options: Add an EnumeratedUserOption class

This will allow us to take choices out of the UserOption class, which
doesn't actually use this attribute.

13 files changed, 93 insertions, 82 deletions

diff --git a/mesonbuild/cmake/interpreter.py b/mesonbuild/cmake/interpreter.py
index fafee86ab..27ce54e20 100644
--- a/mesonbuild/cmake/interpreter.py
+++ b/mesonbuild/cmake/interpreter.py
@@ -19,6 +19,7 @@ from .toolchain import CMakeToolchain, CMakeExecScope
 from .traceparser import CMakeTraceParser
 from .tracetargets import resolve_cmake_trace_targets
 from .. import mlog, mesonlib
+from .. import options
 from ..mesonlib import MachineChoice, OrderedSet, path_is_in_root, relative_to_if_possible
 from ..options import OptionKey
 from ..mesondata import DataFile
@@ -533,17 +534,12 @@ class ConverterTarget:
     @lru_cache(maxsize=None)
     def _all_lang_stds(self, lang: str) -> 'ImmutableListProtocol[str]':
         try:
-            res = self.env.coredata.optstore.get_value_object(OptionKey(f'{lang}_std', machine=MachineChoice.BUILD)).choices
+            opt = self.env.coredata.optstore.get_value_object(OptionKey(f'{lang}_std', machine=MachineChoice.BUILD))
+            assert isinstance(opt, (options.UserStdOption, options.UserComboOption)), 'for mypy'
+            return opt.choices or []
         except KeyError:
             return []
 
-        # TODO: Get rid of this once we have proper typing for options
-        assert isinstance(res, list)
-        for i in res:
-            assert isinstance(i, str)
-
-        return res
-
     def process_inter_target_dependencies(self) -> None:
         # Move the dependencies from all TRANSFER_DEPENDENCIES_FROM to the target
         to_process = list(self.depends)
diff --git a/mesonbuild/compilers/c.py b/mesonbuild/compilers/c.py
index d8ad9f793..4f93ea14e 100644
--- a/mesonbuild/compilers/c.py
+++ b/mesonbuild/compilers/c.py
@@ -124,7 +124,7 @@ class ClangCCompiler(ClangCStds, ClangCompiler, CCompiler):
         opts = super().get_options()
         if self.info.is_windows() or self.info.is_cygwin():
             key = self.form_compileropt_key('winlibs')
-            opts[key] = options.UserArrayOption(
+            opts[key] = options.UserStringArrayOption(
                 self.make_option_name(key),
                 'Standard Windows libraries to link against',
                 gnu_winlibs)
@@ -257,7 +257,7 @@ class GnuCCompiler(GnuCStds, GnuCompiler, CCompiler):
         opts = super().get_options()
         if self.info.is_windows() or self.info.is_cygwin():
             key = self.form_compileropt_key('winlibs')
-            opts[key] = options.UserArrayOption(
+            opts[key] = options.UserStringArrayOption(
                 self.make_option_name(key),
                 'Standard Windows libraries to link against',
                 gnu_winlibs)
@@ -406,7 +406,7 @@ class VisualStudioLikeCCompilerMixin(CompilerMixinBase):
     def get_options(self) -> MutableKeyedOptionDictType:
         opts = super().get_options()
         key = self.form_compileropt_key('winlibs')
-        opts[key] = options.UserArrayOption(
+        opts[key] = options.UserStringArrayOption(
             self.make_option_name(key),
             'Standard Windows libraries to link against',
             msvc_winlibs)
@@ -733,9 +733,7 @@ class MetrowerksCCompilerARM(MetrowerksCompiler, CCompiler):
 
     def get_options(self) -> 'MutableKeyedOptionDictType':
         opts = super().get_options()
-        c_stds = ['c99']
-        key = self.form_compileropt_key('std')
-        opts[key].choices = ['none'] + c_stds
+        self._update_language_stds(opts, ['c99'])
         return opts
 
     def get_option_compile_args(self, options: 'KeyedOptionDictType') -> T.List[str]:
@@ -763,9 +761,7 @@ class MetrowerksCCompilerEmbeddedPowerPC(MetrowerksCompiler, CCompiler):
 
