#define COM(x) /x

void foo () {
    COM(/) this is test
}

void foo () {
    // this is test
}

void foo () {
        / / this is test
}

void foo () {

}

2.1 Phases of translation [lex.phases]
1 The precedence among the syntax rules of translation is specified by the following phases.13)

3 The source file is decomposed into preprocessing tokens (2.4) and sequences of white-space characters
(including comments). A source file shall not end in a partial preprocessing token or partial comment14).
Each comment is replaced by one space character. New-line characters are retained. Whether
each nonempty sequence of white-space characters other than new-line is retained or replaced by one
space character is implementation-defined. The process of dividing a source file’s characters into preprocessing
tokens is context-dependent. [Example: see the handling of < within a #include preprocessing
directive. ]

4 Preprocessing directives are executed and macro invocations are expanded. If a character sequence that
matches the syntax of a universal-character-name is produced by token concatenation (16.3.3), the
behavior is undefined. A #include preprocessing directive causes the named header or source file to
be processed from phase 1 through phase 4, recursively.

#define COM    /##/
void foo () {
    COM: this is test
}

А>#define COM    /##/
А>void foo () {
А>    COM: this is test
А>}
А>

void foo () {
t.cpp:3:5: pasting "/" and "/" does not give a valid preprocessing token
    / /: this is test
}