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+## Add a FeatureOption.enable_if and .disable_if
+
+These are useful when features need to be constrained to pass to [[dependency]],
+as the behavior of an `auto` and `disabled` or `enabled` feature is markedly
+different. consider the following case:
+
+```meson
+opt = get_option('feature').disable_auto_if(not foo)
+if opt.enabled() and not foo
+  error('Cannot enable feat when foo is not also enabled')
+endif
+dep = dependency('foo', required : opt)
+```
+
+This could be simplified to
+```meson
+opt = get_option('feature').disable_if(not foo, error_message : 'Cannot enable feature when foo is not also enabled')
+dep = dependency('foo', required : opt)
+```
+
+For a real life example, here is some code in mesa:
+```meson
+_llvm = get_option('llvm')
+dep_llvm = null_dep
+with_llvm = false
+if _llvm.allowed()
+  dep_llvm = dependency(
+    'llvm',
+    version : _llvm_version,
+    modules : llvm_modules,
+    optional_modules : llvm_optional_modules,
+    required : (
+      with_amd_vk or with_gallium_radeonsi or with_gallium_opencl or with_clc
+      or _llvm.enabled()
+    ),
+    static : not _shared_llvm,
+    fallback : ['llvm', 'dep_llvm'],
+    include_type : 'system',
+  )
+  with_llvm = dep_llvm.found()
+endif
+if with_llvm
+  ...
+elif with_amd_vk and with_aco_tests
+  error('ACO tests require LLVM, but LLVM is disabled.')
+elif with_gallium_radeonsi or with_swrast_vk
+  error('The following drivers require LLVM: RadeonSI, SWR, Lavapipe. One of these is enabled, but LLVM is disabled.')
+elif with_gallium_opencl
+  error('The OpenCL "Clover" state tracker requires LLVM, but LLVM is disabled.')
+elif with_clc
+  error('The CLC compiler requires LLVM, but LLVM is disabled.')
+else
+  draw_with_llvm = false
+endif
+```
+
+simplified to:
+```meson
+_llvm = get_option('llvm') \
+  .enable_if(with_amd_vk and with_aco_tests, error_message : 'ACO tests requires LLVM') \
+  .enable_if(with_gallium_radeonsi, error_message : 'RadeonSI requires LLVM') \
+  .enable_if(with_swrast_vk, error_message : 'Vulkan SWRAST requires LLVM') \
+  .enable_if(with_gallium_opencl, error_message : 'The OpenCL Clover state trackers requires LLVM') \
+  .enable_if(with_clc, error_message : 'CLC library requires LLVM')
+
+dep_llvm = dependency(
+  'llvm',
+  version : _llvm_version,
+  modules : llvm_modules,
+  optional_modules : llvm_optional_modules,
+  required : _llvm,
+  static : not _shared_llvm,
+  fallback : ['llvm', 'dep_llvm'],
+  include_type : 'system',
+)
+with_llvm = dep_llvm.found()
+```