compile features.

Help/variable/CMAKE_CXX_STANDARD_REQUIRED.rst

@@ -6,3 +6,6 @@ Default value for ``CXX_STANDARD_REQUIRED`` property of targets.
 This variable is used to initialize the :prop_tgt:`CXX_STANDARD_REQUIRED`
 property on all targets.  See that target property for additional
 information.
+
+See the :manual:`cmake-compile-features(7)` manual for information on
+compile features.

Help/variable/CMAKE_C_COMPILE_FEATURES.rst

@@ -6,3 +6,6 @@ List of features known to the C compiler
 These features are known to be available for use with the C compiler. This
 list is a subset of the features listed in the :prop_gbl:`CMAKE_C_KNOWN_FEATURES`
 global property.
+
+See the :manual:`cmake-compile-features(7)` manual for information on
+compile features.

Help/variable/CMAKE_C_EXTENSIONS.rst

@@ -6,3 +6,6 @@ Default value for ``C_EXTENSIONS`` property of targets.
 This variable is used to initialize the :prop_tgt:`C_EXTENSIONS`
 property on all targets.  See that target property for additional
 information.
+
+See the :manual:`cmake-compile-features(7)` manual for information on
+compile features.

Help/variable/CMAKE_C_STANDARD.rst

@@ -6,3 +6,6 @@ Default value for ``C_STANDARD`` property of targets.
 This variable is used to initialize the :prop_tgt:`C_STANDARD`
 property on all targets.  See that target property for additional
 information.
+
+See the :manual:`cmake-compile-features(7)` manual for information on
+compile features.

Help/variable/CMAKE_C_STANDARD_REQUIRED.rst

@@ -6,3 +6,6 @@ Default value for ``C_STANDARD_REQUIRED`` property of targets.
 This variable is used to initialize the :prop_tgt:`C_STANDARD_REQUIRED`
 property on all targets.  See that target property for additional
 information.
+
+See the :manual:`cmake-compile-features(7)` manual for information on
+compile features.

Modules/WriteCompilerDetectionHeader.cmake

@@ -46,6 +46,9 @@
 # :prop_gbl:`CMAKE_C_KNOWN_FEATURES` and
 # :prop_gbl:`CMAKE_CXX_KNOWN_FEATURES` global properties.
 #
+# See the :manual:`cmake-compile-features(7)` manual for information on
+# compile features.
+#
 # Feature Test Macros
 # ===================
 #