diff --git a/src/Psalm/Checker/ClassChecker.php b/src/Psalm/Checker/ClassChecker.php
index 834d0f54b..5805797f1 100644
--- a/src/Psalm/Checker/ClassChecker.php
+++ b/src/Psalm/Checker/ClassChecker.php
@@ -380,7 +380,6 @@ class ClassChecker extends ClassLikeChecker
         foreach ($storage->appearing_property_ids as $property_name => $appearing_property_id) {
             $property_class_name = self::getDeclaringClassForProperty($project_checker, $appearing_property_id);
             $property_class_storage = $classlike_storage_provider->get((string)$property_class_name);
-            $property_class_name = self::getDeclaringClassForProperty($project_checker, $appearing_property_id);
 
             $property = $property_class_storage->properties[$property_name];
 
@@ -510,7 +509,6 @@ class ClassChecker extends ClassLikeChecker
             foreach ($storage->appearing_property_ids as $property_name => $appearing_property_id) {
                 $property_class_name = self::getDeclaringClassForProperty($project_checker, $appearing_property_id);
                 $property_class_storage = $classlike_storage_provider->get((string)$property_class_name);
-                $property_class_name = self::getDeclaringClassForProperty($project_checker, $appearing_property_id);
 
                 $property = $property_class_storage->properties[$property_name];
 
diff --git a/src/Psalm/Checker/FunctionChecker.php b/src/Psalm/Checker/FunctionChecker.php
index cbcd0feba..58060d08c 100644
--- a/src/Psalm/Checker/FunctionChecker.php
+++ b/src/Psalm/Checker/FunctionChecker.php
@@ -636,7 +636,6 @@ class FunctionChecker extends FunctionLikeChecker
                     && $stmt instanceof PhpParser\Node\Stmt\Return_
                     && $stmt->expr
                 ) {
-                    $first_param_name = $first_param->name;
                     $assertions = AssertionFinder::getAssertions($stmt->expr, null, $statements_checker);
 
                     if (isset($assertions['$' . $first_param->name])) {
diff --git a/src/Psalm/Checker/FunctionLikeChecker.php b/src/Psalm/Checker/FunctionLikeChecker.php
index c1bbb0dc7..fb8b9fa9e 100644
--- a/src/Psalm/Checker/FunctionLikeChecker.php
+++ b/src/Psalm/Checker/FunctionLikeChecker.php
@@ -32,7 +32,6 @@ use Psalm\Issue\OverriddenMethodAccess;
 use Psalm\Issue\ReservedWord;
 use Psalm\Issue\UntypedParam;
 use Psalm\Issue\UnusedParam;
-use Psalm\Issue\UnusedVariable;
 use Psalm\IssueBuffer;
 use Psalm\StatementsSource;
 use Psalm\Storage\ClassLikeStorage;
@@ -162,17 +161,22 @@ abstract class FunctionLikeChecker extends SourceChecker implements StatementsSo
                 $context->vars_possibly_in_scope['$this'] = true;
             }
 
-            $declaring_method_id = MethodChecker::getDeclaringMethodId($project_checker, $method_id);
-
-            if (!is_string($declaring_method_id)) {
-                throw new \UnexpectedValueException('The declaring method of ' . $method_id . ' should not be null');
-            }
-
             $fq_class_name = (string)$context->self;
 
             $class_storage = $classlike_storage_provider->get($fq_class_name);
 
-            $storage = $codebase->getMethodStorage($declaring_method_id);
+            try {
+                $storage = $codebase->getMethodStorage($real_method_id);
+            } catch (\UnexpectedValueException $e) {
+                if (!$class_storage->parent_classes) {
+                    throw $e;
+                }
+
+                $declaring_method_id = (string) MethodChecker::getDeclaringMethodId($project_checker, $method_id);
+
+                // happens for fake constructors
+                $storage = $codebase->getMethodStorage($declaring_method_id);
+            }
 
             $cased_method_id = $fq_class_name . '::' . $storage->cased_name;
 
