Using the GNU Compiler Collection (GCC): C++ Dialect Options

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### 3.5 Options Controlling C++ Dialect

This section describes the command-line options that are only meaningful for C++ programs. You can also use most of the GNU compiler options regardless of what language your program is in. For example, you might compile a file `firstClass.C` like this:

    g++ -g -fstrict-enums -O -c firstClass.C

In this example, only `-fstrict-enums` is an option meant only for C++ programs; you can use the other options with any language supported by GCC.

Some options for compiling C programs, such as `-std`, are also relevant for C++ programs. See Options Controlling C Dialect.

Here is a list of options that are only for compiling C++ programs:

`-fabi-version=n`

  Use version n of the C++ ABI. The default is version 0.

  Version 0 refers to the version conforming most closely to the C++ ABI specification. Therefore, the ABI obtained using version 0 will change in different versions of G++ as ABI bugs are fixed.

  Version 1 is the version of the C++ ABI that first appeared in G++ 3.2.

  Version 2 is the version of the C++ ABI that first appeared in G++ 3.4, and was the default through G++ 4.9.

  Version 3 corrects an error in mangling a constant address as a template argument.

  Version 4, which first appeared in G++ 4.5, implements a standard mangling for vector types.

  Version 5, which first appeared in G++ 4.6, corrects the mangling of attribute const/volatile on function pointer types, decltype of a plain decl, and use of a function parameter in the declaration of another parameter.

  Version 6, which first appeared in G++ 4.7, corrects the promotion behavior of C++11 scoped enums and the mangling of template argument packs, const/static_cast, prefix ++ and –, and a class scope function used as a template argument.

  Version 7, which first appeared in G++ 4.8, that treats nullptr_t as a builtin type and corrects the mangling of lambdas in default argument scope.

  Version 8, which first appeared in G++ 4.9, corrects the substitution behavior of function types with function-cv-qualifiers.

  Version 9, which first appeared in G++ 5.2, corrects the alignment of `nullptr_t`.

  Version 10, which first appeared in G++ 6.1, adds mangling of attributes that affect type identity, such as ia32 calling convention attributes (e.g. ‘`stdcall`’).

  See also `-Wabi`.

`-fabi-compat-version=n`

  On targets that support strong aliases, G++ works around mangling changes by creating an alias with the correct mangled name when defining a symbol with an incorrect mangled name. This switch specifies which ABI version to use for the alias.

  With `-fabi-version=0` (the default), this defaults to 8 (GCC 5 compatibility). If another ABI version is explicitly selected, this defaults to 0. For compatibility with GCC versions 3.2 through 4.9, use `-fabi-compat-version=2`.

  If this option is not provided but `-Wabi=n` is, that version is used for compatibility aliases. If this option is provided along with `-Wabi` (without the version), the version from this option is used for the warning.

`-fno-access-control`

  Turn off all access checking. This switch is mainly useful for working around bugs in the access control code.

`-fcheck-new`

  Check that the pointer returned by `operator new` is non-null before attempting to modify the storage allocated. This check is normally unnecessary because the C++ standard specifies that `operator new` only returns `0` if it is declared `throw()`, in which case the compiler always checks the return value even without this option. In all other cases, when `operator new` has a non-empty exception specification, memory exhaustion is signalled by throwing `std::bad_alloc`. See also ‘`new (nothrow)`’.

`-fconcepts`

  Enable support for the C++ Extensions for Concepts Technical Specification, ISO 19217 (2015), which allows code like

      template <class T> concept bool Addable = requires (T t) { t + t; };
      template <Addable T> T add (T a, T b) { return a + b; }

`-fconstexpr-depth=n`

  Set the maximum nested evaluation depth for C++11 constexpr functions to n. A limit is needed to detect endless recursion during constant expression evaluation. The minimum specified by the standard is 512.

`-fdeduce-init-list`

  Enable deduction of a template type parameter as `std::initializer_list` from a brace-enclosed initializer list, i.e.

      template <class T> auto forward(T t) -> decltype (realfn (t))
      {
        return realfn (t);
      }

      void f()
      {
        forward({1,2}); // call forward<std::initializer_list<int>>
      }

  This deduction was implemented as a possible extension to the originally proposed semantics for the C++11 standard, but was not part of the final standard, so it is disabled by default. This option is deprecated, and may be removed in a future version of G++.

