Defect Report #009

Submission Date: 10 Dec 92
Submittor: WG14
Source: X3J11/90-023 (Bruce Blodgett)
Question 1
Use of typedef names in parameter declarations
A syntactic ambiguity exists in the draft proposed C standard for which there appears to be no semantic disambiguation. A sequence of examples should explain the ambiguity. This matter needs interpretation by the Committee.
For these examples, let T be declaration specifiers which contai n at least one type specifier, to satisfy the semantics from subclause 6.5.6:

If the identifier is redeclared in an inner scope ..., the type specifiers shall not be omitted in the inner declaration.

Let U be an identifier which is a typedef name at outer scope and which has not (yet) been redeclared at current scope. A caret indicates the position of each abstract declarator. Consider this declaration:
declaration-specifiers direct-declarator (T^(U));
Here U is the type of the single parameter to a function returni ng type T, due to a requirement from subclause 6.5.4.3:

In a parameter declaration, a single typedef name in parentheses is taken to be an abstract declarator that specifies a function with a single parameter, not as redundant parentheses around the identifier for a declarator.

Consider this declaration:
declaration-specifiers direct-declarator (T^(U^(parameter-type-list)));
In this example, U could be the type returned by a function which takes parameter-type-list. This in turn would be the single parameter to a function returning type T.
Alternatively, U could be a redundantly parenthesized name of a function which takes parameter-type-list and returns type T.
Given the spirit of the requirement from subclause 6.5.4.3, the former interpretation seems to be that intended by the Committee. If so, the requirement may be changed to something similar to:

In a parameter declaration, a direct declarator which redeclares a typedef name shall not be redundantly parenthesized.

Of course, parentheses must not be disallowed entirely... [The original had more, but this will suffice.]
Correction
In subclause 6.5.4.3, page 68, lines 2-4, replace:
In a parameter declaration, a single typedef name in parentheses is taken to be an abstract declarator that specifies a function with a single parameter, not as redundant parentheses around the identifier for a declarator.
with:
If, in a parameter declaration, an identifier can be treated as a typedef name or as a parameter name, it shall be taken as a typedef name.
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