Disallow usage of the indexed attribute for modifier parameters Added tests for edge cases of several keywords

Changelog.md

@@ -23,6 +23,7 @@ Compiler Features:
 
 Bugfixes:
  * Parser: Disallow several ``indexed`` attributes for the same event parameter.
+ * Parser: Disallow usage of the ``indexed`` attribute for modifier parameters.
  * Yul Optimizer: Hash hex and decimal literals according to their value instead of their representation, improving the detection of equivalent functions.
  * Solidity Upgrade Tool ``solidity-upgrade``: Fix the tool returning success code on uncaught exceptions.
  * SMTChecker: Fix display error for negative integers that are one more than powers of two.

libsolidity/parsing/Parser.cpp

@@ -879,7 +879,6 @@ ASTPointer<ModifierDefinition> Parser::parseModifierDefinition()
 	if (m_scanner->currentToken() == Token::LParen)
 	{
 		VarDeclParserOptions options;
-		options.allowIndexed = true;
 		options.allowLocationSpecifier = true;
 		parameters = parseParameterList(options);
 	}

test/libsolidity/syntaxTests/modifiers/invalid_parameter_indexed.sol

@@ -0,0 +1,5 @@
+contract B {
+    modifier mod1(uint indexed a) { _; }
+}
+// ----
+// ParserError 2314: (36-43): Expected ',' but got 'indexed'