Conflicting declarations

Unless otherwise specified, two declarations cannot (re)introduce the same entity. The program is ill-formed if such declarations exist.

Corresponding declarations

Two declarations correspond if they (re)introduce the same name, both declare constructors, or both declare destructors, unless

• either is a using declaration,
• one declares a type (not a typedef name) and the other declares a variable, non-static data member other than of an anonymous union, enumerator, function, or function template, or
• each declares a function or function template and they do not declare corresponding overloads (see below).

Two function declarations declare corresponding overloads if both declare functions satisfying all following conditions:

• They have the same parameter-type-list, omitting the types of explicit object parameters(since C++23).

• If both of them are non-static member functions, they need to additionally satisfy one of the following requirements:

• Their object parameters have the same type.

Two function template declarations declare corresponding overloads if both declare function templates satisfying all following conditions:

• Their template parameter lists have the same length.
• Their corresponding template parameters are equivalent.
• They have equivalent parameter-type-lists, omitting the types of explicit object parameters(since C++23).
• They have equivalent return types.

• If both are non-static members function templates, they need to additionally satisfy one of the following requirements:

• Their object parameters have equivalent types.

struct A
{
    friend void c();   // #1
};
 
struct B
{
    friend void c() {} // corresponds to, and defines, #1
};
 
typedef int Int;
 
enum E : int { a };
 
void f(int);   // #2
void f(Int) {} // defines #2
void f(E) {}   // OK, another overload
 
struct X
{
    static void f();
    void f() const;   // error: redeclaration
 
    void g();
    void g() const;   // OK
    void g() &;       // error: redeclaration
 
    void h(this X&, int);
    void h(int) &&;   // OK, another overload
 
    void j(this const X&);
    void j() const &; // error: redeclaration
 
    void k();
    void k(this X&);  // error: redeclaration
};

Potentially-conflict declarations

Unless otherwise specified, two declarations of entities declare the same entity if all following conditions are satisfied, considering declarations of unnamed types to introduce their typedef names and enumeration names for linkage purposes (if any exists):

• They correspond.
• They have the same target scope, which is not a function parameter scope or a template parameter scope.

• One of the following conditions is satisfied:
• They appear in the same translation unit.

• They both declare names with external linkage.

Two declarations potentially conflict if they correspond and cause their shared name to denote different entities. The program is ill-formed if, in any scope, a name is bound to two declarations A and B that potentially conflict and A precedes B, unless B is name-independent.(since C++26)

void f()
{
    int x, y;
    void x(); // error: different entity for x
    int y;    // error: redefinition
}
 
enum { f };   // error: different entity for ::f
 
namespace A {}
namespace B = A;
namespace B = A; // OK, no effect
namespace B = B; // OK, no effect
namespace A = B; // OK, no effect
namespace B {} // error: different entity for B
 
void g()
{
    int _;
    _ = 0; // OK
    int _; // OK since C++26, name-independent declaration
    _ = 0; // error: two non-function declarations in the lookup set
}
 
void h ()
{
    int _;        // #1
    _ ++;         // OK
    static int _; // error: conflicts with #1 because
                  // static variables are not name-independent
}