Consider all grandparents in override analysis.

test/libsolidity/syntaxTests/inheritance/override/correct_choice_for_base_function.sol

@@ -0,0 +1,17 @@
+interface IBase {
+    function foo() external view;
+}
+
+contract Base is IBase {
+    function foo() public virtual view {}
+}
+
+interface IExt is IBase {}
+
+contract Ext is IExt, Base {}
+
+contract T { function foo() public virtual view {} }
+
+contract Impl is Ext, T {
+    function foo() public view override(IBase, Base, T) {}
+}

test/libsolidity/syntaxTests/inheritance/override/correct_choice_for_base_function_abstract_contract.sol

@@ -0,0 +1,17 @@
+abstract contract IBase {
+    function foo() external view virtual;
+}
+
+contract Base is IBase {
+    function foo() public virtual override view {}
+}
+
+abstract contract IExt is IBase {}
+
+contract Ext is IExt, Base {}
+
+contract T { function foo() public virtual view {} }
+
+contract Impl is Ext, T {
+    function foo() public view override(IBase, Base, T) {}
+}

test/libsolidity/syntaxTests/inheritance/override/interface_and_base_override_err.sol

@@ -0,0 +1,18 @@
+interface IBase {
+    function foo() external view;
+}
+
+contract Base is IBase {
+    function foo() public virtual view {}
+}
+
+interface IExt is IBase {}
+
+contract Ext is IExt, Base {}
+
+contract Impl is Ext {
+    function foo() public view {}
+}
+// ----
+// TypeError 9456: (211-240): Overriding function is missing "override" specifier.
+// TypeError 4327: (211-240): Function needs to specify overridden contracts "Base" and "IBase".

test/libsolidity/syntaxTests/inheritance/override/interface_and_base_override_fine.sol

@@ -0,0 +1,16 @@
+interface IBase {
+    function foo() external view;
+}
+
+contract Base is IBase {
+    function foo() public virtual view {}
+}
+
+interface IExt is IBase {}
+
+contract Ext is IExt, Base {}
+
+contract Impl is Ext {
+    function foo() public view override (IBase, Base) {}
+}
+// ----

test/libsolidity/syntaxTests/inheritance/override/override_multi_layered_error.sol

@@ -0,0 +1,22 @@
+interface IBase {
+    function foo() external view;
+}
+
+contract Base1 is IBase { function foo() public virtual view {} }
+contract Base2 is IBase { function foo() public virtual view {} }
+
+interface IExt1a is IBase {}
+interface IExt1b is IBase {}
+interface IExt2a is IBase {}
+interface IExt2b is IBase {}
+
+contract Ext1 is IExt1a, IExt1b, Base1 {}
+contract Ext2 is IExt2a, IExt2b, Base2 {}
+
+contract Impl is Ext1, Ext2 {
+    function foo() public view {}
+}
+// ----
+// TypeError 9456: (424-453): Overriding function is missing "override" specifier.
+// TypeError 9456: (424-453): Overriding function is missing "override" specifier.
+// TypeError 4327: (424-453): Function needs to specify overridden contracts "Base1", "Base2" and "IBase".

test/libsolidity/syntaxTests/inheritance/override/override_multi_layered_fine.sol

@@ -0,0 +1,18 @@
+interface IBase {
+    function foo() external view;
+}
+
+contract Base1 is IBase { function foo() public virtual view {} }
+contract Base2 is IBase { function foo() public virtual view {} }
+
+interface IExt1a is IBase {}
+interface IExt1b is IBase {}
+interface IExt2a is IBase {}
+interface IExt2b is IBase {}
+
+contract Ext1 is IExt1a, IExt1b, Base1 {}
+contract Ext2 is IExt2a, IExt2b, Base2 {}
+
+contract Impl is Ext1, Ext2 {
+    function foo() public view override (IBase, Base1, Base2) {}
+}

test/libsolidity/syntaxTests/inheritance/override/override_multi_layered_fine_abstract.sol

@@ -0,0 +1,18 @@
+interface IBase {
+    function foo() external view;
+}
+
+contract Base1 is IBase { function foo() public virtual view {} }
+contract Base2 is IBase { function foo() public virtual view {} }
+
+interface IExt1a is IBase {}
+abstract contract IExt1b is IBase {}
+abstract contract IExt2a is IBase {}
+interface IExt2b is IBase {}
+
+contract Ext1 is IExt1a, IExt1b, Base1 {}
+contract Ext2 is IExt2a, IExt2b, Base2 {}
+
+contract Impl is Ext1, Ext2 {
+    function foo() public view override (IBase, Base1, Base2) {}
+}