Fix matmul operator parsing (microsoft#1863)

* Remove stale reference

* Don't suppress LHS diagnostics on augmented assign

* Revert "Don't suppress LHS diagnostics on augmented assign"

This reverts commit 6109ac7.

* Add test

* Partial

* Fix based on grouping

* Undo

* Formatting

Author: Mikhail Arkhipov

src/Parsing/Impl/Tokens/Tokenizer.cs

@@ -1822,8 +1822,14 @@ private Token NextOperator(int ch) {
                 case '~':
                     return Tokens.TwiddleToken;
                 case '@':
-                    if (LanguageVersion >= PythonLanguageVersion.V35 && NextChar('=')) {
-                        return Tokens.MatMultiplyEqualToken;
+                    if (LanguageVersion >= PythonLanguageVersion.V35) {
+                        if (NextChar('=')) {
+                            return Tokens.MatMultiplyEqualToken;
+                        }
+                        if (GroupingLevel > 0) {
+                            // @ can't be a decorator here.
+                            return Tokens.MatMultiplyToken;
+                        }
                     }
                     return Tokens.AtToken;
             }

src/Parsing/Test/ParserTests.cs

@@ -1550,6 +1550,24 @@ public void MatMulOperator() {
             }
         }
 
+        [TestMethod, Priority(0)]
+        public void MatMulMultilineOperator() {
+            foreach (var version in V35AndUp) {
+                CheckAst(
+                    ParseFile("MatMulOperator2.py", ErrorSink.Null, version),
+                    CheckSuite(
+                        CheckAssignment(Fob, CheckParenExpr(CheckBinaryExpression(One, PythonOperator.MatMultiply, Two)))
+                    )
+                );
+            }
+
+            foreach (var version in V3Versions.Except(V35AndUp)) {
+                ParseErrors("MatMulOperator2.py", version, new[] {
+                    new ErrorResult("unexpected token '<newline>'", new SourceSpan(2, 6, 3, 1))
+                });
+            }
+        }
+
         [TestMethod, Priority(0)]
         public void GroupingRecovery() {
             foreach (var version in AllVersions) {
@@ -4851,7 +4869,7 @@ private Action<Statement> CheckDelStmt(params Action<Expression>[] deletes) {
             };
         }
 
-        private Action<Expression> CheckParenExpr(Action<Expression> value) {
+        private static Action<Expression> CheckParenExpr(Action<Expression> value) {
             return expr => {
                 Assert.AreEqual(typeof(ParenthesisExpression), expr.GetType());
                 var paren = (ParenthesisExpression)expr;

src/UnitTests/TestData/Grammar/MatMulOperator2.py

@@ -0,0 +1,4 @@
+fob = (
+    1
+    @ 2
+)