Sizeof should fail on incomplete types

And so we check for it when type-checking the `sizeof` expression.

Note: previously, the `size-of-expr` function would return zero (sum of the
struct components sizes).

Author: petr-bauch

regression/cbmc/incomplete-sizeof/main.c

@@ -0,0 +1,11 @@
+#include <assert.h>
+#include <stdlib.h>
+
+struct foo;
+
+int main(int argc, char **argv)
+{
+  struct foo *thefoo = malloc(sizeof(struct foo));
+  size_t s = sizeof(struct foo);
+  assert(s == 0);
+}

regression/cbmc/incomplete-sizeof/test.desc

@@ -0,0 +1,9 @@
+CORE
+main.c
+
+^EXIT=6$
+^SIGNAL=0$
+^file main.c line 8 function main: calling sizeof for incomplete type: struct foo$
+^CONVERSION ERROR$
+--
+^warning: ignoring

src/ansi-c/c_typecheck_expr.cpp

@@ -951,6 +951,16 @@ void c_typecheck_baset::typecheck_expr_sizeof(exprt &expr)
   }
   else
   {
+    if(
+      type.id() == ID_struct_tag &&
+      to_struct_type(follow_tag(to_struct_tag_type(type))).is_incomplete())
+    {
+      error().source_location = expr.source_location();
+      error() << "calling sizeof for incomplete type: " << to_string(type)
+              << eom;
+      throw 0;
+    }
+
     auto size_of_opt = size_of_expr(type, *this);
 
     if(!size_of_opt.has_value())