[Bug c/112556] New: Null pointer constants with enumeration type are not accepted

[luigighiron at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112556

            Bug ID: 112556
           Summary: Null pointer constants with enumeration type are not
                    accepted
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: luigighiron at gmail dot com
  Target Milestone: ---

The following code is not accepted in GCC:

void*p=(enum{E})E;

To make it clear why this code should be accepted, I have
provided some references to the current C standard.

> An integer constant expression with the value 0,
> or such an expression cast to type void *, is called
> a null pointer constant. If a null pointer constant is
> converted to a pointer type, the resulting pointer,
> called a null pointer, is guaranteed to compare unequal
> to a pointer to any object or function.
Section 6.3.2.3 "Pointers" Paragraph 1 ISO/IEC 9899:2018

> An integer constant expression shall have integer type
> and shall only have operands that are integer constants,
> enumeration constants, character constants, sizeof
> expressions whose results are integer constants,
> _Alignof expressions, and floating constants that are
> the immediate operands of casts. Cast operators in an
> integer constant expression shall only convert arithmetic
> types to integer types, except as part of an operand
> to the sizeof or _Alignof operator.
Section 6.6 "Constant expressions" Paragraph 6 ISO/IEC 9899:2018

> The type char, the signed and unsigned integer types,
> and the enumerated types are collectively called
> integer types. The integer and real floating types are
> collectively called real types.
> 
> Integer and floating types are collectively called
> arithmetic types. Each arithmetic type belongs to
> one type domain: the real type domain comprises the
> real types, the complex type domain comprises the
> complex types.
Section 6.2.5 "Types" Paragraphs 17 and 18 ISO/IEC 9899:2018

(enum{E})E is an integer constant expression because
E is an enumeration constant, and the cast is converting
an arithmetic type (int) to an integer type (the enumeration
type). This integer constant expression has the value zero,
so it is a null pointer constant.

This problem seems to be specific to null pointer constants,
for example the type int(*)[(enum{A,B})B] is not considered
to be a variable length array type (which is correct).