     def get_options(self) -> 'MutableKeyedOptionDictType':
         opts = super().get_options()
-        c_stds = ['c99']
-        key = self.form_compileropt_key('std')
-        opts[key].choices = ['none'] + c_stds
+        self._update_language_stds(opts, ['c99'])
         return opts
 
     def get_option_compile_args(self, options: 'KeyedOptionDictType') -> T.List[str]:
diff --git a/mesonbuild/compilers/compilers.py b/mesonbuild/compilers/compilers.py
index 1432afb85..5777f19f1 100644
--- a/mesonbuild/compilers/compilers.py
+++ b/mesonbuild/compilers/compilers.py
@@ -1359,6 +1359,15 @@ class Compiler(HoldableObject, metaclass=abc.ABCMeta):
     def form_compileropt_key(self, basename: str) -> OptionKey:
         return OptionKey(f'{self.language}_{basename}', machine=self.for_machine)
 
+    def _update_language_stds(self, opts: MutableKeyedOptionDictType, value: T.List[str]) -> None:
+        key = self.form_compileropt_key('std')
+        std = opts[key]
+        assert isinstance(std, (options.UserStdOption, options.UserComboOption)), 'for mypy'
+        if 'none' not in value:
+            value = ['none'] + value
+        std.choices = value
+
+
 def get_global_options(lang: str,
                        comp: T.Type[Compiler],
                        for_machine: MachineChoice,
@@ -1374,12 +1383,12 @@ def get_global_options(lang: str,
     comp_options = env.options.get(comp_key, [])
     link_options = env.options.get(largkey, [])
 
-    cargs = options.UserArrayOption(
+    cargs = options.UserStringArrayOption(
         f'{lang}_{argkey.name}',
         description + ' compiler',
         comp_options, split_args=True, allow_dups=True)
 
-    largs = options.UserArrayOption(
+    largs = options.UserStringArrayOption(
         f'{lang}_{largkey.name}',
         description + ' linker',
         link_options, split_args=True, allow_dups=True)
diff --git a/mesonbuild/compilers/cpp.py b/mesonbuild/compilers/cpp.py
index 7fa9aa833..80f84b38e 100644
--- a/mesonbuild/compilers/cpp.py
+++ b/mesonbuild/compilers/cpp.py
@@ -243,7 +243,7 @@ class ClangCPPCompiler(_StdCPPLibMixin, ClangCPPStds, ClangCompiler, CPPCompiler
             self.make_option_name(key),
             'C++ exception handling type.',
             'default',
-            ['none', 'default', 'a', 's', 'sc'])
+            choices=['none', 'default', 'a', 's', 'sc'])
 
         key = self.form_compileropt_key('rtti')
         opts[key] = options.UserBooleanOption(
@@ -259,7 +259,7 @@ class ClangCPPCompiler(_StdCPPLibMixin, ClangCPPStds, ClangCompiler, CPPCompiler
 
         if self.info.is_windows() or self.info.is_cygwin():
             key = self.form_compileropt_key('winlibs')
-            opts[key] = options.UserArrayOption(
+            opts[key] = options.UserStringArrayOption(
                 self.make_option_name(key),
                 'Standard Win libraries to link against',
                 gnu_winlibs)
@@ -399,7 +399,7 @@ class ArmclangCPPCompiler(ArmclangCompiler, CPPCompiler):
             self.make_option_name(key),
             'C++ exception handling type.',
             'default',
-            ['none', 'default', 'a', 's', 'sc'])
+            choices=['none', 'default', 'a', 's', 'sc'])
 
         key = self.form_compileropt_key('std')
         std_opt = opts[key]
@@ -449,7 +449,7 @@ class GnuCPPCompiler(_StdCPPLibMixin, GnuCPPStds, GnuCompiler, CPPCompiler):
             self.make_option_name(key),
             'C++ exception handling type.',
             'default',
-            ['none', 'default', 'a', 's', 'sc'])
+            choices=['none', 'default', 'a', 's', 'sc'])
 
