[vala/0.46] vala: Allow to override virtual interface implementations

[Rico Tzschichholz <ricotz src gnome org>]

commit 12db9b7eeb1f53975010d89839d554b5fcf99831
Author: Rico Tzschichholz <ricotz ubuntu com>
Date:   Thu Sep 26 11:08:31 2019 +0200

    vala: Allow to override virtual interface implementations
    
    Fixes https://gitlab.gnome.org/GNOME/vala/issues/852

 codegen/valaccodemethodmodule.vala            |  2 +-
 tests/Makefile.am                             |  1 +
 tests/objects/interface-virtual-override.vala | 16 ++++++++++++++++
 vala/valamethod.vala                          |  2 +-
 4 files changed, 19 insertions(+), 2 deletions(-)
---
diff --git a/codegen/valaccodemethodmodule.vala b/codegen/valaccodemethodmodule.vala
index 4727dfec8..adcc62876 100644
--- a/codegen/valaccodemethodmodule.vala
+++ b/codegen/valaccodemethodmodule.vala
@@ -536,7 +536,7 @@ public abstract class Vala.CCodeMethodModule : CCodeStructModule {
                                        if (m.overrides || (m.base_interface_method != null && !m.is_abstract 
&& !m.is_virtual)) {
                                                Method base_method;
                                                ReferenceType base_expression_type;
-                                               if (m.overrides) {
+                                               if (m.overrides && m.base_method != null) {
                                                        base_method = m.base_method;
                                                        base_expression_type = new ObjectType ((Class) 
base_method.parent_symbol);
                                                } else {
diff --git a/tests/Makefile.am b/tests/Makefile.am
index bb374221a..f6e4b0881 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -314,6 +314,7 @@ TESTS = \
        objects/interface_only.vala \
        objects/interfaces.vala \
        objects/interface-generics.vala \
+       objects/interface-virtual-override.vala \
        objects/methods.vala \
        objects/paramspec.vala \
        objects/plugin-module-init.vala \
diff --git a/tests/objects/interface-virtual-override.vala b/tests/objects/interface-virtual-override.vala
new file mode 100644
index 000000000..62fcb6597
--- /dev/null
+++ b/tests/objects/interface-virtual-override.vala
@@ -0,0 +1,16 @@
+interface IFoo : Object {
+       public virtual int foo () {
+               assert_not_reached ();
+       }
+}
+
+class Bar : Object, IFoo {
+       public override int foo () {
+               return 42;
+       }
+}
+
+void main () {
+       var bar = new Bar ();
+       assert (bar.foo () == 42);
+}
diff --git a/vala/valamethod.vala b/vala/valamethod.vala
index 3d7518f6a..2e0c5f7d8 100644
--- a/vala/valamethod.vala
+++ b/vala/valamethod.vala
@@ -877,7 +877,7 @@ public class Vala.Method : Subroutine, Callable {
                                Report.error (source_reference, "A struct member `%s' cannot be marked as 
override, virtual, or abstract".printf (get_full_name ()));
                                return false;
                        }
-               } else if (overrides && base_method == null) {
+               } else if (overrides && base_method == null && base_interface_method == null) {
                        Report.error (source_reference, "`%s': no suitable method found to override".printf 
(get_full_name ()));
                } else if ((is_abstract || is_virtual || overrides) && access == SymbolAccessibility.PRIVATE) 
{
                        error = true;