Disallow empty structs

Changelog.md

@@ -12,6 +12,7 @@ Bugfixes:
  * Commandline interface: Support ``--evm-version constantinople`` properly.
  * DocString Parser: Fix error message for empty descriptions.
  * Standard JSON: Support ``constantinople`` as ``evmVersion`` properly.
+ * Syntax Checker: Issue error for empty structs.
  * Type System: Make external library functions accessible.
 
 ### 0.4.21 (2018-03-07)

docs/grammar.txt

@@ -19,7 +19,7 @@ InheritanceSpecifier = UserDefinedTypeName ( '(' Expression ( ',' Expression )*
 StateVariableDeclaration = TypeName ( 'public' | 'internal' | 'private' | 'constant' )? Identifier ('=' Expression)? ';'
 UsingForDeclaration = 'using' Identifier 'for' ('*' | TypeName) ';'
 StructDefinition = 'struct' Identifier '{'
-                     ( VariableDeclaration ';' (VariableDeclaration ';')* )? '}'
+                     ( VariableDeclaration ';' (VariableDeclaration ';')* ) '}'
 
 ModifierDefinition = 'modifier' Identifier ParameterList? Block
 ModifierInvocation = Identifier ( '(' ExpressionList? ')' )?

libsolidity/analysis/SyntaxChecker.cpp

@@ -255,3 +255,10 @@ bool SyntaxChecker::visit(VariableDeclaration const& _declaration)
 	}
 	return true;
 }
+
+bool SyntaxChecker::visit(StructDefinition const& _struct)
+{
+	if (_struct.members().empty())
+		m_errorReporter.syntaxError(_struct.location(), "Defining empty structs is disallowed.");
+	return true;
+}

libsolidity/analysis/SyntaxChecker.h

@@ -71,6 +71,8 @@ private:
 
 	virtual bool visit(VariableDeclaration const& _declaration) override;
 
+	virtual bool visit(StructDefinition const& _struct) override;
+
 	ErrorReporter& m_errorReporter;
 
 	/// Flag that indicates whether a function modifier actually contains '_'.

test/libsolidity/SolidityNameAndTypeResolution.cpp

@@ -7279,7 +7279,7 @@ BOOST_AUTO_TEST_CASE(modifiers_access_storage_pointer)
 {
 	char const* text = R"(
 		contract C {
-			struct S { }
+			struct S { uint a; }
 			modifier m(S storage x) {
 				x;
 				_;

test/libsolidity/syntaxTests/empty_struct.sol

@@ -0,0 +1,5 @@
+contract test {
+	struct A {}
+}
+// ----
+// SyntaxError: Defining empty structs is disallowed.