Turns it into warning (error for 0.5.0) and adds Changelog entry.

Changelog.md

@@ -5,7 +5,7 @@ Features:
  * Type Checker: Make literals (without explicit type casting) an error for tight packing as experimental 0.5.0 feature.
 
 Bugfixes:
-
+ * Type Checker: Warn about empty tuple components (this will turn into an error with version 0.5.0).
 
 
 ### 0.4.23 (2018-04-19)

libsolidity/analysis/TypeChecker.cpp

@@ -1406,8 +1406,10 @@ bool TypeChecker::visit(TupleExpression const& _tuple)
 	}
 	else
 	{
+		bool const v050 = m_scope->sourceUnit().annotation().experimentalFeatures.count(ExperimentalFeature::V050);
 		bool isPure = true;
 		TypePointer inlineArrayType;
+
 		for (size_t i = 0; i < components.size(); ++i)
 		{
 			// Outside of an lvalue-context, the only situation where a component can be empty is (x,).
@@ -1420,7 +1422,12 @@ bool TypeChecker::visit(TupleExpression const& _tuple)
 
 				if (types[i]->category() == Type::Category::Tuple)
 					if (dynamic_cast<TupleType const&>(*types[i]).components().empty())
-						m_errorReporter.fatalTypeError(components[i]->location(), "Type of tuple component cannot be null.");
+					{
+						if (v050)
+							m_errorReporter.fatalTypeError(components[i]->location(), "Tuple component cannot be empty.");
+						else
+							m_errorReporter.warning(components[i]->location(), "Tuple component cannot be empty.");
+					}
 
 				// Note: code generation will visit each of the expression even if they are not assigned from.
 				if (types[i]->category() == Type::Category::RationalNumber && components.size() > 1)

test/libsolidity/syntaxTests/types/empty_tuple_event.sol

@@ -7,4 +7,4 @@ contract C {
 }
 // ----
 // Warning: (95-106): Invoking events without "emit" prefix is deprecated.
-// TypeError: (95-106): Type of tuple component cannot be null.
+// Warning: (95-106): Tuple component cannot be empty.

test/libsolidity/syntaxTests/types/empty_tuple_event_050.sol

@@ -7,4 +7,4 @@ contract C {
 }
 // ----
 // TypeError: (101-112): Event invocations have to be prefixed by "emit".
-// TypeError: (101-112): Type of tuple component cannot be null.
+// TypeError: (101-112): Tuple component cannot be empty.

test/libsolidity/syntaxTests/types/empty_tuple_function.sol

@@ -1,11 +1,12 @@
 pragma solidity ^0.4.3;
 contract C {
-    function f() {}
-    function a() public {
+    function f() private pure {}
+    function a() public pure {
         bool x = true;
         bool y = true;
         (x) ? (f(), y = false) : (f(), y = false);
     }
 }
 // ----
-// TypeError: (144-147): Type of tuple component cannot be null.
+// Warning: (162-165): Tuple component cannot be empty.
+// Warning: (181-184): Tuple component cannot be empty.

test/libsolidity/syntaxTests/types/empty_tuple_function_050.sol

@@ -0,0 +1,11 @@
+pragma experimental "v0.5.0";
+contract C {
+    function f() private pure {}
+    function a() public pure {
+        bool x = true;
+        bool y = true;
+        (x) ? (f(), y = false) : (f(), y = false);
+    }
+}
+// ----
+// TypeError: (168-171): Tuple component cannot be empty.

test/libsolidity/syntaxTests/types/empty_tuple_lvalue.sol

@@ -1,11 +1,13 @@
 pragma solidity ^0.4.3;
 contract C {
-    function f() public pure {}
+    function f() private pure {}
     function a() public {
         uint x;
         uint y;
         (x, y) = (f(), f());
     }
 }
 // ----
-// TypeError: (145-148): Type of tuple component cannot be null.
+// Warning: (146-149): Tuple component cannot be empty.
+// Warning: (151-154): Tuple component cannot be empty.
+// TypeError: (145-155): Type tuple(tuple(),tuple()) is not implicitly convertible to expected type tuple(uint256,uint256).

test/libsolidity/syntaxTests/types/empty_tuple_lvalue_050.sol

@@ -0,0 +1,11 @@
+pragma experimental "v0.5.0";
+contract C {
+    function f() private pure {}
+    function a() public {
+        uint x;
+        uint y;
+        (x, y) = (f(), f());
+    }
+}
+// ----
+// TypeError: (152-155): Tuple component cannot be empty.

test/libsolidity/syntaxTests/types/empty_tuple_lvalue_array.sol

@@ -0,0 +1,13 @@
+pragma solidity ^0.4.3;
+contract C {
+    function f() private pure {}
+    function a() public {
+        uint x;
+        uint y;
+        (x, y) = [f(), f()];
+    }
+}
+// ----
+// Warning: (146-149): Tuple component cannot be empty.
+// Warning: (151-154): Tuple component cannot be empty.
+// TypeError: (145-155): Type tuple()[2] memory is not implicitly convertible to expected type tuple(uint256,uint256).