@@ -285,6 +289,10 @@ abstract class FunctionLikeChecker extends SourceChecker implements StatementsSo
             $context->vars_in_scope['$' . $function_param->name] = $param_type;
             $context->vars_possibly_in_scope['$' . $function_param->name] = true;
 
+            if ($context->collect_references && $function_param->location) {
+                $context->unreferenced_vars['$' . $function_param->name] = $function_param->location;
+            }
+
             if (!$function_param->type_location || !$function_param->location) {
                 continue;
             }
@@ -404,8 +412,9 @@ abstract class FunctionLikeChecker extends SourceChecker implements StatementsSo
                 }
             }
 
-            if ($function_param->by_ref && !$param_type->isMixed()) {
-                $context->byref_constraints['$' . $function_param->name] = new \Psalm\ReferenceConstraint($param_type);
+            if ($function_param->by_ref) {
+                $context->byref_constraints['$' . $function_param->name]
+                    = new \Psalm\ReferenceConstraint(!$param_type->isMixed() ? $param_type : null);
             }
 
             if ($function_param->by_ref) {
@@ -555,71 +564,57 @@ abstract class FunctionLikeChecker extends SourceChecker implements StatementsSo
             }
         }
 
-        if ($context->collect_references &&
-            !$this->getFileChecker()->project_checker->find_references_to &&
-            $context->check_variables
+        if ($context->collect_references
+            && !$project_checker->find_references_to
+            && $context->check_variables
         ) {
-            foreach ($context->vars_possibly_in_scope as $var_name => $_) {
-                if (strpos($var_name, '->') === false &&
-                    $var_name !== '$this' &&
-                    strpos($var_name, '::$') === false &&
-                    strpos($var_name, '[') === false &&
-                    $var_name !== '$_'
+            foreach ($context->unreferenced_vars as $var_name => $original_location) {
+                if (!array_key_exists(substr($var_name, 1), $storage->param_types)) {
+                    continue;
+                }
+
+                if (!($storage instanceof MethodStorage)
+                    || $storage->visibility === ClassLikeChecker::VISIBILITY_PRIVATE
                 ) {
-                    $original_location = $statements_checker->getFirstAppearance($var_name);
+                    if (IssueBuffer::accepts(
+                        new UnusedParam(
+                            'Param ' . $var_name . ' is never referenced in this method',
+                            $original_location
+                        ),
+                        $this->getSuppressedIssues()
+                    )) {
+                        // fall through
+                    }
+                } else {
+                    $fq_class_name = (string)$context->self;
 
-                    if (!isset($context->referenced_var_ids[$var_name]) && $original_location) {
-                        if (!array_key_exists(substr($var_name, 1), $storage->param_types)) {
-                            if (IssueBuffer::accepts(
-                                new UnusedVariable(
-                                    'Variable ' . $var_name . ' is never referenced',
-                                    $original_location
-                                ),
-                                $this->getSuppressedIssues()
-                            )) {
-                                // fall through
-                            }
-                        } elseif (!$storage instanceof MethodStorage
-                            || $storage->visibility === ClassLikeChecker::VISIBILITY_PRIVATE
+                    $class_storage = $codebase->classlike_storage_provider->get($fq_class_name);
+
+                    if (!$class_storage || $storage->abstract) {
+                        continue;
+                    }
+
+                    $method_name_lc = strtolower($storage->cased_name);
+                    $parent_method_id = end($class_storage->overridden_method_ids[$method_name_lc]);
+
+                    $position = array_search(substr($var_name, 1), array_keys($storage->param_types), true);
+
+                    if ($position === false) {
+                        throw new \UnexpectedValueException('$position should not be false here');
+                    }
+
+                    if ($parent_method_id) {
+                        $parent_method_storage = $codebase->getMethodStorage($parent_method_id);
+
+                        // if the parent method has a param at that position and isn't abstract
+                        if (!$parent_method_storage->abstract
+                            && isset($parent_method_storage->params[$position])
                         ) {
-                            if (IssueBuffer::accepts(
-                                new UnusedParam(
-                                    'Param ' . $var_name . ' is never referenced in this method',
-                                    $original_location
-                                ),
-                                $this->getSuppressedIssues()
-                            )) {
-                                // fall through
-                            }
-                        } else {
-                            if (!$class_storage || $storage->abstract) {
-                                continue;
-                            }
-
-                            /** @var ClassMethod $this->function */
-                            $method_name_lc = strtolower((string)$this->function->name);
-                            $parent_method_id = end($class_storage->overridden_method_ids[$method_name_lc]);
-
-                            $position = array_search(substr($var_name, 1), array_keys($storage->param_types), true);
-
-                            if ($position === false) {
-                                throw new \UnexpectedValueException('$position should not be false here');
-                            }
-
-                            if ($parent_method_id) {
-                                $parent_method_storage = $codebase->getMethodStorage($parent_method_id);
-
-                                // if the parent method has a param at that position and isn't abstract
-                                if (!$parent_method_storage->abstract
-                                    && isset($parent_method_storage->params[$position])
-                                ) {
-                                    continue;
-                                }
-                            }
-
-                            $storage->unused_params[$position] = $original_location;
+                            continue;
                         }
                     }
+
+                    $storage->unused_params[$position] = $original_location;
                 }
             }
 
