What standard clause mandates this lvalue-to-rvalue conversion?

Question

Given:

int main() {
   int x = 0;
   int y = x; // <---
}

Could someone please tell me which clause of the standard (2003 preferred) man

Answer 1

3.10 Lvalues and rvalues

1 Every expression is either an lvalue or an rvalue.

2 An lvalue refers to an object or function. Some rvalue expressions—those of class or cvqualified class type—also refer to objects.47)

3 [Note: some builtin operators and function calls yield lvalues. [Example: if E is an expression of pointer type, then *E is an lvalue expression referring to the object or function to which E points. As another example, the function int& f(); yields an lvalue, so the call f() is an lvalue expression. ]

1. [Note: some builin operators expect lvalue operands. [Example: builtin assignment operators all expect their left hand operands to be lvalues. ] Other builtin operators yield rvalues, and some expect them. [Example: the unary and binary + operators expect rvalue arguments and yield rvalue results. ] The discussion of each builtin operator in clause 5 indicates whether it expects lvalue operands and whether it yieldsan lvalue. ]

5 The result of calling a function that does not return a reference is an rvalue. User defined operators are functions, and whether such operators expect or yield lvalues is determined by their parameter and return types.

6 An expression which holds a temporary object resulting from a cast to a nonreference type is an rvalue (this includes the explicit creation of an object using functional notation (5.2.3)).

7 Whenever an lvalue appears in a context where an rvalue is expected, the lvalue is converted to an rvalue; see 4.1, 4.2, and 4.3.

8 The discussion of reference initialization in 8.5.3 and of temporaries in 12.2 indicates the behavior of lvalues and rvalues in other significant contexts.

9 Class rvalues can have cvqualified types; nonclass rvalues always have cvunqualified types. Rvalues shall always have complete types or the void type; in addition to these types, lvalues can also have incomplete types.

10 An lvalue for an object is necessary in order to modify the object except that an rvalue of class type can also be used to modify its referent under certain circumstances. [Example: a member function called for an object (9.3) can modify the object. ]

11 Functions cannot be modified, but pointers to functions can be modifiable.

12 A pointer to an incomplete type can be modifiable. At some point in the program when the pointed to type is complete, the object at which the pointer points can also be modified.

13 The referent of a constqualified expression shall not be modified (through that expression), except that if it is of class type and has a mutable component, that component can be modified (7.1.5.1).

14 If an expression can be used to modify the object to which it refers, the expression is called modifiable. A program that attempts to modify an object through a nonmodifiable lvalue or rvalue expression is illformed.

15 If a program attempts to access the stored value of an object through an lvalue of other than one of the following types the behavior is undefined48): — the dynamic type of the object, — a cvqualified version of the dynamic type of the object, — a type that is the signed or unsigned type corresponding to the dynamic type of the object, — a type that is the signed or unsigned type corresponding to a cvqualified version of the dynamic type of the object, — an aggregate or union type that includes one of the aforementioned types among its members (including, recursively, a member of a subaggregate or contained union), — a type that is a (possibly cvqualified) base class type of the dynamic type of the object, — a char or unsigned char type.