Tests, Changelog and updated grammar

test/libsolidity/semanticTests/modifiers/function_modifier_empty.sol

@@ -0,0 +1,17 @@
+abstract contract A {
+    function f() public mod returns (bool r) {
+        return true;
+    }
+
+    modifier mod virtual;
+}
+
+
+contract C is A {
+    modifier mod override {
+        if (false) _;
+    }
+}
+
+// ----
+// f() -> false

test/libsolidity/syntaxTests/modifiers/empty_modifier_body.sol

@@ -0,0 +1,14 @@
+abstract contract A { modifier mod(uint a) virtual;}
+contract B is A { modifier mod(uint a) override { _; } }
+
+abstract contract C {
+	modifier m virtual;
+	function f() m public {
+
+	}
+}
+contract D is C {
+	modifier m override {
+		_;
+	}
+}

test/libsolidity/syntaxTests/modifiers/empty_modifier_err.sol

@@ -0,0 +1,10 @@
+contract A {modifier m virtual;}
+
+abstract contract B {modifier m virtual;}
+contract C is B { }
+
+abstract contract D {modifier m;}
+// ----
+// TypeError: (0-32): Contract "A" should be marked as abstract.
+// TypeError: (76-95): Contract "C" should be marked as abstract.
+// TypeError: (118-129): Modifiers without implementation must be marked virtual.

test/libsolidity/syntaxTests/modifiers/unimplemented_function_and_modifier.sol

@@ -0,0 +1,31 @@
+abstract contract A {
+  function foo() public virtual;
+  function foo(uint x) virtual public returns(uint);
+  modifier mod() virtual;
+}
+
+contract B is A {
+  function foo(uint x) override public returns(uint) {return x;}
+  modifier mod() override { _; }
+}
+
+contract C is A {
+  function foo() public override {}
+  modifier mod() override { _; }
+}
+
+contract D is A {
+  function foo() public override {}
+  function foo(uint x) override public returns(uint) {return x;}
+}
+
+/* No errors */
+contract E is A {
+  function foo() public override {}
+  function foo(uint x) override public returns(uint) {return x;}
+  modifier mod() override { _;}
+}
+// ----
+// TypeError: (137-254): Contract "B" should be marked as abstract.
+// TypeError: (256-344): Contract "C" should be marked as abstract.
+// TypeError: (346-466): Contract "D" should be marked as abstract.