         key = self.form_compileropt_key('rtti')
         opts[key] = options.UserBooleanOption(
@@ -465,7 +465,7 @@ class GnuCPPCompiler(_StdCPPLibMixin, GnuCPPStds, GnuCompiler, CPPCompiler):
 
         if self.info.is_windows() or self.info.is_cygwin():
             key = key.evolve(name='cpp_winlibs')
-            opts[key] = options.UserArrayOption(
+            opts[key] = options.UserStringArrayOption(
                 self.make_option_name(key),
                 'Standard Win libraries to link against',
                 gnu_winlibs)
@@ -578,7 +578,7 @@ class ElbrusCPPCompiler(ElbrusCompiler, CPPCompiler):
             self.make_option_name(key),
             'C++ exception handling type.',
             'default',
-            ['none', 'default', 'a', 's', 'sc'])
+            choices=['none', 'default', 'a', 's', 'sc'])
 
         key = self.form_compileropt_key('debugstl')
         opts[key] = options.UserBooleanOption(
@@ -661,7 +661,7 @@ class IntelCPPCompiler(IntelGnuLikeCompiler, CPPCompiler):
             self.make_option_name(key),
             'C++ exception handling type.',
             'default',
-            ['none', 'default', 'a', 's', 'sc'])
+            choices=['none', 'default', 'a', 's', 'sc'])
 
         key = self.form_compileropt_key('rtti')
         opts[key] = options.UserBooleanOption(
@@ -691,9 +691,7 @@ class IntelCPPCompiler(IntelGnuLikeCompiler, CPPCompiler):
             c_stds += ['c++2a']
             g_stds += ['gnu++2a']
 
-        std_opt = opts[self.form_compileropt_key('std')]
-        assert isinstance(std_opt, options.UserStdOption), 'for mypy'
-        std_opt.set_versions(c_stds + g_stds)
+        self._update_language_stds(opts, c_stds + g_stds)
         return opts
 
     def get_option_compile_args(self, options: 'KeyedOptionDictType') -> T.List[str]:
@@ -754,7 +752,7 @@ class VisualStudioLikeCPPCompilerMixin(CompilerMixinBase):
             self.make_option_name(key),
             'C++ exception handling type.',
             'default',
-            ['none', 'default', 'a', 's', 'sc'])
+            choices=['none', 'default', 'a', 's', 'sc'])
 
         key = self.form_compileropt_key('rtti')
         opts[key] = options.UserBooleanOption(
@@ -763,7 +761,7 @@ class VisualStudioLikeCPPCompilerMixin(CompilerMixinBase):
             True)
 
         key = self.form_compileropt_key('winlibs')
-        opts[key] = options.UserArrayOption(
+        opts[key] = options.UserStringArrayOption(
             self.make_option_name(key),
             'Standard Win libraries to link against',
             msvc_winlibs)
@@ -1040,8 +1038,7 @@ class MetrowerksCPPCompilerARM(MetrowerksCompiler, CPPCompiler):
 
     def get_options(self) -> 'MutableKeyedOptionDictType':
         opts = super().get_options()
-        key = self.form_compileropt_key('std')
-        opts[key].choices = ['none']
+        self._update_language_stds(opts, [])
         return opts
 
     def get_option_compile_args(self, options: 'KeyedOptionDictType') -> T.List[str]:
@@ -1069,8 +1066,7 @@ class MetrowerksCPPCompilerEmbeddedPowerPC(MetrowerksCompiler, CPPCompiler):
 
     def get_options(self) -> 'MutableKeyedOptionDictType':
         opts = super().get_options()
-        key = self.form_compileropt_key('std')
-        opts[key].choices = ['none']
+        self._update_language_stds(opts, [])
         return opts
 
     def get_option_compile_args(self, options: 'KeyedOptionDictType') -> T.List[str]:
diff --git a/mesonbuild/compilers/cuda.py b/mesonbuild/compilers/cuda.py
index aefcc512b..6a49d95ae 100644
--- a/mesonbuild/compilers/cuda.py
+++ b/mesonbuild/compilers/cuda.py
@@ -648,13 +648,12 @@ class CudaCompiler(Compiler):
 
         opts = super().get_options()
 
