macro expand error

cat test_macro.c

#define TEST_MACRO(b)   chip->##b

int main(void)
{
        TEST_MACRO(yyy)

        return 0;
}

gcc -E test_macro.c

# 1 "test_macro.c"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "/usr/include/stdc-predef.h" 1 3 4
# 1 "<command-line>" 2
# 1 "test_macro.c"


int main(void)
{
test_macro.c:1:29: error: pasting "->" and "yyy" does not give a valid preprocessing token
 #define TEST_MACRO(b)   chip->##b
                             ^
test_macro.c:5:9: note: in expansion of macro ‘TEST_MACRO’
         TEST_MACRO(yyy)
         ^
        chip->yyy

        return 0;
}

What's pasting in above block ?
It is often useful to merge two tokens into one while expanding macros. This is called token pasting or token concatenation.

Reference :

3.5 Concatenation in [The C Preprocessor] for GCC version 8.1.0]

Question :

What's valid preprocessing token ?
preprocessing-token:

• head-name
• identifer
• pp-number
• character-constant
• string-literal
• punctuator
• each non-white-space character that cannot be one of the above

Error message mean that "->" is not a valid preprocessing token ?
or
"->" is not a valid preprocessing token for operator "##" ?

Solve :

cat test_macro.c

#define TEST_MACRO(b)   chip->b

int main(void)
{
        TEST_MACRO(yyy)

        return 0;
}

gcc -E test_macro.c

# 1 "test_macro.c"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "/usr/include/stdc-predef.h" 1 3 4
# 1 "<command-line>" 2
# 1 "test_macro.c"


int main(void)
{
        chip->yyy

        return 0;
}

Reference :

https://stackoverflow.com/questions/13216423/error-pasting-and-red-does-not-give-a-valid-preprocessing-token

[To be continued]