Fixes #188: Fix parsing empty variables

Previously, the fallback case when something beginning
with the token `$` wasn't a simple variable
would be to assume it would be a complex variable
such as `$$x`

Check that the first token would be valid before recursing.

src/Parser.php

@@ -2066,10 +2066,19 @@ class Parser {
                 $variable->dollar = $this->eat1(TokenKind::DollarToken);
                 $token = $this->getCurrentToken();
 
-                $variable->name =
-                    $token->kind === TokenKind::OpenBraceToken ?
-                        $this->parseBracedExpression($variable) :
-                        $this->parseSimpleVariable($variable);
+                switch ($token->kind) {
+                    case TokenKind::OpenBraceToken:
+                        $variable->name = $this->parseBracedExpression($variable);
+                        break;
+                    case TokenKind::VariableName:
+                    case TokenKind::StringVarname:
+                    case TokenKind::DollarToken:
+                        $variable->name = $this->parseSimpleVariable($variable);
+                        break;
+                    default:
+                        $variable->name = new MissingToken(TokenKind::VariableName, $token->fullStart);
+                        break;
+                }
             } elseif ($token->kind === TokenKind::VariableName || $token->kind === TokenKind::StringVarname) {
                 // TODO consider splitting into dollar and name.
                 // StringVarname is the variable name without $, used in a template string e.g. `"${foo}"`
@@ -2089,7 +2098,6 @@ class Parser {
             TokenKind::YieldFromKeyword,
             TokenKind::YieldKeyword
             );
-
         $yieldExpression->arrayElement = $this->parseArrayElement($yieldExpression);
         return $yieldExpression;
     }

tests/cases/parser/variables6.php

@@ -0,0 +1,3 @@
+<?php
+
+$x = $;

tests/cases/parser/variables6.php.diag

@@ -0,0 +1,8 @@
+[
+    {
+        "kind": 0,
+        "message": "'VariableName' expected.",
+        "start": 13,
+        "length": 0
+    }
+]

tests/cases/parser/variables6.php.tree

@@ -0,0 +1,59 @@
+{
+    "SourceFileNode": {
+        "statementList": [
+            {
+                "InlineHtml": {
+                    "scriptSectionEndTag": null,
+                    "text": null,
+                    "scriptSectionStartTag": {
+                        "kind": "ScriptSectionStartTag",
+                        "textLength": 6
+                    }
+                }
+            },
+            {
+                "ExpressionStatement": {
+                    "expression": {
+                        "AssignmentExpression": {
+                            "leftOperand": {
+                                "Variable": {
+                                    "dollar": null,
+                                    "name": {
+                                        "kind": "VariableName",
+                                        "textLength": 2
+                                    }
+                                }
+                            },
+                            "operator": {
+                                "kind": "EqualsToken",
+                                "textLength": 1
+                            },
+                            "byRef": null,
+                            "rightOperand": {
+                                "Variable": {
+                                    "dollar": {
+                                        "kind": "DollarToken",
+                                        "textLength": 1
+                                    },
+                                    "name": {
+                                        "error": "MissingToken",
+                                        "kind": "VariableName",
+                                        "textLength": 0
+                                    }
+                                }
+                            }
+                        }
+                    },
+                    "semicolon": {
+                        "kind": "SemicolonToken",
+                        "textLength": 1
+                    }
+                }
+            }
+        ],
+        "endOfFileToken": {
+            "kind": "EndOfFileToken",
+            "textLength": 0
+        }
+    }
+}