Prevent void param type (#1142)

PHP 7.1 introduced the `void` type. Consequently, it is no longer
allowed to specify `void` as the parameter type:

```php
<?php
function foo(void $bar) {}
```

* This pull request adds detection for this behavior and rejects
functions that have the _identifier_ `void` as parameter type.
* Pre-7.1, it was allowed to create a class `class void {}` and use that
as a parameter type. In this case, the parameter's type is the _name_
`void`, so the error isn't thrown.
* Two test cases are added to verify the above behavior.

Fixes #1137.

Author: WoutervdBrink

lib/PhpParser/ParserAbstract.php

@@ -1078,6 +1078,13 @@ protected function checkParam(Param $node): void {
                 $node->default->getAttributes()
             ));
         }
+
+        if ($node->type instanceof Identifier && $node->type->name === 'void') {
+            $this->emitError(new Error(
+                'void cannot be used as a parameter type',
+                $node->type->getAttributes()
+            ));
+        }
     }
 
     protected function checkTryCatch(TryCatch $node): void {

test/code/parser/stmt/function/invalidVoidParam.test

@@ -0,0 +1,37 @@
+Invalid parameter with type void
+-----
+<?php
+function foo(void $foo) {}
+-----
+void cannot be used as a parameter type from 2:14 to 2:17
+array(
+    0: Stmt_Function(
+        attrGroups: array(
+        )
+        byRef: false
+        name: Identifier(
+            name: foo
+        )
+        params: array(
+            0: Param(
+                attrGroups: array(
+                )
+                flags: 0
+                type: Identifier(
+                    name: void
+                )
+                byRef: false
+                variadic: false
+                var: Expr_Variable(
+                    name: foo
+                )
+                default: null
+                hooks: array(
+                )
+            )
+        )
+        returnType: null
+        stmts: array(
+        )
+    )
+)

test/code/parser/stmt/function/validVoidParam.test

@@ -0,0 +1,37 @@
+Valid parameter with type void
+-----
+<?php
+function foo(void $foo) {}
+-----
+!!version=7.0
+array(
+    0: Stmt_Function(
+        attrGroups: array(
+        )
+        byRef: false
+        name: Identifier(
+            name: foo
+        )
+        params: array(
+            0: Param(
+                attrGroups: array(
+                )
+                flags: 0
+                type: Name(
+                    name: void
+                )
+                byRef: false
+                variadic: false
+                var: Expr_Variable(
+                    name: foo
+                )
+                default: null
+                hooks: array(
+                )
+            )
+        )
+        returnType: null
+        stmts: array(
+        )
+    )
+)