Why is it legal to have white space before builtin function in awk?

Question

I am trying to understand how busybox's awk works so I'm looking into the standard and hit weird thing which I do not fully understand why is legal. Standard ( https://pubs.opengroup.org/onlinepubs/9699919799/utilities/awk.html , in section User-Defined Functions ) states that

When invoking a function, no white space can be placed between the function name and the opening parenthesis.

The grammar shown later on is prefixed with:

This formal syntax shall take precedence over the preceding text syntax description.

non_unary_expr   : '(' expr ')'
                 | '!' expr
                 ...
                 | FUNC_NAME '(' expr_list_opt ')'
                      /* no white space allowed before '(' */
                 | BUILTIN_FUNC_NAME '(' expr_list_opt ')'
                 | BUILTIN_FUNC_NAME

The grammar is completely same for both BUILTIN_FUNC_NAME and FUNC_NAME. Yet despite that, it behaves differently for user and builtin functions:

+$echo | awk -P '{ print length() 1 }'
01
+$echo | awk -P '{ print length () 1 }'
01
+$echo | awk -P 'function foo() { return 0 } ; { print foo() 1 }'
01
+$echo | awk -P 'function foo() { return 0 } ; { print foo () 1 }'
awk: cmd. line:1: error: function `foo' called with space between name and `(',
or used as a variable or an array
awk: cmd. line:1: function foo() { return 0 } ; { print foo () 1 }
awk: cmd. line:1:                                            ^ syntax error
awk: cmd. line:1: function foo() { return 0 } ; { print foo () 1 }
awk: cmd. line:1:                                              ^ syntax error

Which part of the grammar does specify this behaviour?

Answer 1

Check the definition of the FUNC_NAME in the same spec you're quoting from:

12. The token NAME shall consist of a word that is not a keyword or a name of a built-in function and is not followed immediately (without any delimiters) by the ( character.

13. The token FUNC_NAME shall consist of a word that is not a keyword or a name of a built-in function, followed immediately (without any delimiters) by the ( character. The ( character shall not be included as part of the token.

So the difference is already made in the lexer, and a word like foo will turn into a NAME, not a FUNC_NAME token when not immediatedly followed by a (.