Fix PSUseDeclaredVarsMoreThanAssignments to not give false positives when using += operator (#935)

* fix false positives such as '$a=@(); $b | foreach-object { $a+=$c}' because the rule misunderstood += as assignment

* add test (it turns out a regression happened for 'flags strongly typed variables'

* fix regression for strongly typed variables by being more selective about the if/else decision

* add additional test case for unassigned variable

Author: bergmeister

Rules/UseDeclaredVarsMoreThanAssignments.cs

@@ -173,9 +173,19 @@ private IEnumerable<DiagnosticRecord> AnalyzeScriptBlockAst(ScriptBlockAst scrip
                             // Try casting to AssignmentStatementAst to be able to catch case where a variable is assigned more than once (https://github.com/PowerShell/PSScriptAnalyzer/issues/833)
                             var varInAssignmentAsStatementAst = varInAssignment.Parent as AssignmentStatementAst;
                             var varAstAsAssignmentStatementAst = varAst.Parent as AssignmentStatementAst;
-                            if (varInAssignmentAsStatementAst != null && varAstAsAssignmentStatementAst != null)
+                            if (varAstAsAssignmentStatementAst != null)
                             {
-                                inAssignment = varInAssignmentAsStatementAst.Left.Extent.Text.Equals(varAstAsAssignmentStatementAst.Left.Extent.Text, StringComparison.OrdinalIgnoreCase);
+                                if (varAstAsAssignmentStatementAst.Operator == TokenKind.Equals)
+                                {
+                                    if (varInAssignmentAsStatementAst != null)
+                                    {
+                                        inAssignment = varInAssignmentAsStatementAst.Left.Extent.Text.Equals(varAstAsAssignmentStatementAst.Left.Extent.Text, StringComparison.OrdinalIgnoreCase);
+                                    }
+                                    else
+                                    {
+                                        inAssignment = varInAssignment.Equals(varAst);
+                                    }
+                                }
                             }
                             else
                             {

Tests/Rules/UseDeclaredVarsMoreThanAssignments.tests.ps1

@@ -73,5 +73,15 @@ function MyFunc2() {
         It "returns no violations" {
             $noViolations.Count | Should -Be 0
         }
+        
+        It "Does not flag += operator" {
+            $results = Invoke-ScriptAnalyzer -ScriptDefinition '$array=@(); $list | ForEach-Object { $array += $c }' | Where-Object { $_.RuleName -eq $violationName }
+            $results.Count | Should -Be 0
+        }
+
+        It "Does not flag += operator when using unassigned variable" {
+            $results = Invoke-ScriptAnalyzer -ScriptDefinition '$list | ForEach-Object { $array += $c }' | Where-Object { $_.RuleName -eq $violationName }
+            $results.Count | Should -Be 0
+        }
     }
 }