cmStandardLevelResolver: Avoid unnecessary flags, fix unset level logic

The changes are part of CMP0128.

When the standard level is unset:
* Flags are added if extension mode doesn't match the compiler's default.
  Previously logic only worked if LANG_EXTENSIONS was ON. Fixes #22224.
* The full flag is used. Previously CMAKE_LANG_EXTENSION_COMPILE_OPTION was
  used. This was only supported for IAR.

Otherwise:
* Avoid adding flags if not necessary per the detected compiler defaults.
* Fixed check for when the requested standard is older. It now matches the
  nearby comments.

I reworded the fallback comment as its logic was a bit difficult to wrap my
head around.

5 files changed, 96 insertions, 6 deletions

diff --git a/Help/manual/cmake-compile-features.7.rst b/Help/manual/cmake-compile-features.7.rst
index 2b0de11..a0459fa 100644
--- a/Help/manual/cmake-compile-features.7.rst
+++ b/Help/manual/cmake-compile-features.7.rst
@@ -118,14 +118,13 @@ as well as any dependents (that may include headers from ``mylib``).
 Availability of Compiler Extensions
 -----------------------------------
 
-Because the :prop_tgt:`CXX_EXTENSIONS` target property is ``ON`` by default,
-CMake uses extended variants of language dialects by default, such as
-``-std=gnu++11`` instead of ``-std=c++11``.  That target property may be
-set to ``OFF`` to use the non-extended variant of the dialect flag.  Note
-that because most compilers enable extensions by default, this could
-expose cross-platform bugs in user code or in the headers of third-party
+The :prop_tgt:`<LANG>_EXTENSIONS` target property defaults to the compiler's
+efault. Note that because most compilers enable extensions by default, this
+may expose cross-platform bugs in user code or in the headers of third-party
 dependencies.
 
+:prop_tgt:`<LANG>_EXTENSIONS` used to default to ``ON``. See :policy:`CMP0128`.
+
 Optional Compile Features
 =========================
 
diff --git a/Help/manual/cmake-policies.7.rst b/Help/manual/cmake-policies.7.rst
index 0f0c0ab..3df4f9f 100644
--- a/Help/manual/cmake-policies.7.rst
+++ b/Help/manual/cmake-policies.7.rst
@@ -57,6 +57,7 @@ Policies Introduced by CMake 3.22
 .. toctree::
    :maxdepth: 1
 
+   CMP0128: Selection of language standard and extension flags improved. </policy/CMP0128>
    CMP0127: cmake_dependent_option() supports full Condition Syntax. </policy/CMP0127>
 
