Clang Flag: -Wc++1z-compat / -Wno-c++1z-compat

The Clang compiler offers specialized flags for enabling or disabling warnings that pertain to compatibility with certain versions of the C++ standard. This article describes two such flags, -Wc++1z-compat and -Wno-c++1z-compat, which are specifically related to features and behaviors that might affect compatibility with the C++17 standard (previously referred to as C++1z during its draft phase). The flag -Wc++1z-compat is used to activate warnings about code constructs that may be incompatible with the C++17 standard. This option can help developers identify and modify parts of the codebase that are not fully compliant with this version of the standard, or that might behave differently when compiled with a C++17-compliant compiler. Although this flag is not activated by default, it can be explicitly enabled to ensure better adherence to C++17 specifications. On the other hand, -Wno-c++1z-compat serves the opposite purpose. It is active by default and suppresses warnings related to C++17 compatibility. By using this flag, developers can intentionally ignore compatibility issues with the C++17 standard, possibly because they are targeting a different version of C++ or because they have assessed that these warnings are not relevant to their project. Deactivating these warnings can be useful in situations where the focus is on backward compatibility with earlier C++ standards or when using features from newer standards that are known to be backward compatible. Both flags are part of a broader set of diagnostic tools provided by Clang to assist developers in writing code that is portable across different versions of the C++ standard. By carefully considering when to enable or disable these warnings, developers can control the level of strictness applied to their code with respect to C++17 compatibility, thus facilitating a smoother transition between different versions of the standard and ensuring that their codebase remains robust and future-proof.

AI Generated

Subroups

• -Wc++17-compat (45 elements)

Warnings/Remarks

Default Active (Deactivate with -Wno-c++1z-compat)

warn_deprecated_register	warning: 'register' storage class specifier is deprecated and incompatible with C++17
warn_increment_bool	warning: incrementing expression of type bool is deprecated and incompatible with C++17
warn_cxx17_compat_exception_spec_in_signature	warning: mangled name of A will change in C++17 due to non-throwing exception specification in function signature

Default Inactive (Activate with -Wc++1z-compat)