`-ffriend-injection`

  Inject friend functions into the enclosing namespace, so that they are visible outside the scope of the class in which they are declared. Friend functions were documented to work this way in the old Annotated C++ Reference Manual. However, in ISO C++ a friend function that is not declared in an enclosing scope can only be found using argument dependent lookup. GCC defaults to the standard behavior.

  This option is for compatibility, and may be removed in a future release of G++.

`-fno-elide-constructors`

  The C++ standard allows an implementation to omit creating a temporary that is only used to initialize another object of the same type. Specifying this option disables that optimization, and forces G++ to call the copy constructor in all cases.

`-fno-enforce-eh-specs`

  Don’t generate code to check for violation of exception specifications at run time. This option violates the C++ standard, but may be useful for reducing code size in production builds, much like defining `NDEBUG`. This does not give user code permission to throw exceptions in violation of the exception specifications; the compiler still optimizes based on the specifications, so throwing an unexpected exception results in undefined behavior at run time.

`-fextern-tls-init`
`-fno-extern-tls-init`

  The C++11 and OpenMP standards allow `thread_local` and `threadprivate` variables to have dynamic (runtime) initialization. To support this, any use of such a variable goes through a wrapper function that performs any necessary initialization. When the use and definition of the variable are in the same translation unit, this overhead can be optimized away, but when the use is in a different translation unit there is significant overhead even if the variable doesn’t actually need dynamic initialization. If the programmer can be sure that no use of the variable in a non-defining TU needs to trigger dynamic initialization (either because the variable is statically initialized, or a use of the variable in the defining TU will be executed before any uses in another TU), they can avoid this overhead with the `-fno-extern-tls-init` option.

  On targets that support symbol aliases, the default is `-fextern-tls-init`. On targets that do not support symbol aliases, the default is `-fno-extern-tls-init`.

`-ffor-scope`
`-fno-for-scope`

  If `-ffor-scope` is specified, the scope of variables declared in a for-init-statement is limited to the `for` loop itself, as specified by the C++ standard. If `-fno-for-scope` is specified, the scope of variables declared in a for-init-statement extends to the end of the enclosing scope, as was the case in old versions of G++, and other (traditional) implementations of C++.

  If neither flag is given, the default is to follow the standard, but to allow and give a warning for old-style code that would otherwise be invalid, or have different behavior.

`-fno-gnu-keywords`

  Do not recognize `typeof` as a keyword, so that code can use this word as an identifier. You can use the keyword `__typeof__` instead. This option is implied by the strict ISO C++ dialects: `-ansi`, `-std=c++98`, `-std=c++11`, etc.

`-fno-implicit-templates`

  Never emit code for non-inline templates that are instantiated implicitly (i.e. by use); only emit code for explicit instantiations. See Template Instantiation, for more information.

`-fno-implicit-inline-templates`

  Don’t emit code for implicit instantiations of inline templates, either. The default is to handle inlines differently so that compiles with and without optimization need the same set of explicit instantiations.

`-fno-implement-inlines`

  To save space, do not emit out-of-line copies of inline functions controlled by `#pragma implementation`. This causes linker errors if these functions are not inlined everywhere they are called.

`-fms-extensions`

  Disable Wpedantic warnings about constructs used in MFC, such as implicit int and getting a pointer to member function via non-standard syntax.

`-fno-nonansi-builtins`

  Disable built-in declarations of functions that are not mandated by ANSI/ISO C. These include `ffs`, `alloca`, `_exit`, `index`, `bzero`, `conjf`, and other related functions.

`-fnothrow-opt`

  Treat a `throw()` exception specification as if it were a `noexcept` specification to reduce or eliminate the text size overhead relative to a function with no exception specification. If the function has local variables of types with non-trivial destructors, the exception specification actually makes the function smaller because the EH cleanups for those variables can be optimized away. The semantic effect is that an exception thrown out of a function with such an exception specification results in a call to `terminate` rather than `unexpected`.

`-fno-operator-names`

  Do not treat the operator name keywords `and`, `bitand`, `bitor`, `compl`, `not`, `or` and `xor` as synonyms as keywords.

