Re: [C PATCH] Don't accept a cast as a valid LHS (PR c/66341)

[Marek Polacek <polacek at redhat dot com>]

On Wed, Jun 03, 2015 at 06:02:12PM +0000, Joseph Myers wrote:
> I'm not clear on what trees you get in the problem case (or which of the 
> tests in the testcase you added was broken before - they don't seem that 
> similar to the tests in the bug).  It would seem safer to me to use 
> lvalue_p (value) as the condition for adding a NON_VALUE_EXPR.

E.g.
int *p;
(int *) (char *) p = 0;
wasn't rejected before.  In this case build_c_cast tried to cast
"(char *) p", which is a NOP_EXPR, to "(int *)".  There's this

  value = convert (type, value);

which trims the NOP_EXPR and we get "p".  Then there's

  if (value == expr)
    value = non_lvalue_loc (loc, value);

but that's false since it's comparing "p" and "(char *) p", so we don't
wrap the "p" in NON_VALUE_EXPR.  Hopefully this makes the issue a tad
clearer.

Using lvalue_p is fine as well, so adjusted.

> That's not a cast to a qualified type.  A cast to qualified type would be 
> (int *const) p.

Duh, adjusted.  Thanks.

Bootstrapped/regtested on x86_64-linux pending, ok if it passes?

2015-06-03  Marek Polacek  <polacek@redhat.com>

	PR c/66341
	* c-typeck.c (build_c_cast): Wrap VALUE into NON_LVALUE_EXPR if
	it is a lvalue.

	* gcc.dg/lvalue-8.c: New test.

diff --git gcc/c/c-typeck.c gcc/c/c-typeck.c
index f55d4c6..6b313f3 100644
--- gcc/c/c-typeck.c
+++ gcc/c/c-typeck.c
@@ -5195,7 +5195,7 @@ build_c_cast (location_t loc, tree type, tree expr)
     }
 
   /* Don't let a cast be an lvalue.  */
-  if (value == expr)
+  if (lvalue_p (value))
     value = non_lvalue_loc (loc, value);
 
   /* Don't allow the results of casting to floating-point or complex
diff --git gcc/testsuite/gcc.dg/lvalue-8.c gcc/testsuite/gcc.dg/lvalue-8.c
index e69de29..04eeb71 100644
--- gcc/testsuite/gcc.dg/lvalue-8.c
+++ gcc/testsuite/gcc.dg/lvalue-8.c
@@ -0,0 +1,19 @@
+/* PR c/66341 */
+/* { dg-do compile } */
+
+void
+foo (int *p)
+{
+  p = 0;
+  /* A cast does not yield an lvalue.  */
+  (int *) p = 0; /* { dg-error "lvalue required as left operand of assignment" } */
+  /* A cast to a qualified type has the same effect as a cast
+     to the unqualified version of the type.  */
+  (int *const) p = 0; /* { dg-error "lvalue required as left operand of assignment" } */
+  (int *) (char *) p = 0; /* { dg-error "lvalue required as left operand of assignment" } */
+  (int *) (char *) (int *) p = 0; /* { dg-error "lvalue required as left operand of assignment" } */
+  (int *) (char *) (int *) (char *) p = 0; /* { dg-error "lvalue required as left operand of assignment" } */
+  (int *) (double *) p = 0; /* { dg-error "lvalue required as left operand of assignment" } */
+  (int *) (int *) p = 0; /* { dg-error "lvalue required as left operand of assignment" } */
+  (int *) (int *const) p = 0; /* { dg-error "lvalue required as left operand of assignment" } */
+}


	Marek