What C90 constraint does it violate? Both gcc and clang reject itThe "constraint" in C89/90 is simply the fact that C89/90 _requires_ an lvalue (of array type) in order to apply array to pointer conversion. Here's is the original wording:
with "-std=c90 -pedantic-errors", with an error message "ISO C90
forbids subscripting non-lvalue array", but I don't see a relevant
constraint in the C90 standard.
Except when it is the operand of the sizeof operator or the unary & operator, or is a character string literal used to initialize an array of character type, or is a wide string literal used to initialize an array with element type compatible with wchar-t, an *lvalue* that has type “array of type” is converted to an expression that has type “pointer to rype” that points to the initial element of the array object and is not an lvalue.
The presence of that "*lvalue*" requirement is what prevented up from using `[]` operator on non-lvalue arrays in C89/90, because `[]` critically relies on that conversion.
In C11 the wording has changed:
Except when it is the operand of the sizeof operator, the _Alignof operator, or the unary & operator, or is a string literal used to initialize an array, an expression that has type ‘‘array of type’’ is converted to an expression with type ‘‘pointer to type’’ that points to the initial element of the array object and is not an lvalue. If the array object has register storage class, the behavior is undefined.
Note that the "lvalue" requirement has disappeared from this wording. That is exactly why since C99 we can apply `[]` to non-lvalue arrays.
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Best regards,
Andrey
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