-        # XXX: cpp_std is correct, the annotations are wrong
         key = self.form_compileropt_key('std')
         opts[key] = options.UserComboOption(
             self.make_option_name(key),
             'C++ language standard to use with CUDA',
             'none',
-            cpp_stds)
+            choices=cpp_stds)
 
         key = self.form_compileropt_key('ccbindir')
         opts[key] = options.UserStringOption(
diff --git a/mesonbuild/compilers/cython.py b/mesonbuild/compilers/cython.py
index ba04aea0b..ed0ab31ad 100644
--- a/mesonbuild/compilers/cython.py
+++ b/mesonbuild/compilers/cython.py
@@ -74,14 +74,14 @@ class CythonCompiler(Compiler):
             self.make_option_name(key),
             'Python version to target',
             '3',
-            ['2', '3'])
+            choices=['2', '3'])
 
         key = self.form_compileropt_key('language')
         opts[key] = options.UserComboOption(
             self.make_option_name(key),
             'Output C or C++ files',
             'c',
-            ['c', 'cpp'])
+            choices=['c', 'cpp'])
 
         return opts
 
diff --git a/mesonbuild/compilers/fortran.py b/mesonbuild/compilers/fortran.py
index b004727b4..72c9a5a97 100644
--- a/mesonbuild/compilers/fortran.py
+++ b/mesonbuild/compilers/fortran.py
@@ -121,7 +121,7 @@ class FortranCompiler(CLikeCompiler, Compiler):
             self.make_option_name(key),
             'Fortran language standard to use',
             'none',
-            ['none'])
+            choices=['none'])
 
         return opts
 
@@ -281,8 +281,7 @@ class GnuFortranCompiler(GnuCompiler, FortranCompiler):
             fortran_stds += ['f2008']
         if version_compare(self.version, '>=8.0.0'):
             fortran_stds += ['f2018']
-        key = self.form_compileropt_key('std')
-        opts[key].choices = ['none'] + fortran_stds
+        self._update_language_stds(opts, fortran_stds)
         return opts
 
     def get_option_compile_args(self, options: 'KeyedOptionDictType') -> T.List[str]:
@@ -338,9 +337,7 @@ class ElbrusFortranCompiler(ElbrusCompiler, FortranCompiler):
 
     def get_options(self) -> 'MutableKeyedOptionDictType':
         opts = super().get_options()
-        fortran_stds = ['f95', 'f2003', 'f2008', 'gnu', 'legacy', 'f2008ts']
-        key = self.form_compileropt_key('std')
-        opts[key].choices = ['none'] + fortran_stds
+        self._update_language_stds(opts, ['f95', 'f2003', 'f2008', 'gnu', 'legacy', 'f2008ts'])
         return opts
 
     def get_module_outdir_args(self, path: str) -> T.List[str]:
@@ -418,8 +415,7 @@ class IntelFortranCompiler(IntelGnuLikeCompiler, FortranCompiler):
 
     def get_options(self) -> 'MutableKeyedOptionDictType':
         opts = super().get_options()
-        key = self.form_compileropt_key('std')
-        opts[key].choices = ['none', 'legacy', 'f95', 'f2003', 'f2008', 'f2018']
+        self._update_language_stds(opts, ['none', 'legacy', 'f95', 'f2003', 'f2008', 'f2018'])
         return opts
 
     def get_option_compile_args(self, options: 'KeyedOptionDictType') -> T.List[str]:
@@ -473,8 +469,7 @@ class IntelClFortranCompiler(IntelVisualStudioLikeCompiler, FortranCompiler):
 
     def get_options(self) -> 'MutableKeyedOptionDictType':
         opts = super().get_options()
-        key = self.form_compileropt_key('std')
-        opts[key].choices = ['none', 'legacy', 'f95', 'f2003', 'f2008', 'f2018']
+        self._update_language_stds(opts, ['none', 'legacy', 'f95', 'f2003', 'f2008', 'f2018'])
         return opts
 
