coredata: Warn on usage of both -Dbuildtype and -Doptimization/-Ddebug

It may not be obvious to users that these two ways to set build-types
override each other and specifying both is redundant, and conflicts
are resolved based on whichever is specified later.

Closes https://github.com/mesonbuild/meson/issues/6742

docs/markdown/Builtin-options.md

@@ -87,6 +87,7 @@ Using the option as-is with no prefix affects all machines. For example:
 | werror                               | false         | Treat warnings as errors                                       | no             |
 | wrap_mode {default, nofallback,<br>nodownload, forcefallback} | default | Wrap mode to use                            | no             |
 
+<a name="build-type-options"></a>
 For setting optimization levels and toggling debug, you can either set the
 `buildtype` option, or you can set the `optimization` and `debug` options which
 give finer control over the same. Whichever you decide to use, the other will

mesonbuild/coredata.py

@@ -718,6 +718,13 @@ class CoreData:
             self.copy_build_options_from_regular_ones()
 
     def set_default_options(self, default_options, subproject, env):
+        # Warn if the user is using two different ways of setting build-type
+        # options that override each other
+        if 'buildtype' in env.cmd_line_options and \
+           ('optimization' in env.cmd_line_options or 'debug' in env.cmd_line_options):
+            mlog.warning('Recommend using either -Dbuildtype or -Doptimization + -Ddebug. '
+                         'Using both is redundant since they override each other. '
+                         'See: https://mesonbuild.com/Builtin-options.html#build-type-options')
         cmd_line_options = OrderedDict()
         # Set project default_options as if they were passed to the cmdline.
         # Subprojects can only define default for user options and not yielding

run_unittests.py

@@ -3842,10 +3842,11 @@ recommended as it is not supported on some platforms''')
         self.assertEqual(opts['debug'], True)
         self.assertEqual(opts['optimization'], '2')
         self.assertEqual(opts['buildtype'], 'debugoptimized')
-        # Setting both buildtype and debug on the command-line should work
-        # Also test that --debug is parsed as -Ddebug=true
+        # Setting both buildtype and debug on the command-line should work, and
+        # should warn not to do that. Also test that --debug is parsed as -Ddebug=true
         self.new_builddir()
-        self.init(testdir, extra_args=['-Dbuildtype=debugoptimized', '--debug'])
+        out = self.init(testdir, extra_args=['-Dbuildtype=debugoptimized', '--debug'])
+        self.assertRegex(out, 'Recommend using either.*buildtype.*debug.*redundant')
         opts = self.get_opts_as_dict()
         self.assertEqual(opts['debug'], True)
         self.assertEqual(opts['optimization'], '2')