[cfe-dev] A doubt about cvr-qualified array types.

[Eli Friedman]

On Sat, Oct 23, 2010 at 2:15 AM, Enea Zaffanella <zaffanella at cs.unipr.it> wrote:
> Hello.
>
> We have a doubt regarding qualified array types.
>
> Consider the following program:
> ===============================
> struct S {
>  int a[2];
> };
>
> void foo(void) {
>  const struct S* ps;
>  const int (*b)[2];
>  b = &ps->a;
> }
> ===============================
> as well as its ast dump:
> ===============================
> struct S {
>    int a[2];
> };
> void foo() (CompoundStmt 0x3c48bf8 <arraytype.c:5:16, line:9:1>
>  (DeclStmt 0x3c1be20 <line:6:3, col:21>
>    0x3c1bdd0 "const struct S *ps")
>  (DeclStmt 0x3c1bf60 <line:7:3, col:20>
>    0x3c1bf10 "const int (*b)[2]")
>  (BinaryOperator 0x3c48bc8 <line:8:3, col:12> 'const int (*)[2]' '='
>    (DeclRefExpr 0x3c1bf80 <col:3> 'const int (*)[2]' Var='b' 0x3c1bf10)
>    (UnaryOperator 0x3c48ba0 <col:7, col:12> 'int const (*)[2]' prefix '&'
>      (MemberExpr 0x3c48b30 <col:8, col:12> 'int const[2]' ->a 0x3c1bc10
>        (DeclRefExpr 0x3c1bfb8 <col:8> 'const struct S *' Var='ps'
> 0x3c1bdd0)))))
> ===============================
>
> The dump shows a mismatch between the types of the lhs and the rhs of
> the assignment expression:
>  - the lhs has type    const int (*)[2]
>  - the rhs has type    int const (*)[2]
> that is, in the rhs the const qualifier is applied to the array type,
> rather than to the array element type. The two types are anyway detected
> as being equivalent as witnessed by the absence of an implicit cast.

Per C99, those are the same type.

> We thought there was an implementation invariant whereby qualifiers of
> array types, when stored in clang::Expr nodes, were automatically pushed
> on the corresponding array element type.
> Is this NOT the case?

For canonical types, yes... for non-canonical types, no.  The two
types in question here should probably be fixed to print the same way,
though... it's confusing to have what appears to be the same type
print in different ways.

-Eli