@@ -629,7 +624,7 @@ abstract class FunctionLikeChecker extends SourceChecker implements StatementsSo
                         $storage->used_params[$i] = true;
 
                         /** @var ClassMethod $this->function */
-                        $method_name_lc = strtolower((string)$this->function->name);
+                        $method_name_lc = strtolower($storage->cased_name);
 
                         if (!isset($class_storage->overridden_method_ids[$method_name_lc])) {
                             continue;
@@ -1066,22 +1061,23 @@ abstract class FunctionLikeChecker extends SourceChecker implements StatementsSo
      */
     public function getFunctionLikeStorage(StatementsChecker $statements_checker)
     {
-        $function_id = $this->getMethodId();
-
         $project_checker = $this->getFileChecker()->project_checker;
         $codebase = $project_checker->codebase;
 
-        if (strpos($function_id, '::')) {
-            $declaring_method_id = MethodChecker::getDeclaringMethodId($project_checker, $function_id);
+        if ($this->function instanceof ClassMethod) {
+            $method_id = (string) $this->getMethodId();
 
-            if (!$declaring_method_id) {
-                throw new \UnexpectedValueException('The declaring method of ' . $function_id . ' should not be null');
+            try {
+                return $codebase->getMethodStorage($method_id);
+            } catch (\UnexpectedValueException $e) {
+                $declaring_method_id = (string) MethodChecker::getDeclaringMethodId($project_checker, $method_id);
+
+                // happens for fake constructors
+                return $codebase->getMethodStorage($declaring_method_id);
             }
-
-            return $codebase->getMethodStorage($declaring_method_id);
         }
 
-        return $codebase->getFunctionStorage($statements_checker, $function_id);
+        return $codebase->getFunctionStorage($statements_checker, (string) $this->getMethodId());
     }
 
     /**
diff --git a/src/Psalm/Checker/Statements/Block/DoChecker.php b/src/Psalm/Checker/Statements/Block/DoChecker.php
index 810995e68..86c4f4848 100644
--- a/src/Psalm/Checker/Statements/Block/DoChecker.php
+++ b/src/Psalm/Checker/Statements/Block/DoChecker.php
@@ -72,6 +72,13 @@ class DoChecker
             $do_context->referenced_var_ids
         );
 
+        if ($context->collect_references) {
+            $context->unreferenced_vars = array_intersect_key(
+                $do_context->unreferenced_vars,
+                $context->unreferenced_vars
+            );
+        }
+
         return ExpressionChecker::analyze($statements_checker, $stmt->cond, $context);
     }
 }
diff --git a/src/Psalm/Checker/Statements/Block/ForChecker.php b/src/Psalm/Checker/Statements/Block/ForChecker.php
index 960d984ed..f538fad04 100644
--- a/src/Psalm/Checker/Statements/Block/ForChecker.php
+++ b/src/Psalm/Checker/Statements/Block/ForChecker.php
@@ -98,6 +98,13 @@ class ForChecker
             $context->referenced_var_ids
         );
 