`-fno-optional-diags`

  Disable diagnostics that the standard says a compiler does not need to issue. Currently, the only such diagnostic issued by G++ is the one for a name having multiple meanings within a class.

`-fpermissive`

  Downgrade some diagnostics about nonconformant code from errors to warnings. Thus, using `-fpermissive` allows some nonconforming code to compile.

`-fno-pretty-templates`

  When an error message refers to a specialization of a function template, the compiler normally prints the signature of the template followed by the template arguments and any typedefs or typenames in the signature (e.g. `void f(T) [with T = int]` rather than `void f(int)`) so that it’s clear which template is involved. When an error message refers to a specialization of a class template, the compiler omits any template arguments that match the default template arguments for that template. If either of these behaviors make it harder to understand the error message rather than easier, you can use `-fno-pretty-templates` to disable them.

`-frepo`

  Enable automatic template instantiation at link time. This option also implies `-fno-implicit-templates`. See Template Instantiation, for more information.

`-fno-rtti`

  Disable generation of information about every class with virtual functions for use by the C++ run-time type identification features (`dynamic_cast` and `typeid`). If you don’t use those parts of the language, you can save some space by using this flag. Note that exception handling uses the same information, but G++ generates it as needed. The `dynamic_cast` operator can still be used for casts that do not require run-time type information, i.e. casts to `void *` or to unambiguous base classes.

`-fsized-deallocation`

  Enable the built-in global declarations

      void operator delete (void *, std::size_t) noexcept;
      void operator delete[] (void *, std::size_t) noexcept;

  as introduced in C++14. This is useful for user-defined replacement deallocation functions that, for example, use the size of the object to make deallocation faster. Enabled by default under `-std=c++14` and above. The flag `-Wsized-deallocation` warns about places that might want to add a definition.

`-fstrict-enums`

  Allow the compiler to optimize using the assumption that a value of enumerated type can only be one of the values of the enumeration (as defined in the C++ standard; basically, a value that can be represented in the minimum number of bits needed to represent all the enumerators). This assumption may not be valid if the program uses a cast to convert an arbitrary integer value to the enumerated type.

`-ftemplate-backtrace-limit=n`

  Set the maximum number of template instantiation notes for a single warning or error to n. The default value is 10.

`-ftemplate-depth=n`

  Set the maximum instantiation depth for template classes to n. A limit on the template instantiation depth is needed to detect endless recursions during template class instantiation. ANSI/ISO C++ conforming programs must not rely on a maximum depth greater than 17 (changed to 1024 in C++11). The default value is 900, as the compiler can run out of stack space before hitting 1024 in some situations.

`-fno-threadsafe-statics`

  Do not emit the extra code to use the routines specified in the C++ ABI for thread-safe initialization of local statics. You can use this option to reduce code size slightly in code that doesn’t need to be thread-safe.

`-fuse-cxa-atexit`

  Register destructors for objects with static storage duration with the `__cxa_atexit` function rather than the `atexit` function. This option is required for fully standards-compliant handling of static destructors, but only works if your C library supports `__cxa_atexit`.

`-fno-use-cxa-get-exception-ptr`

  Don’t use the `__cxa_get_exception_ptr` runtime routine. This causes `std::uncaught_exception` to be incorrect, but is necessary if the runtime routine is not available.

`-fvisibility-inlines-hidden`

  This switch declares that the user does not attempt to compare pointers to inline functions or methods where the addresses of the two functions are taken in different shared objects.

  The effect of this is that GCC may, effectively, mark inline methods with `__attribute__ ((visibility ("hidden")))` so that they do not appear in the export table of a DSO and do not require a PLT indirection when used within the DSO. Enabling this option can have a dramatic effect on load and link times of a DSO as it massively reduces the size of the dynamic export table when the library makes heavy use of templates.

  The behavior of this switch is not quite the same as marking the methods as hidden directly, because it does not affect static variables local to the function or cause the compiler to deduce that the function is defined in only one shared object.

  You may mark a method as having a visibility explicitly to negate the effect of the switch for that method. For example, if you do want to compare pointers to a particular inline method, you might mark it as having default visibility. Marking the enclosing class with explicit visibility has no effect.

  Explicitly instantiated inline methods are unaffected by this option as their linkage might otherwise cross a shared library boundary. See Template Instantiation.

