C++ PATCH for c++/91548 - fix detecting modifying const objects for ARRAY_REF

[Jason Merrill]

On 10/23/19 2:10 PM, Marek Polacek wrote:
> This fixes a bogus "modifying a const object" error for an array that actually
> isn't declared const.  The problem was how I handled ARRAY_REFs here; we
> shouldn't look at the ARRAY_REF itself, but at the array its accessing.
> 
> Bootstrapped/regtested on x86_64-linux, ok for trunk?

OK.

> 2019-10-23  Marek Polacek  <polacek@redhat.com>
> 
> 	PR c++/91548 - fix detecting modifying const objects for ARRAY_REF.
> 	* constexpr.c (cxx_eval_store_expression): Don't call
> 	modifying_const_object_p for ARRAY_REF.
> 
> 	* g++.dg/cpp1y/constexpr-tracking-const15.C: New test.
> 	* g++.dg/cpp1y/constexpr-tracking-const16.C: New test.
> 	* g++.dg/cpp1z/constexpr-tracking-const1.C: New test.
> 
> diff --git gcc/cp/constexpr.c gcc/cp/constexpr.c
> index 11a1eaa0e82..6b4e854e35c 100644
> --- gcc/cp/constexpr.c
> +++ gcc/cp/constexpr.c
> @@ -3910,10 +3910,6 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
>   	    tree elt = TREE_OPERAND (probe, 1);
>   	    if (TREE_CODE (elt) == FIELD_DECL && DECL_MUTABLE_P (elt))
>   	      mutable_p = true;
> -	    if (evaluated
> -		&& modifying_const_object_p (TREE_CODE (t), probe, mutable_p)
> -		&& const_object_being_modified == NULL_TREE)
> -	      const_object_being_modified = probe;
>   	    if (TREE_CODE (probe) == ARRAY_REF)
>   	      {
>   		elt = eval_and_check_array_index (ctx, probe, false,
> @@ -3921,6 +3917,15 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
>   		if (*non_constant_p)
>   		  return t;
>   	      }
> +	    /* We don't check modifying_const_object_p for ARRAY_REFs.  Given
> +	       "int a[10]", an ARRAY_REF "a[2]" can be "const int", even though
> +	       the array isn't const.  Instead, check "a" in the next iteration;
> +	       that will detect modifying "const int a[10]".  */
> +	    else if (evaluated
> +		     && modifying_const_object_p (TREE_CODE (t), probe,
> +						  mutable_p)
> +		     && const_object_being_modified == NULL_TREE)
> +	      const_object_being_modified = probe;
>   	    vec_safe_push (refs, elt);
>   	    vec_safe_push (refs, TREE_TYPE (probe));
>   	    probe = ob;
> diff --git gcc/testsuite/g++.dg/cpp1y/constexpr-tracking-const15.C gcc/testsuite/g++.dg/cpp1y/constexpr-tracking-const15.C
> new file mode 100644
> index 00000000000..db1b2bb7ea6
> --- /dev/null
> +++ gcc/testsuite/g++.dg/cpp1y/constexpr-tracking-const15.C
> @@ -0,0 +1,21 @@
> +// PR c++/91548 - fix detecting modifying const objects for ARRAY_REF.
> +// { dg-do compile { target c++14 } }
> +
> +constexpr int& impl(const int (&array)[10], int index) {
> +  return const_cast<int&>(array[index]);
> +}
> +
> +struct A {
> +  constexpr int& operator[](int i) { return impl(elems, i); }
> +  int elems[10];
> +};
> +
> +constexpr bool
> +f()
> +{
> +  A arr = {};
> +  arr[2] = true;
> +  return false;
> +}
> +
> +constexpr bool b = f();
> diff --git gcc/testsuite/g++.dg/cpp1y/constexpr-tracking-const16.C gcc/testsuite/g++.dg/cpp1y/constexpr-tracking-const16.C
> new file mode 100644
> index 00000000000..5a5b92bc8cc
> --- /dev/null
> +++ gcc/testsuite/g++.dg/cpp1y/constexpr-tracking-const16.C
> @@ -0,0 +1,22 @@
> +// PR c++/91548 - fix detecting modifying const objects for ARRAY_REF.
> +// { dg-do compile { target c++14 } }
> +
> +constexpr int& impl(const int (&array)[10], int index) {
> +  return const_cast<int&>(array[index]);
> +}
> +
> +struct A {
> +  constexpr int& operator[](int i) { return impl(elems, i); }
> +  const int elems[10];
> +};
> +
> +constexpr bool
> +f()
> +{
> +  A arr = {};
> +  arr[2] = 1; // { dg-error "modifying a const object" }
> +  return false;
> +}
> +
> +constexpr bool b = f(); // { dg-message "in .constexpr. expansion of " }
> +// { dg-message "originally declared" "" { target *-*-* } .-1 }
> diff --git gcc/testsuite/g++.dg/cpp1z/constexpr-tracking-const1.C gcc/testsuite/g++.dg/cpp1z/constexpr-tracking-const1.C
> new file mode 100644
> index 00000000000..a3856b8e7ec
> --- /dev/null
> +++ gcc/testsuite/g++.dg/cpp1z/constexpr-tracking-const1.C
> @@ -0,0 +1,25 @@
> +// PR c++/91548 - fix detecting modifying const objects for ARRAY_REF.
> +// { dg-do compile { target c++17 } }
> +
> +using size_t = decltype(sizeof(0));
> +
> +template <typename T, size_t N>
> +constexpr T& impl(T const (&array)[N], size_t index) {
> +    return const_cast<T&>(array[index]);
> +}
> +
> +template <typename T, size_t N>
> +struct my_array {
> +    constexpr T& operator[](size_t i) { return impl(elems, i); }
> +    constexpr T const& operator[](size_t i) const { return elems[i]; }
> +    T elems[N];
> +};
> +
> +bool f(int i) {
> +    static constexpr auto table = []() {
> +        my_array<bool, 256> arr = {};
> +        arr[2] = true;
> +        return arr;
> +    }();
> +    return table[i];
> +}
>