vala: Restore DestroysInstance behaviour for non-compact classes

Although this attribute is not meant to be applied on methods of reference
counted classes, the original behaviour should not be changed silently.

Regression of 3d83f31

Fixes https://gitlab.gnome.org/GNOME/vala/issues/873

tests/Makefile.am

@@ -304,6 +304,7 @@ TESTS = \
 	delegates/bug792077.vala \
 	objects/chainup.vala \
 	objects/class_only.vala \
+	objects/class-destroysinstance.vala \
 	objects/classes.vala \
 	objects/classes-interfaces.vala \
 	objects/classes-interfaces-virtuals.vala \

tests/objects/class-destroysinstance.vala

@@ -0,0 +1,15 @@
+class Foo : Object {
+	[DestroysInstance]
+	public void free () {
+		assert (this.ref_count == 2);
+		this.unref ();
+	}
+}
+
+void main () {
+	var foo = new Foo ();
+	{
+		foo.free ();
+	}
+	assert (foo.ref_count == 1);
+}

vala/valamemberaccess.vala

@@ -916,7 +916,8 @@ public class Vala.MemberAccess : Expression {
 		}
 
 		if (symbol_reference is Method && ((Method) symbol_reference).get_attribute ("DestroysInstance") != null) {
-			if (ma != null) {
+			unowned Class? cl = ((Method) symbol_reference).parent_symbol as Class;
+			if (cl != null && cl.is_compact && ma != null) {
 				ma.lvalue = true;
 				ma.check_lvalue_access ();
 			}