`-fvisibility-ms-compat`

  This flag attempts to use visibility settings to make GCC’s C++ linkage model compatible with that of Microsoft Visual Studio.

  The flag makes these changes to GCC’s linkage model:

  1. It sets the default visibility to `hidden`, like `-fvisibility=hidden`.
  2. Types, but not their members, are not hidden by default.
  3. The One Definition Rule is relaxed for types without explicit visibility specifications that are defined in more than one shared object: those declarations are permitted if they are permitted when this option is not used.

  In new code it is better to use `-fvisibility=hidden` and export those classes that are intended to be externally visible. Unfortunately it is possible for code to rely, perhaps accidentally, on the Visual Studio behavior.

  Among the consequences of these changes are that static data members of the same type with the same name but defined in different shared objects are different, so changing one does not change the other; and that pointers to function members defined in different shared objects may not compare equal. When this flag is given, it is a violation of the ODR to define types with the same name differently.

`-fno-weak`

  Do not use weak symbol support, even if it is provided by the linker. By default, G++ uses weak symbols if they are available. This option exists only for testing, and should not be used by end-users; it results in inferior code and has no benefits. This option may be removed in a future release of G++.

`-nostdinc++`

  Do not search for header files in the standard directories specific to C++, but do still search the other standard directories. (This option is used when building the C++ library.)

In addition, these optimization, warning, and code generation options have meanings only for C++ programs:

`-Wabi (C, Objective-C, C++ and Objective-C++ only)`

  Warn when G++ it generates code that is probably not compatible with the vendor-neutral C++ ABI. Since G++ now defaults to updating the ABI with each major release, normally `-Wabi` will warn only if there is a check added later in a release series for an ABI issue discovered since the initial release. `-Wabi` will warn about more things if an older ABI version is selected (with `-fabi-version=n`).

  `-Wabi` can also be used with an explicit version number to warn about compatibility with a particular `-fabi-version` level, e.g. `-Wabi=2` to warn about changes relative to `-fabi-version=2`.

  If an explicit version number is provided and `-fabi-compat-version` is not specified, the version number from this option is used for compatibility aliases. If no explicit version number is provided with this option, but `-fabi-compat-version` is specified, that version number is used for ABI warnings.

  Although an effort has been made to warn about all such cases, there are probably some cases that are not warned about, even though G++ is generating incompatible code. There may also be cases where warnings are emitted even though the code that is generated is compatible.

  You should rewrite your code to avoid these warnings if you are concerned about the fact that code generated by G++ may not be binary compatible with code generated by other compilers.

  Known incompatibilities in `-fabi-version=2` (which was the default from GCC 3.4 to 4.9) include:

  - A template with a non-type template parameter of reference type was mangled incorrectly:

        extern int N;
        template <int &> struct S {};
        void n (S<N>) {2}

    This was fixed in `-fabi-version=3`.

  - SIMD vector types declared using `__attribute ((vector_size))` were mangled in a non-standard way that does not allow for overloading of functions taking vectors of different sizes.

    The mangling was changed in `-fabi-version=4`.

  - `__attribute ((const))` and `noreturn` were mangled as type qualifiers, and `decltype` of a plain declaration was folded away.

    These mangling issues were fixed in `-fabi-version=5`.

  - Scoped enumerators passed as arguments to a variadic function are promoted like unscoped enumerators, causing `va_arg` to complain. On most targets this does not actually affect the parameter passing ABI, as there is no way to pass an argument smaller than `int`.

    Also, the ABI changed the mangling of template argument packs, `const_cast`, `static_cast`, prefix increment/decrement, and a class scope function used as a template argument.

    These issues were corrected in `-fabi-version=6`.

  - Lambdas in default argument scope were mangled incorrectly, and the ABI changed the mangling of `nullptr_t`.

    These issues were corrected in `-fabi-version=7`.

  - When mangling a function type with function-cv-qualifiers, the un-qualified function type was incorrectly treated as a substitution candidate.

    This was fixed in `-fabi-version=8`, the default for GCC 5.1.

  - `decltype(nullptr)` incorrectly had an alignment of 1, leading to unaligned accesses. Note that this did not affect the ABI of a function with a `nullptr_t` parameter, as parameters have a minimum alignment.