warn_cxx17_compat_adl_only_template_id	warning: use of function template name with no prior function template declaration in function call with explicit template arguments is incompatible with C++ standards before C++20
warn_cxx17_compat_bitfield_member_init	warning: default member initializer for bit-field is incompatible with C++ standards before C++20
warn_cxx17_compat_capture_binding	warning: captured structured bindings are incompatible with C++ standards before C++20
warn_cxx17_compat_constexpr_body_invalid_stmt	warning: use of this statement in a constexpr function constructor is incompatible with C++ standards before C++20
warn_cxx17_compat_constexpr_ctor_missing_init	warning: constexpr constructor that does not initialize all members is incompatible with C++ standards before C++20
warn_cxx17_compat_constexpr_function_try_block	warning: function try block in constexpr function constructor is incompatible with C++ standards before C++20
warn_cxx17_compat_constexpr_local_var_no_init	warning: uninitialized variable in a constexpr function constructor is incompatible with C++ standards before C++20
warn_cxx17_compat_constexpr_union_ctor_no_init	warning: constexpr union constructor that does not initialize any member is incompatible with C++ standards before C++20
warn_cxx17_compat_constexpr_virtual	warning: virtual constexpr functions are incompatible with C++ standards before C++20
warn_cxx17_compat_decomp_decl_spec	warning: decomposition declaration declared 'B' with 'B' specifiers is incompatible with C++ standards before C++20
warn_cxx17_compat_defaulted_comparison	warning: defaulted comparison operators are incompatible with C++ standards before C++20
warn_cxx17_compat_defaulted_method_type_mismatch	warning: explicitly defaulting this default constructor copy constructor move constructor copy assignment operator move assignment operator destructor with a type different from the implicit type is incompatible with C++ standards before C++20
warn_cxx17_compat_equals_this_lambda_capture	warning: explicit capture of 'this' with a capture default of '=' is incompatible with C++ standards before C++20
warn_cxx17_compat_explicit_bool	warning: explicit(bool) is incompatible with C++ standards before C++20
warn_cxx17_compat_for_range_init_stmt	warning: range-based for loop initialization statements are incompatible with C++ standards before C++20
warn_cxx17_compat_init_capture_pack	warning: initialized lambda capture packs are incompatible with C++ standards before C++20
warn_cxx17_compat_inline_nested_namespace_definition	warning: inline nested namespace definition is incompatible with C++ standards before C++20
warn_cxx17_compat_lambda_def_ctor_assign	warning: default construction assignment of lambda is incompatible with C++ standards before C++20
warn_cxx17_compat_lambda_template_parameter_list	warning: explicit template parameter list for lambdas is incompatible with C++ standards before C++20
warn_cxx17_compat_missing_varargs_arg	warning: passing no argument for the '...' parameter of a variadic macro is incompatible with C++ standards before C++20
warn_cxx17_compat_spaceship	warning: '<=>' operator is incompatible with C++ standards before C++20
warn_cxx17_compat_template_nontype_parm_type	warning: non-type template parameter of type A is incompatible with C++ standards before C++20
warn_cxx17_compat_unicode_type	warning: 'char8_t' type specifier is incompatible with C++ standards before C++20
warn_cxx17_compat_using_decl_class_member_enumerator	warning: member using declaration naming a non-member enumerator is incompatible with C++ standards before C++20
warn_cxx17_compat_using_decl_non_member_enumerator	warning: member using declaration naming non-class 'A' enumerator is incompatible with C++ standards before C++20
warn_cxx17_compat_using_decl_scoped_enumerator	warning: using declaration naming a scoped enumerator is incompatible with C++ standards before C++20
warn_cxx17_compat_using_enum_declaration	warning: using enum declaration is incompatible with C++ standards before C++20
ext_subscript_overload	warning: overloaded A with no a defaulted more than one parameter is a C++23 extension
warn_cxx20_alias_in_init_statement	warning: alias declaration in this context is incompatible with C++ standards before C++23
warn_cxx20_compat_auto_expr	warning: 'auto' as a functional-style cast is incompatible with C++ standards before C++23
warn_cxx20_compat_consteval_if	warning: consteval if is incompatible with C++ standards before C++23
warn_cxx20_compat_constexpr_body_invalid_stmt	warning: use of this statement in a constexpr function constructor is incompatible with C++ standards before C++23
warn_cxx20_compat_constexpr_var	warning: definition of a static variable thread_local variable variable of non-literal type in a constexpr function constructor is incompatible with C++ standards before C++23
warn_cxx20_compat_decl_attrs_on_lambda	warning: an attribute specifier sequence B in this position is incompatible with C++ standards before C++23
warn_cxx20_compat_label_end_of_compound_statement	warning: label at end of compound statement is incompatible with C++ standards before C++23
warn_cxx20_compat_operator_overload_static	warning: declaring overloaded A as 'static' is incompatible with C++ standards before C++23
warn_cxx20_compat_size_t_suffix	warning: 'size_t' suffix for literals is incompatible with C++ standards before C++23
warn_cxx20_compat_static_lambda	warning: static lambdas are incompatible with C++ standards before C++23
warn_cxx23_compat_defaulted_comparison_constexpr_mismatch	warning: defaulted definition of equality three-way equality relational comparison operator three-way comparison operator that is declared constexpr consteval but   for which the corresponding implicit 'operator==' invokes a non-constexpr comparison function is incompatible with C++ standards before C++23
warn_cxx23_compat_pp_directive	warning: use of a '# elifdef elifndef ' directive is incompatible with C++ standards before C++23
warn_cxx23_compat_warning_directive	warning: #warning is incompatible with C++ standards before C++23
warn_cxx23_delimited_escape_sequence	warning: delimited named escape sequences are incompatible with C++ standards before C++23