(correct email)
> We did not propagate C_TYPE_VARIABLY_MODIFIED to pointers in all
> cases. I added this directly in two places, but maybe we should
> check all cases of build_pointer_type or integrate this into
> c_build_pointer_type and use this everywhere (but I do not fully
> understand the pointer mode logic there).
>
>
> Bootstrapped and regession tested on x86_64.
>
>
> c: Fix for some variably modified types not being recognized [PR114831]
>
> We did not evaluate expressions with variably modified types correctly
> in typeof and did not produce warnings when jumping over declarations
> using typeof. After addressof or array-to-pointer decay we construct
> new pointer types that have to be marked variably modified if the pointer
> target is variably modified.
>
> 2024-05-18 Martin Uecker <uec...@tugraz.at>
>
> PR c/114831
> gcc/c/
> * c-typeck.cc (array_to_pointer_conversion, build_unary_op):
> Propagate flag to pointer target.
>
> gcc/testsuite/
> * gcc.dg/pr114831-1.c: New test.
> * gcc.dg/pr114831-2.c: New test.
> * gcc.dg/gnu23-varmod-1.c: New test.
> * gcc.dg/gnu23-varmod-2.c: New test.
>
> diff --git a/gcc/c/c-typeck.cc b/gcc/c/c-typeck.cc
> index 7ecca9f58c6..2d092357e0f 100644
> --- a/gcc/c/c-typeck.cc
> +++ b/gcc/c/c-typeck.cc
> @@ -1891,8 +1891,12 @@ array_to_pointer_conversion (location_t loc, tree exp)
>
> copy_warning (exp, orig_exp);
>
> + bool varmod = C_TYPE_VARIABLY_MODIFIED (restype);
> +
> ptrtype = build_pointer_type (restype);
>
> + C_TYPE_VARIABLY_MODIFIED (ptrtype) = varmod;
> +
> if (INDIRECT_REF_P (exp))
> return convert (ptrtype, TREE_OPERAND (exp, 0));
>
> @@ -4630,6 +4634,7 @@ build_unary_op (location_t location, enum tree_code
> code, tree xarg,
> tree eptype = NULL_TREE;
> const char *invalid_op_diag;
> bool int_operands;
> + bool varmod;
>
> int_operands = EXPR_INT_CONST_OPERANDS (xarg);
> if (int_operands)
> @@ -5113,8 +5118,12 @@ build_unary_op (location_t location, enum tree_code
> code, tree xarg,
> gcc_assert (TREE_CODE (arg) != COMPONENT_REF
> || !DECL_C_BIT_FIELD (TREE_OPERAND (arg, 1)));
>
> + varmod = C_TYPE_VARIABLY_MODIFIED (argtype);
> +
> argtype = build_pointer_type (argtype);
>
> + C_TYPE_VARIABLY_MODIFIED (argtype) = varmod;
> +
> /* ??? Cope with user tricks that amount to offsetof. Delete this
> when we have proper support for integer constant expressions. */
> val = get_base_address (arg);
> diff --git a/gcc/testsuite/gcc.dg/gnu23-varmod-1.c
> b/gcc/testsuite/gcc.dg/gnu23-varmod-1.c
> new file mode 100644
> index 00000000000..add10d13573
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/gnu23-varmod-1.c
> @@ -0,0 +1,12 @@
> +/* { dg-do compile }
> + * { dg-options "-std=gnu23" } */
> +
> +int foo(int n)
> +{
> + int (*a(void))[n] { return 0; };
> + goto err; /* { dg-error "jump into scope" "variably modified" } */
> + typeof((n++,a)) b2;
> +err:
> + return n;
> +}
> +
> diff --git a/gcc/testsuite/gcc.dg/gnu23-varmod-2.c
> b/gcc/testsuite/gcc.dg/gnu23-varmod-2.c
> new file mode 100644
> index 00000000000..c36af1d1647
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/gnu23-varmod-2.c
> @@ -0,0 +1,16 @@
> +/* { dg-do run }
> + * { dg-options "-std=gnu23" } */
> +
> +int foo(int n)
> +{
> + int (*a(void))[n] { return 0; };
> + typeof((n++,a)) b2;
> + return n;
> +}
> +
> +int main()
> +{
> + if (2 != foo(1))
> + __builtin_abort();
> +}
> +
> diff --git a/gcc/testsuite/gcc.dg/pr114831-1.c
> b/gcc/testsuite/gcc.dg/pr114831-1.c
> new file mode 100644
> index 00000000000..ed30a494b3c
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/pr114831-1.c
> @@ -0,0 +1,27 @@
> +/* { dg-do compile }
> + * { dg-options "-std=c23" } */
> +
> +void f(int n)
> +{
> + int a[n];
> + goto foo; /* { dg-error "jump into scope" "variably modified" } */
> + typeof(a) b1;
> +foo:
> +}
> +
> +void g(int n)
> +{
> + int a2[1][n];
> + goto foo; /* { dg-error "jump into scope" "variably modified" } */
> + typeof((n++,a2)) b2;
> +foo:
> +}
> +
> +void h(int n)
> +{
> + int a[n];
> + typeof(a) b1;
> + goto foo; /* { dg-error "jump into scope" "variably modified" } */
> + typeof(&b1) b;
> +foo:
> +}
> diff --git a/gcc/testsuite/gcc.dg/pr114831-2.c
> b/gcc/testsuite/gcc.dg/pr114831-2.c
> new file mode 100644
> index 00000000000..ecfd87988c2
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/pr114831-2.c
> @@ -0,0 +1,16 @@
> +/* { dg-do run }
> + * { dg-options "-std=c23" } */
> +
> +int foo(int n)
> +{
> + int a[1][n];
> + typeof((n++,a)) b2;
> + return n;
> +}
> +
> +int main()
> +{
> + if (2 != foo(1))
> + __builtin_abort();
> +}
> +
>