    This was fixed in `-fabi-version=9`, the default for GCC 5.2.

  - Target-specific attributes that affect the identity of a type, such as ia32 calling conventions on a function type (stdcall, regparm, etc.), did not affect the mangled name, leading to name collisions when function pointers were used as template arguments.

    This was fixed in `-fabi-version=10`, the default for GCC 6.1.

  It also warns about psABI-related changes. The known psABI changes at this point include:

  - For SysV/x86-64, unions with `long double` members are passed in memory as specified in psABI. For example:

        union U {
          long double ld;
          int i;
        };

    `union U` is always passed in memory.

`-Wabi-tag (C++ and Objective-C++ only)`

  Warn when a type with an ABI tag is used in a context that does not have that ABI tag. See C++ Attributes for more information about ABI tags.

`-Wctor-dtor-privacy (C++ and Objective-C++ only)`

  Warn when a class seems unusable because all the constructors or destructors in that class are private, and it has neither friends nor public static member functions. Also warn if there are no non-private methods, and there’s at least one private member function that isn’t a constructor or destructor.

`-Wdelete-non-virtual-dtor (C++ and Objective-C++ only)`

  Warn when `delete` is used to destroy an instance of a class that has virtual functions and non-virtual destructor. It is unsafe to delete an instance of a derived class through a pointer to a base class if the base class does not have a virtual destructor. This warning is enabled by `-Wall`.

`-Wliteral-suffix (C++ and Objective-C++ only)`

  Warn when a string or character literal is followed by a ud-suffix which does not begin with an underscore. As a conforming extension, GCC treats such suffixes as separate preprocessing tokens in order to maintain backwards compatibility with code that uses formatting macros from `<inttypes.h>`. For example:

      #define __STDC_FORMAT_MACROS
      #include <inttypes.h>
      #include <stdio.h>

      int main() {
        int64_t i64 = 123;
        printf("My int64: %" PRId64"\n", i64);
      }

  In this case, `PRId64` is treated as a separate preprocessing token.

  This warning is enabled by default.

`-Wlto-type-mismatch`

  During the link-time optimization warn about type mismatches in global declarations from different compilation units. Requires `-flto` to be enabled. Enabled by default.

`-Wnarrowing (C++ and Objective-C++ only)`

  With `-std=gnu++98` or `-std=c++98`, warn when a narrowing conversion prohibited by C++11 occurs within ‘`{ }`’, e.g.

      int i = { 2.2 }; // error: narrowing from double to int

  This flag is included in `-Wall` and `-Wc++11-compat`.

  When a later standard is in effect, e.g. when using `-std=c++11`, narrowing conversions are diagnosed by default, as required by the standard. A narrowing conversion from a constant produces an error, and a narrowing conversion from a non-constant produces a warning, but `-Wno-narrowing` suppresses the diagnostic. Note that this does not affect the meaning of well-formed code; narrowing conversions are still considered ill-formed in SFINAE contexts.

`-Wnoexcept (C++ and Objective-C++ only)`

  Warn when a noexcept-expression evaluates to false because of a call to a function that does not have a non-throwing exception specification (i.e. `throw()` or `noexcept`) but is known by the compiler to never throw an exception.

`-Wnon-virtual-dtor (C++ and Objective-C++ only)`

  Warn when a class has virtual functions and an accessible non-virtual destructor itself or in an accessible polymorphic base class, in which case it is possible but unsafe to delete an instance of a derived class through a pointer to the class itself or base class. This warning is automatically enabled if `-Weffc++` is specified.

`-Wreorder (C++ and Objective-C++ only)`

  Warn when the order of member initializers given in the code does not match the order in which they must be executed. For instance:

      struct A {
        int i;
        int j;
        A(): j (0), i (1) { }
      };

  The compiler rearranges the member initializers for `i` and `j` to match the declaration order of the members, emitting a warning to that effect. This warning is enabled by `-Wall`.

`-fext-numeric-literals (C++ and Objective-C++ only)`

  Accept imaginary, fixed-point, or machine-defined literal number suffixes as GNU extensions. When this option is turned off these suffixes are treated as C++11 user-defined literal numeric suffixes. This is on by default for all pre-C++11 dialects and all GNU dialects: `-std=c++98`, `-std=gnu++98`, `-std=gnu++11`, `-std=gnu++14`. This option is off by default for ISO C++11 onwards (`-std=c++11`, ...).

