fix(diagnostics): update checking of $this usage to only error in static methods (#545)

Author: MaartenStaa

fixtures/diagnostics/errors/this_in_function.php

@@ -99,11 +99,9 @@ private function collectDiagnostics($node)
  // Find the first ancestor that's a class method. Return an error
  // if there is none, or if the method is static.
  $method = $node->getFirstAncestor(Node\MethodDeclaration::class);
- if ($method === null || $method->isStatic()) {
+ if ($method->isStatic()) {
  $this->diagnostics[] = new Diagnostic(
- $method === null
- ? "\$this can only be used in an object context."
- : "\$this can not be used in static methods.",
+ "\$this can not be used in static methods.",
  Range::fromNode($node),
  null,
  DiagnosticSeverity::ERROR,

fixtures/diagnostics/errors/this_in_root.php

@@ -71,42 +71,6 @@ public function testThisInStaticMethodProducesError()
  );
  }
 
- public function testThisInFunctionProducesError()
- {
- $diagnostics = $this->collectDiagnostics(
- __DIR__ . '/../../fixtures/diagnostics/errors/this_in_function.php'
- );
-
- $this->assertCount(1, $diagnostics);
- $this->assertDiagnostic(
- $diagnostics[0],
- '$this can only be used in an object context.',
- DiagnosticSeverity::ERROR,
- new Range(
- new Position(4, 11),
- new Position(4, 16)
- )
- );
- }
-
- public function testThisInRoot()
- {
- $diagnostics = $this->collectDiagnostics(
- __DIR__ . '/../../fixtures/diagnostics/errors/this_in_root.php'
- );
-
- $this->assertCount(1, $diagnostics);
- $this->assertDiagnostic(
- $diagnostics[0],
- '$this can only be used in an object context.',
- DiagnosticSeverity::ERROR,
- new Range(
- new Position(2, 5),
- new Position(2, 10)
- )
- );
- }
-
  public function testThisInMethodProducesNoError()
  {
  $diagnostics = $this->collectDiagnostics(