     def get_option_compile_args(self, options: 'KeyedOptionDictType') -> T.List[str]:
diff --git a/mesonbuild/compilers/rust.py b/mesonbuild/compilers/rust.py
index 7fe16c6f8..aacdc07d7 100644
--- a/mesonbuild/compilers/rust.py
+++ b/mesonbuild/compilers/rust.py
@@ -241,7 +241,7 @@ class RustCompiler(Compiler):
             self.make_option_name(key),
             'Rust edition to use',
             'none',
-            ['none', '2015', '2018', '2021', '2024'])
+            choices=['none', '2015', '2018', '2021', '2024'])
 
         return opts
 
diff --git a/mesonbuild/coredata.py b/mesonbuild/coredata.py
index d295858dc..5469616f8 100644
--- a/mesonbuild/coredata.py
+++ b/mesonbuild/coredata.py
@@ -599,7 +599,7 @@ class CoreData:
             oldval = self.optstore.get_value_object(key)
             if type(oldval) is not type(value):
                 self.optstore.set_value(key, value.value)
-            elif oldval.choices != value.choices:
+            elif oldval.printable_choices() != value.printable_choices():
                 # If the choices have changed, use the new value, but attempt
                 # to keep the old options. If they are not valid keep the new
                 # defaults but warn.
diff --git a/mesonbuild/mconf.py b/mesonbuild/mconf.py
index 3a6343ba1..7c2270edc 100644
--- a/mesonbuild/mconf.py
+++ b/mesonbuild/mconf.py
@@ -240,7 +240,7 @@ class Conf:
                 printable_value = '<inherited from main project>'
             if isinstance(o, options.UserFeatureOption) and o.is_auto():
                 printable_value = auto.printable_value()
-            self.add_option(str(root), o.description, printable_value, o.choices)
+            self.add_option(str(root), o.description, printable_value, o.printable_choices())
 
     def print_conf(self, pager: bool) -> None:
         if pager:
diff --git a/mesonbuild/mintro.py b/mesonbuild/mintro.py
index cd68911ce..cf76b6012 100644
--- a/mesonbuild/mintro.py
+++ b/mesonbuild/mintro.py
@@ -319,7 +319,7 @@ def list_buildoptions(coredata: cdata.CoreData, subprojects: T.Optional[T.List[s
                 typestr = 'combo'
             elif isinstance(opt, options.UserIntegerOption):
                 typestr = 'integer'
-            elif isinstance(opt, options.UserArrayOption):
+            elif isinstance(opt, options.UserStringArrayOption):
                 typestr = 'array'
                 c = opt.printable_choices()
                 if c:
diff --git a/mesonbuild/optinterpreter.py b/mesonbuild/optinterpreter.py
index 99efd511e..8c0d1daec 100644
--- a/mesonbuild/optinterpreter.py
+++ b/mesonbuild/optinterpreter.py
@@ -213,7 +213,7 @@ class OptionInterpreter:
         KwargInfo('value', str, default=''),
     )
     def string_parser(self, name: str, description: str, args: T.Tuple[bool, _DEPRECATED_ARGS], kwargs: StringArgs) -> options.UserOption:
-        return options.UserStringOption(name, description, kwargs['value'], None, *args)
+        return options.UserStringOption(name, description, kwargs['value'], *args)
 
     @typed_kwargs(
         'boolean option',
@@ -239,7 +239,7 @@ class OptionInterpreter:
         value = kwargs['value']
         if value is None:
             value = kwargs['choices'][0]
-        return options.UserComboOption(name, description, value, choices, *args)
+        return options.UserComboOption(name, description, value, *args, choices)
 
     @typed_kwargs(
         'integer option',
@@ -255,7 +255,7 @@ class OptionInterpreter:
     )
     def integer_parser(self, name: str, description: str, args: T.Tuple[bool, _DEPRECATED_ARGS], kwargs: IntegerArgs) -> options.UserOption:
         return options.UserIntegerOption(
-            name, description, kwargs['value'], None, *args, min_value=kwargs['min'], max_value=kwargs['max'])
+            name, description, kwargs['value'], *args, min_value=kwargs['min'], max_value=kwargs['max'])
 