The following `-W…` options are not affected by `-Wall`.

`-Weffc++ (C++ and Objective-C++ only)`

  Warn about violations of the following style guidelines from Scott Meyers’ Effective C++ series of books:

  - Define a copy constructor and an assignment operator for classes with dynamically-allocated memory.
  - Prefer initialization to assignment in constructors.
  - Have `operator=` return a reference to `*this`.
  - Don’t try to return a reference when you must return an object.
  - Distinguish between prefix and postfix forms of increment and decrement operators.
  - Never overload `&&`, `||`, or `,`.

  This option also enables `-Wnon-virtual-dtor`, which is also one of the effective C++ recommendations. However, the check is extended to warn about the lack of virtual destructor in accessible non-polymorphic bases classes too.

  When selecting this option, be aware that the standard library headers do not obey all of these guidelines; use ‘`grep -v`’ to filter out those warnings.

`-Wstrict-null-sentinel (C++ and Objective-C++ only)`

  Warn about the use of an uncasted `NULL` as sentinel. When compiling only with GCC this is a valid sentinel, as `NULL` is defined to `__null`. Although it is a null pointer constant rather than a null pointer, it is guaranteed to be of the same size as a pointer. But this use is not portable across different compilers.

`-Wno-non-template-friend (C++ and Objective-C++ only)`

  Disable warnings when non-templatized friend functions are declared within a template. Since the advent of explicit template specification support in G++, if the name of the friend is an unqualified-id (i.e., ‘`friend foo(int)`’), the C++ language specification demands that the friend declare or define an ordinary, nontemplate function. (Section 14.5.3). Before G++ implemented explicit specification, unqualified-ids could be interpreted as a particular specialization of a templatized function. Because this non-conforming behavior is no longer the default behavior for G++, `-Wnon-template-friend` allows the compiler to check existing code for potential trouble spots and is on by default. This new compiler behavior can be turned off with `-Wno-non-template-friend`, which keeps the conformant compiler code but disables the helpful warning.

`-Wold-style-cast (C++ and Objective-C++ only)`

  Warn if an old-style (C-style) cast to a non-void type is used within a C++ program. The new-style casts (`dynamic_cast`, `static_cast`, `reinterpret_cast`, and `const_cast`) are less vulnerable to unintended effects and much easier to search for.

`-Woverloaded-virtual (C++ and Objective-C++ only)`

  Warn when a function declaration hides virtual functions from a base class. For example, in:

      struct A {
        virtual void f();
      };

      struct B: public A {
        void f(int);
      };

  the `A` class version of `f` is hidden in `B`, and code like:

      B* b;
      b->f();

  fails to compile.

`-Wno-pmf-conversions (C++ and Objective-C++ only)`

  Disable the diagnostic for converting a bound pointer to member function to a plain pointer.

`-Wsign-promo (C++ and Objective-C++ only)`

  Warn when overload resolution chooses a promotion from unsigned or enumerated type to a signed type, over a conversion to an unsigned type of the same size. Previous versions of G++ tried to preserve unsignedness, but the standard mandates the current behavior.

`-Wtemplates (C++ and Objective-C++ only)`

  Warn when a primary template declaration is encountered. Some coding rules disallow templates, and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so one can still use the STL. One may also instantiate or specialize templates.

`-Wmultiple-inheritance (C++ and Objective-C++ only)`

  Warn when a class is defined with multiple direct base classes. Some coding rules disallow multiple inheritance, and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so one can still use the STL. One may also define classes that indirectly use multiple inheritance.

`-Wvirtual-inheritance`

  Warn when a class is defined with a virtual direct base classe. Some coding rules disallow multiple inheritance, and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so one can still use the STL. One may also define classes that indirectly use virtual inheritance.

`-Wnamespaces`

  Warn when a namespace definition is opened. Some coding rules disallow namespaces, and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so one can still use the STL. One may also use using directives and qualified names.

`-Wno-terminate (C++ and Objective-C++ only)`

  Disable the warning about a throw-expression that will immediately result in a call to `terminate`.

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