+        if ($context->collect_references) {
+            $context->unreferenced_vars = array_intersect_key(
+                $for_context->unreferenced_vars,
+                $context->unreferenced_vars
+            );
+        }
+
         return null;
     }
 }
diff --git a/src/Psalm/Checker/Statements/Block/ForeachChecker.php b/src/Psalm/Checker/Statements/Block/ForeachChecker.php
index 8a2f45e66..90cddaa39 100644
--- a/src/Psalm/Checker/Statements/Block/ForeachChecker.php
+++ b/src/Psalm/Checker/Statements/Block/ForeachChecker.php
@@ -272,6 +272,15 @@ class ForeachChecker
             }
         }
 
+        if ($context->collect_references
+            && $stmt->byRef
+            && $stmt->valueVar instanceof PhpParser\Node\Expr\Variable
+            && is_string($stmt->valueVar->name)
+        ) {
+            $foreach_context->byref_constraints['$' . $stmt->valueVar->name]
+                = new \Psalm\ReferenceConstraint($value_type);
+        }
+
         AssignmentChecker::analyze(
             $statements_checker,
             $stmt->valueVar,
@@ -335,6 +344,10 @@ class ForeachChecker
             $context->referenced_var_ids
         );
 
+        if ($context->collect_references) {
+            $context->unreferenced_vars = $foreach_context->unreferenced_vars;
+        }
+
         return null;
     }
 }
diff --git a/src/Psalm/Checker/Statements/Block/IfChecker.php b/src/Psalm/Checker/Statements/Block/IfChecker.php
index 1d00ff0cb..c7199b5c9 100644
--- a/src/Psalm/Checker/Statements/Block/IfChecker.php
+++ b/src/Psalm/Checker/Statements/Block/IfChecker.php
@@ -283,6 +283,22 @@ class IfChecker
             return false;
         }
 
+        $newly_unreferenced_vars = [];
+        $old_unreferenced_vars = $context->unreferenced_vars;
+
+        if ($context->collect_references) {
+            foreach ($context->unreferenced_vars as $var_id => $_) {
+                if (!isset($if_context->unreferenced_vars[$var_id])) {
+                    unset($context->unreferenced_vars[$var_id]);
+                }
+            }
+
+            $newly_unreferenced_vars = array_diff_key(
+                $if_context->unreferenced_vars,
+                $old_unreferenced_vars
+            );
+        }
+
         // check the elseifs
         foreach ($stmt->elseifs as $elseif) {
             $elseif_context = clone $original_context;
@@ -302,6 +318,22 @@ class IfChecker
             ) === false) {
                 return false;
             }
+
+            if ($context->collect_references) {
+                foreach ($context->unreferenced_vars as $var_id => $_) {
+                    if (!isset($elseif_context->unreferenced_vars[$var_id])) {
+                        unset($context->unreferenced_vars[$var_id]);
+                    }
+                }
+
+                $newly_unreferenced_vars = array_merge(
+                    $newly_unreferenced_vars,
+                    array_diff_key(
+                        $elseif_context->unreferenced_vars,
+                        $old_unreferenced_vars
+                    )
+                );
+            }
         }
 
         // check the else
@@ -323,6 +355,22 @@ class IfChecker
             ) === false) {
                 return false;
             }
+
+            if ($context->collect_references) {
+                foreach ($context->unreferenced_vars as $var_id => $_) {
+                    if (!isset($else_context->unreferenced_vars[$var_id])) {
+                        unset($context->unreferenced_vars[$var_id]);
+                    }
+                }
+
+                $newly_unreferenced_vars = array_merge(
+                    $newly_unreferenced_vars,
+                    array_diff_key(
+                        $else_context->unreferenced_vars,
+                        $old_unreferenced_vars
+                    )
+                );
+            }
         } else {
             $if_scope->final_actions[] = ScopeChecker::ACTION_NONE;
         }
@@ -385,6 +433,13 @@ class IfChecker
             }
         }
 