 Policies Introduced by CMake 3.21
diff --git a/Help/policy/CMP0128.rst b/Help/policy/CMP0128.rst
new file mode 100644
index 0000000..f341f24
--- /dev/null
+++ b/Help/policy/CMP0128.rst
@@ -0,0 +1,78 @@
+CMP0128
+-------
+
+.. versionadded:: 3.22
+
+When this policy is set to ``NEW``:
+
+* :prop_tgt:`<LANG>_EXTENSIONS` is initialized to
+  :variable:`CMAKE_<LANG>_EXTENSIONS_DEFAULT`.
+
+* Extensions are correctly disabled/enabled if :prop_tgt:`<LANG>_STANDARD` is
+  unset.
+
+* Standard mode-affecting flags aren't added unless necessary to achieve the
+  specified mode.
+
+The ``OLD`` behavior:
+
+* Initializes :prop_tgt:`<LANG>_EXTENSIONS` to ``ON``.
+
+* Always adds a flag if :prop_tgt:`<LANG>_STANDARD` is set and
+  :prop_tgt:`<LANG>_STANDARD_REQUIRED` is ``OFF``.
+
+* If :prop_tgt:`<LANG>_STANDARD` is unset:
+
+  * Doesn't disable extensions even if :prop_tgt:`<LANG>_EXTENSIONS` is
+    ``OFF``.
+
+  * Fails to enable extensions if :prop_tgt:`<LANG>_EXTENSIONS` is ``ON``
+    except for the ``IAR`` compiler.
+
+Code may need to be updated for the ``NEW`` behavior in the following cases:
+
+* If :prop_tgt:`<LANG>_EXTENSIONS` matches
+  :variable:`CMAKE_<LANG>_EXTENSIONS_DEFAULT` or is unset and the compiler's
+  default satisfies :prop_tgt:`<LANG>_STANDARD` but the compiled code requires
+  the exact standard specified.
+  Such code should set :prop_tgt:`<LANG>_STANDARD_REQUIRED` to ``ON``.
+
+  For example:
+
+  .. code-block:: cmake
+
+    cmake_minimum_required(VERSION |release|)
+    project(example C)
+
+    add_executable(exe main.c)
+    set_property(TARGET exe PROPERTY C_STANDARD 99)
+
+  If the compiler defaults to C11 then the standard specification for C99 is
+  satisfied and CMake will pass no flags. ``main.c`` will no longer compile if
+  it is incompatible with C11.
+
+* If a standard mode flag previously overridden by CMake's and not used during
+  compiler detection now takes effect due to CMake no longer adding one as the
+  default detected is appropriate.
+
+  Such code should be converted to either:
+
+  * Use :prop_tgt:`<LANG>_STANDARD` and :prop_tgt:`<LANG>_EXTENSIONS` instead
+    of manually adding flags.
+
+  * Or ensure the manually-specified flags are used during compiler detection.
+
+If compiler flags affecting the standard mode are used during compiler
+detection (for example in :manual:`a toolchain file <cmake-toolchains(7)>`
+using :variable:`CMAKE_<LANG>_FLAGS_INIT`) then they will affect the detected
+default :variable:`standard <CMAKE_<LANG>_STANDARD_DEFAULT>` and
+:variable:`extensions <CMAKE_<LANG>_EXTENSIONS_DEFAULT>`.
+
+Unlike many policies, CMake version |release| does *not* warn when the policy
+is not set and simply uses the ``OLD`` behavior. Use the
+:command:`cmake_policy` command to set it to ``OLD`` or ``NEW`` explicitly.
+See documentation of the
+:variable:`CMAKE_POLICY_WARNING_CMP0128 <CMAKE_POLICY_WARNING_CMP<NNNN>>`
+variable to control the warning.
+
+.. include:: DEPRECATED.txt
diff --git a/Help/release/dev/compile-features-standard-logic-rework.rst b/Help/release/dev/compile-features-standard-logic-rework.rst
new file mode 100644
index 0000000..432a756
--- /dev/null
+++ b/Help/release/dev/compile-features-standard-logic-rework.rst
@@ -0,0 +1,10 @@
+compile-features-standard-logic-rework
+--------------------------------------
+
+* The :manual:`Compile Features <cmake-compile-features(7)>` functionality now
+  correctly disables or enables compiler extensions when no standard level is
+  specified and avoids unnecessarily adding language standard flags if the
+  requested settings match the compiler's defaults. See :policy:`CMP0128`.
+
+* :prop_tgt:`<LANG>_EXTENSIONS` is initialized to
+  :variable:`CMAKE_<LANG>_EXTENSIONS_DEFAULT`. See :policy:`CMP0128`.
diff --git a/Help/variable/CMAKE_POLICY_WARNING_CMPNNNN.rst b/Help/variable/CMAKE_POLICY_WARNING_CMPNNNN.rst
index 11dd194..0231668 100644
--- a/Help/variable/CMAKE_POLICY_WARNING_CMPNNNN.rst
+++ b/Help/variable/CMAKE_POLICY_WARNING_CMPNNNN.rst
@@ -32,6 +32,8 @@ only for the policies that do not warn by default:
   policy :policy:`CMP0116`.
 * ``CMAKE_POLICY_WARNING_CMP0126`` controls the warning for
   policy :policy:`CMP0126`.
+* ``CMAKE_POLICY_WARNING_CMP0128`` controls the warning for
+  policy :policy:`CMP0128`.
 
 This variable should not be set by a project in CMake code.  Project
 developers running CMake may set this variable in their cache to