Fixed const qualifiers on named structs/unions members that should prevent assignments to the whole structs/unions.

Added warning on ignored qualifiers on anonymous structs/unions.

test/err/bug2018.c

@@ -0,0 +1,25 @@
+/* Bug #2018 - Compiler has problems with const struct fields */
+
+struct X {
+    struct {
+        int a;
+    } a;
+    union {
+        int a;
+        const int b;
+    } b;
+};
+
+struct X f(void)
+{
+    struct X x = { 42 };
+    return x;
+}
+
+int main(void)
+{
+    struct X x = { 0 };
+    x = f();            /* Error since X is read only */
+
+    return 0;
+}

test/val/bug2018-ok.c

@@ -0,0 +1,61 @@
+/* Bug #2018 - Compiler has problems with const struct fields */
+
+#include <stdio.h>
+
+unsigned failures;
+
+struct X {
+    const struct {              /* Qualifier ignored in cc65 */
+        int a;
+    };
+    const union {               /* Qualifier ignored in cc65 */
+        int b;
+    };
+};
+
+union Y {
+    const struct {              /* Qualifier ignored in cc65 */
+        int a;
+    };
+    const union {               /* Qualifier ignored in cc65 */
+        int b;
+    };
+};
+
+struct X f(struct X a)
+{
+    struct X x = { 42 };
+    return --a.a ? a : x;
+}
+
+union Y g(union Y a)
+{
+    union Y y = { 42 };
+    return --a.a ? a : y;
+}
+
+int main(void)
+{
+    struct X x = { 1 };
+    union Y y = { 1 };
+
+    x = f(x);                   /* Allowed in cc65 since X is not read only */
+    y = g(y);                   /* Allowed in cc65 since Y is not read only */
+
+    if (x.a != 42)
+    {
+        ++failures;
+    }
+
+    if (y.a != 42)
+    {
+        ++failures;
+    }
+
+    if (failures > 0)
+    {
+        printf("failures: %u\n", failures);
+    }
+
+    return failures;
+}