     @typed_kwargs(
         'string array option',
@@ -270,12 +270,11 @@ class OptionInterpreter:
                 FeatureDeprecated('String value for array option', '1.3.0').use(self.subproject)
             else:
                 raise mesonlib.MesonException('Value does not define an array: ' + value)
-        # XXX: the value of choices is correct, the annotation is wrong.
-        #      the annotation will be fixed in a later commit
-        return options.UserArrayOption(name, description, value,
-                                       choices=choices,  # type: ignore[arg-type]
-                                       yielding=args[0],
-                                       deprecated=args[1])
+        return options.UserStringArrayOption(
+            name, description, value,
+            choices=choices,
+            yielding=args[0],
+            deprecated=args[1])
 
     @typed_kwargs(
         'feature option',
diff --git a/mesonbuild/options.py b/mesonbuild/options.py
index a53cb91a4..ae580785e 100644
--- a/mesonbuild/options.py
+++ b/mesonbuild/options.py
@@ -253,7 +253,6 @@ class UserOption(T.Generic[_T], HoldableObject):
     name: str
     description: str
     value_: dataclasses.InitVar[_T]
-    choices: T.Optional[T.Union[str, T.List[_T]]] = None
     yielding: bool = DEFAULT_YIELDING
     deprecated: DeprecatedType = False
     readonly: bool = dataclasses.field(default=False, init=False)
@@ -269,11 +268,7 @@ class UserOption(T.Generic[_T], HoldableObject):
         return self.value
 
     def printable_choices(self) -> T.Optional[T.List[str]]:
-        if not self.choices:
-            return None
-        if isinstance(self.choices, str):
-            return [self.choices]
-        return [str(c) for c in self.choices]
+        return None
 
     # Check that the input is a valid value and return the
     # "cleaned" or "native" version. For example the Boolean
@@ -288,6 +283,17 @@ class UserOption(T.Generic[_T], HoldableObject):
 
 
 @dataclasses.dataclass
+class EnumeratedUserOption(UserOption[_T]):
+
+    """A generic UserOption that has enumerated values."""
+
+    choices: T.List[_T] = dataclasses.field(default_factory=list)
+
+    def printable_choices(self) -> T.Optional[T.List[str]]:
+        return [str(c) for c in self.choices]
+
+
+@dataclasses.dataclass
 class UserStringOption(UserOption[str]):
 
     def validate_value(self, value: T.Any) -> str:
@@ -296,7 +302,7 @@ class UserStringOption(UserOption[str]):
         return value
 
 @dataclasses.dataclass
-class UserBooleanOption(UserOption[bool]):
+class UserBooleanOption(EnumeratedUserOption[bool]):
 
     choices: T.List[bool] = dataclasses.field(default_factory=lambda: [True, False])
 
@@ -327,7 +333,10 @@ class UserIntegerOption(UserOption[int]):
             choices.append(f'>= {self.min_value!s}')
         if self.max_value is not None:
             choices.append(f'<= {self.max_value!s}')
-        self.choices = ', '.join(choices)
+        self.__choices: str = ', '.join(choices)
+
+    def printable_choices(self) -> T.Optional[T.List[str]]:
+        return [self.__choices]
 
     def validate_value(self, value: T.Any) -> int:
         if isinstance(value, str):
@@ -376,7 +385,7 @@ class UserUmaskOption(UserIntegerOption, UserOption[T.Union[str, OctalInt]]):
             raise MesonException(f'Invalid mode for option "{self.name}" {e}')
 
 @dataclasses.dataclass
-class UserComboOption(UserOption[str]):
+class UserComboOption(EnumeratedUserOption[str]):
 
     def validate_value(self, value: T.Any) -> str:
         if value not in self.choices:
@@ -390,15 +399,32 @@ class UserComboOption(UserOption[str]):
             raise MesonException('Value "{}" (of type "{}") for option "{}" is not one of the choices.'
                                  ' Possible choices are (as string): {}.'.format(
                                      value, _type, self.name, optionsstring))
+
+        assert isinstance(value, str), 'for mypy'
         return value
 
 @dataclasses.dataclass
-class UserArrayOption(UserOption[T.List[str]]):
+class UserArrayOption(UserOption[T.List[_T]]):
 
