>>500
規格の文法上不可能だから。
ISO/IEC 14882:1998 Annex A Grammar summaryより

for ( for-init-statement condition(opt) ; expression(opt) ) statement

for-init-statement:
    expression-statement
    simple-declaration
expression-statement:
    expression(opt) ;
simple-declaration:
    decl-specifier-seq(opt) init-declarator-list(opt) ;
decl-specifier-seq:
    decl-specifier-seq(opt) decl-specifier
decl-specifier:
    storage-class-specifier
    type-specifier
    function-specifier
    friend
    typedef
init-declarator-list:
    init-declarator
    init-declarator-list , init-declarator
init-declarator:
    declarator initializer(opt)