+        if ($context->collect_references) {
+            $context->unreferenced_vars = array_merge(
+                $context->unreferenced_vars,
+                $newly_unreferenced_vars
+            );
+        }
+
         return null;
     }
 
@@ -441,12 +496,14 @@ class IfChecker
 
         if ($if_context->byref_constraints !== null) {
             foreach ($if_context->byref_constraints as $var_id => $byref_constraint) {
-                if ($outer_context->byref_constraints !== null &&
-                    isset($outer_context->byref_constraints[$var_id]) &&
-                    !TypeChecker::isContainedBy(
+                if ($outer_context->byref_constraints !== null
+                    && isset($outer_context->byref_constraints[$var_id])
+                    && $byref_constraint->type
+                    && ($outer_constraint_type = $outer_context->byref_constraints[$var_id]->type)
+                    && !TypeChecker::isContainedBy(
                         $project_checker,
                         $byref_constraint->type,
-                        $outer_context->byref_constraints[$var_id]->type
+                        $outer_constraint_type
                     )
                 ) {
                     if (IssueBuffer::accepts(
@@ -829,12 +886,14 @@ class IfChecker
 
         if ($elseif_context->byref_constraints !== null) {
             foreach ($elseif_context->byref_constraints as $var_id => $byref_constraint) {
-                if ($outer_context->byref_constraints !== null &&
-                    isset($outer_context->byref_constraints[$var_id]) &&
-                    !TypeChecker::isContainedBy(
+                if ($outer_context->byref_constraints !== null
+                    && isset($outer_context->byref_constraints[$var_id])
+                    && ($outer_constraint_type = $outer_context->byref_constraints[$var_id]->type)
+                    && $byref_constraint->type
+                    && !TypeChecker::isContainedBy(
                         $project_checker,
                         $byref_constraint->type,
-                        $outer_context->byref_constraints[$var_id]->type
+                        $outer_constraint_type
                     )
                 ) {
                     if (IssueBuffer::accepts(
@@ -1087,12 +1146,14 @@ class IfChecker
             $project_checker = $statements_checker->getFileChecker()->project_checker;
 
             foreach ($else_context->byref_constraints as $var_id => $byref_constraint) {
-                if ($outer_context->byref_constraints !== null &&
-                    isset($outer_context->byref_constraints[$var_id]) &&
-                    !TypeChecker::isContainedBy(
+                if ($outer_context->byref_constraints !== null
+                    && isset($outer_context->byref_constraints[$var_id])
+                    && ($outer_constraint_type = $outer_context->byref_constraints[$var_id]->type)
+                    && $byref_constraint->type
+                    && !TypeChecker::isContainedBy(
                         $project_checker,
                         $byref_constraint->type,
-                        $outer_context->byref_constraints[$var_id]->type
+                        $outer_constraint_type
                     )
                 ) {
                     if (IssueBuffer::accepts(
diff --git a/src/Psalm/Checker/Statements/Block/LoopChecker.php b/src/Psalm/Checker/Statements/Block/LoopChecker.php
index 8b35133d2..691481ee7 100644
--- a/src/Psalm/Checker/Statements/Block/LoopChecker.php
+++ b/src/Psalm/Checker/Statements/Block/LoopChecker.php
@@ -365,6 +365,13 @@ class LoopChecker
             $inner_context->referenced_var_ids,
             $loop_scope->loop_context->referenced_var_ids
         );
+
+        if ($inner_context->collect_references) {
+            $loop_scope->loop_context->unreferenced_vars = array_diff_key(
+                $inner_context->unreferenced_vars,
+                $inner_context->referenced_var_ids
+            );
+        }
     }
 
