On Thu, 8 Aug 2024, Jens Gustedt wrote:
> No, the ambiguity is there because the first ( after the keyword could
> start either a type in parenthesis or an expression, and among these a
> compound literal. If that first parenthesis would be part of the
> construct (as for the typeof or offsetof constructs) there would be no
> ambiguity a the only look ahead would be balanced parenthesis parsing.
I don't consider this ambiguity / unbounded lookahead in any problematic
sense. There are the following cases for sizeof:
* Not followed by '(': sizeof unary-expression.
* Followed by '(' then a token that does not start a type-name: sizeof
unary-expression.
* Followed by '(' then a token that does start a type-name: sizeof
(type-name) later-tokens, where if later-tokens start with '{' then it's
sizeof unary-expression and otherwise it's sizeof (type-name).
The last case is not problematic because the parsing of the type-name
doesn't depend at all on what comes after it; it's parsed exactly the same
whether it's part of sizeof (type-name) or a compound literal.
Fundamentally this is exactly the same as if a cast-expression starts with
(type-name): until the end of the type name, you don't know whether it's a
cast, or whether the cast-expression is actually a unary-expression which
is a postfix-expression which is a compound-literal. In both cases, the
parsing of a compound-literal is entered only after the initial
(type-name) has been seen, because until after the (type-name) it's not
known which construct is being parsed.
--
Joseph S. Myers
[email protected]
