force to 0 expressions which are erroneously non-constant

When an expression that needs to be constant but is, in fact,
not constant, sparse throws an error and leaves it as-is.
But some code makes the assumption that the expression is
constant and uses its value, with some random result.

One situation where this happens is in code like:
	switch (x) {
	case <some non-const expression>: ...

In this case, the linearization of the switch/case statement
will unconditionally use the value of the case expression
but the expression has no value.

One way to avoid this would be to add defensive checks each
time a value is retrieved but this is a lot of work and time
for no benefits.

So, change this by forcing the expression to be a constant
value of 0 just after the error message has been issued.

Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>

1 files changed, 5 insertions, 1 deletions

diff --git a/expand.c b/expand.c
index 623b1800..c4f806de 100644
--- a/expand.c
+++ b/expand.c
@@ -1177,8 +1177,12 @@ static void expand_const_expression(struct expression *expr, const char *where)
 {
 	if (expr) {
 		expand_expression(expr);
-		if (expr->type != EXPR_VALUE)
+		if (expr->type != EXPR_VALUE) {
 			expression_error(expr, "Expected constant expression in %s", where);
+			expr->ctype = &int_ctype;
+			expr->type = EXPR_VALUE;
+			expr->value = 0;
+		}
 	}
 }