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[dcl.fct.def.replace]
9 Declarations [dcl]
9.6 Function definitions [dcl.fct.def]
9.6.5 Replaceable function definitions [dcl.fct.def.replace]
Certain functions for which a definition is supplied by the implementation are replaceable.
A C++ program may provide a definition with the signature of a replaceable function, called a replacement function.
The replacement function is used instead of the default version supplied by the implementation.
Such replacement occurs prior to program startup ([basic.def.odr], [basic.start]).
A declaration of the replacement function
shall not be inline,
shall be attached to the global module,
shall have C++ language linkage,
shall have the same return type as the replaceable function, and
if the function is declared in a standard library header, shall be such that it would be valid as a redeclaration of the declaration in that header;
no diagnostic is required.
[Note 1:
The one-definition rule ([basic.def.odr]) applies to the definitions of a replaceable function provided by the program.
The implementation-supplied function definition is an otherwise-unnamed function with no linkage.
â end note]