Change error from directive to keyword.

docs/090-breaking-changes.rst

@@ -0,0 +1,28 @@
+********************************
+Solidity v0.8.0 Breaking Changes
+********************************
+
+This section highlights the main breaking changes introduced in Solidity
+version 0.9.0.
+For the full list check
+`the release changelog <https://github.com/ethereum/solidity/releases/tag/v0.9.0>`_.
+
+Silent Changes of the Semantics
+===============================
+
+...
+
+New Restrictions
+================
+
+- `error` is now a keyword and cannot be used as identifier anymore.
+
+Interface Changes
+=================
+
+...
+
+How to update your code
+=======================
+
+...

docs/grammar/SolidityLexer.g4

@@ -29,7 +29,7 @@ Do: 'do';
 Else: 'else';
 Emit: 'emit';
 Enum: 'enum';
-Error: 'error'; // not a real keyword
+Error: 'error';
 Revert: 'revert'; // not a real keyword
 Event: 'event';
 External: 'external';

docs/grammar/SolidityParser.g4

@@ -13,6 +13,7 @@ sourceUnit: (
 	pragmaDirective
 	| importDirective
 	| contractDefinition
+	| errorDefinition
 	| interfaceDefinition
 	| libraryDefinition
 	| functionDefinition
@@ -379,9 +380,9 @@ tupleExpression: LParen (expression? ( Comma expression?)* ) RParen;
 inlineArrayExpression: LBrack (expression ( Comma expression)* ) RBrack;
 
 /**
- * Besides regular non-keyword Identifiers, some keywords like 'from' and 'error' can also be used as identifiers.
+ * Besides regular non-keyword Identifiers, some keywords like 'from' can also be used as identifiers.
  */
-identifier: Identifier | From | Error | Revert;
+identifier: Identifier | From | Revert;
 
 literal: stringLiteral | numberLiteral | booleanLiteral | hexStringLiteral | unicodeStringLiteral;
 booleanLiteral: True | False;

docs/index.rst

@@ -28,8 +28,8 @@ a 0.x version number `to indicate this fast pace of change <https://semver.org/#
 
 .. warning::
 
-  Solidity recently released the 0.8.x version that introduced a lot of breaking
-  changes. Make sure you read :doc:`the full list <080-breaking-changes>`.
+  Solidity recently released the 0.9.x version that introduced a lot of breaking
+  changes. Make sure you read :doc:`the full list <090-breaking-changes>`.
 
 Ideas for improving Solidity or this documentation are always welcome,
 read our :doc:`contributors guide <contributing>` for more details.
@@ -155,6 +155,7 @@ Contents
    060-breaking-changes.rst
    070-breaking-changes.rst
    080-breaking-changes.rst
+   090-breaking-changes.rst
    natspec-format.rst
    security-considerations.rst
    smtchecker.rst
@@ -165,4 +166,4 @@ Contents
    bugs.rst
    contributing.rst
    brand-guide.rst
-   language-influences.rst
+   language-influences.rst