[C PATCH] Fix for some variably modified types not being recognized [PR114831]

[Martin Uecker <uecker@tugraz.at>]

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  <uecker@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();
+}
+