     /**
diff --git a/src/Psalm/Checker/Statements/Block/SwitchChecker.php b/src/Psalm/Checker/Statements/Block/SwitchChecker.php
index 5d4424558..8bfe0103f 100644
--- a/src/Psalm/Checker/Statements/Block/SwitchChecker.php
+++ b/src/Psalm/Checker/Statements/Block/SwitchChecker.php
@@ -198,6 +198,13 @@ class SwitchChecker
                 $case_context->referenced_var_ids
             );
 
+            if ($context->collect_references) {
+                $context->unreferenced_vars = array_intersect_key(
+                    $case_context->unreferenced_vars,
+                    $context->unreferenced_vars
+                );
+            }
+
             if ($case_exit_type !== 'return_throw') {
                 $vars = array_diff_key(
                     $case_context->vars_possibly_in_scope,
diff --git a/src/Psalm/Checker/Statements/Block/TryChecker.php b/src/Psalm/Checker/Statements/Block/TryChecker.php
index af95cec06..14d0f3ada 100644
--- a/src/Psalm/Checker/Statements/Block/TryChecker.php
+++ b/src/Psalm/Checker/Statements/Block/TryChecker.php
@@ -75,6 +75,18 @@ class TryChecker
             }
 
             $try_context->vars_possibly_in_scope = $context->vars_possibly_in_scope;
+
+            $context->referenced_var_ids = array_merge(
+                $try_context->referenced_var_ids,
+                $context->referenced_var_ids
+            );
+
+            if ($context->collect_references) {
+                $context->unreferenced_vars = array_intersect_key(
+                    $try_context->unreferenced_vars,
+                    $context->unreferenced_vars
+                );
+            }
         }
 
         $try_leaves_loop = $loop_scope
@@ -200,6 +212,13 @@ class TryChecker
                 $context->referenced_var_ids
             );
 
+            if ($context->collect_references) {
+                $context->unreferenced_vars = array_intersect_key(
+                    $catch_context->unreferenced_vars,
+                    $context->unreferenced_vars
+                );
+            }
+
             if ($catch_actions[$i] !== [ScopeChecker::ACTION_END]) {
                 foreach ($catch_context->vars_in_scope as $var_id => $type) {
                     if ($catch->var !== $var_id &&
diff --git a/src/Psalm/Checker/Statements/Block/WhileChecker.php b/src/Psalm/Checker/Statements/Block/WhileChecker.php
index f9ebbc000..1d64b71e8 100644
--- a/src/Psalm/Checker/Statements/Block/WhileChecker.php
+++ b/src/Psalm/Checker/Statements/Block/WhileChecker.php
@@ -83,6 +83,13 @@ class WhileChecker
             $while_context->referenced_var_ids
         );
 
+        if ($context->collect_references) {
+            $context->unreferenced_vars = array_intersect_key(
+                $while_context->unreferenced_vars,
+                $context->unreferenced_vars
+            );
+        }
+
         return null;
     }
 }
diff --git a/src/Psalm/Checker/Statements/Expression/ArrayChecker.php b/src/Psalm/Checker/Statements/Expression/ArrayChecker.php
index dba5e43fb..096cdf5f6 100644
--- a/src/Psalm/Checker/Statements/Expression/ArrayChecker.php
+++ b/src/Psalm/Checker/Statements/Expression/ArrayChecker.php
@@ -120,7 +120,7 @@ class ArrayChecker
                 ) {
                     $array_key = $item->key ? $item->key->value : $int_offset;
 