-    value_: dataclasses.InitVar[T.Union[str, T.List[str]]]
+    value_: dataclasses.InitVar[T.Union[_T, T.List[_T]]]
+    choices: T.Optional[T.List[_T]] = None
     split_args: bool = False
     allow_dups: bool = False
 
+    def extend_value(self, value: T.Union[str, T.List[str]]) -> None:
+        """Extend the value with an additional value."""
+        new = self.validate_value(value)
+        self.set_value(self.value + new)
+
+    def printable_choices(self) -> T.Optional[T.List[str]]:
+        if self.choices is None:
+            return None
+        return [str(c) for c in self.choices]
+
+
+@dataclasses.dataclass
+class UserStringArrayOption(UserArrayOption[str]):
+
     def listify(self, value: T.Any) -> T.List[T.Any]:
         try:
             return listify_array_value(value, self.split_args)
@@ -427,11 +453,6 @@ class UserArrayOption(UserOption[T.List[str]]):
                 )
         return newvalue
 
-    def extend_value(self, value: T.Union[str, T.List[str]]) -> None:
-        """Extend the value with an additional value."""
-        new = self.validate_value(value)
-        self.set_value(self.value + new)
-
 
 @dataclasses.dataclass
 class UserFeatureOption(UserComboOption):
@@ -471,7 +492,7 @@ class UserStdOption(UserComboOption):
         # Map a deprecated std to its replacement. e.g. gnu11 -> c11.
         self.deprecated_stds: T.Dict[str, str] = {}
         opt_name = 'cpp_std' if lang == 'c++' else f'{lang}_std'
-        super().__init__(opt_name, f'{lang} language standard to use', 'none', ['none'])
+        super().__init__(opt_name, f'{lang} language standard to use', 'none', choices=['none'])
 
     def set_versions(self, versions: T.List[str], gnu: bool = False, gnu_deprecated: bool = False) -> None:
         assert all(std in self.all_stds for std in versions)
@@ -637,7 +658,7 @@ BUILTIN_CORE_OPTIONS: T.Dict['OptionKey', 'BuiltinOption'] = OrderedDict([
     (OptionKey('warning_level'),   BuiltinOption(UserComboOption, 'Compiler warning level to use', '1', choices=['0', '1', '2', '3', 'everything'], yielding=False)),
     (OptionKey('werror'),          BuiltinOption(UserBooleanOption, 'Treat warnings as errors', False, yielding=False)),
     (OptionKey('wrap_mode'),       BuiltinOption(UserComboOption, 'Wrap mode', 'default', choices=['default', 'nofallback', 'nodownload', 'forcefallback', 'nopromote'])),
-    (OptionKey('force_fallback_for'), BuiltinOption(UserArrayOption, 'Force fallback for those subprojects', [])),
+    (OptionKey('force_fallback_for'), BuiltinOption(UserStringArrayOption, 'Force fallback for those subprojects', [])),
     (OptionKey('vsenv'),           BuiltinOption(UserBooleanOption, 'Activate Visual Studio environment', False, readonly=True)),
 
     # Pkgconfig module
@@ -660,8 +681,8 @@ BUILTIN_CORE_OPTIONS: T.Dict['OptionKey', 'BuiltinOption'] = OrderedDict([
 BUILTIN_OPTIONS = OrderedDict(chain(BUILTIN_DIR_OPTIONS.items(), BUILTIN_CORE_OPTIONS.items()))
 
 BUILTIN_OPTIONS_PER_MACHINE: T.Dict['OptionKey', 'BuiltinOption'] = OrderedDict([
-    (OptionKey('pkg_config_path'), BuiltinOption(UserArrayOption, 'List of additional paths for pkg-config to search', [])),
-    (OptionKey('cmake_prefix_path'), BuiltinOption(UserArrayOption, 'List of additional prefixes for cmake to search', [])),
+    (OptionKey('pkg_config_path'), BuiltinOption(UserStringArrayOption, 'List of additional paths for pkg-config to search', [])),
+    (OptionKey('cmake_prefix_path'), BuiltinOption(UserStringArrayOption, 'List of additional prefixes for cmake to search', [])),
 ])
 
 # Special prefix-dependent defaults for installation directories that reside in