-                    $property_types[$item->key ? $item->key->value : $int_offset] = $item_value_type;
+                    $property_types[$array_key] = $item_value_type;
                 } else {
                     $can_create_objectlike = false;
                 }
diff --git a/src/Psalm/Checker/Statements/Expression/AssignmentChecker.php b/src/Psalm/Checker/Statements/Expression/AssignmentChecker.php
index 47115f26e..22d35fab6 100644
--- a/src/Psalm/Checker/Statements/Expression/AssignmentChecker.php
+++ b/src/Psalm/Checker/Statements/Expression/AssignmentChecker.php
@@ -165,11 +165,14 @@ class AssignmentChecker
             )) {
                 // fall through
             }
-        } elseif ($var_id && isset($context->byref_constraints[$var_id])) {
+        } elseif ($var_id
+            && isset($context->byref_constraints[$var_id])
+            && ($outer_constraint_type = $context->byref_constraints[$var_id]->type)
+        ) {
             if (!TypeChecker::isContainedBy(
                 $statements_checker->getFileChecker()->project_checker,
                 $assign_value_type,
-                $context->byref_constraints[$var_id]->type,
+                $outer_constraint_type,
                 $assign_value_type->ignore_nullable_issues,
                 $assign_value_type->ignore_falsable_issues
             )
@@ -214,6 +217,10 @@ class AssignmentChecker
             $context->vars_possibly_in_scope[$var_id] = true;
             $context->assigned_var_ids[$var_id] = true;
 
+            if ($context->collect_references && !isset($context->byref_constraints[$var_id])) {
+                $context->unreferenced_vars[$var_id] = new CodeLocation($statements_checker, $assign_var);
+            }
+
             if (!$statements_checker->hasVariable($var_id)) {
                 $statements_checker->registerVariable(
                     $var_id,
diff --git a/src/Psalm/Checker/Statements/Expression/BinaryOpChecker.php b/src/Psalm/Checker/Statements/Expression/BinaryOpChecker.php
index 6d54d3f1f..52d37221a 100644
--- a/src/Psalm/Checker/Statements/Expression/BinaryOpChecker.php
+++ b/src/Psalm/Checker/Statements/Expression/BinaryOpChecker.php
@@ -105,6 +105,13 @@ class BinaryOpChecker
                 $context->referenced_var_ids
             );
 
+            if ($context->collect_references) {
+                $context->unreferenced_vars = array_intersect_key(
+                    $op_context->unreferenced_vars,
+                    $context->unreferenced_vars
+                );
+            }
+
             foreach ($op_context->vars_in_scope as $var_id => $type) {
                 if (isset($context->vars_in_scope[$var_id])) {
                     $context->vars_in_scope[$var_id] = Type::combineUnionTypes($context->vars_in_scope[$var_id], $type);
@@ -230,6 +237,13 @@ class BinaryOpChecker
                 $context->referenced_var_ids
             );
 
+            if ($context->collect_references) {
+                $context->unreferenced_vars = array_intersect_key(
+                    $op_context->unreferenced_vars,
+                    $context->unreferenced_vars
+                );
+            }
+
             $context->vars_possibly_in_scope = array_merge(
                 $op_context->vars_possibly_in_scope,
                 $context->vars_possibly_in_scope
@@ -296,6 +310,13 @@ class BinaryOpChecker
                 $t_if_context->referenced_var_ids
             );
 
+            if ($context->collect_references) {
+                $context->unreferenced_vars = array_intersect_key(
+                    $t_if_context->unreferenced_vars,
+                    $context->unreferenced_vars
+                );
+            }
+
             $t_else_context = clone $context;
 
             if ($negated_if_types) {
@@ -325,6 +346,13 @@ class BinaryOpChecker
                 $t_else_context->referenced_var_ids
             );
 
+            if ($context->collect_references) {
+                $context->unreferenced_vars = array_intersect_key(
+                    $t_else_context->unreferenced_vars,
+                    $context->unreferenced_vars
+                );
+            }
+
             $lhs_type = null;
 
             if (isset($stmt->left->inferredType)) {
diff --git a/src/Psalm/Checker/Statements/Expression/TernaryChecker.php b/src/Psalm/Checker/Statements/Expression/TernaryChecker.php
index 42c67d388..21d104865 100644
--- a/src/Psalm/Checker/Statements/Expression/TernaryChecker.php
+++ b/src/Psalm/Checker/Statements/Expression/TernaryChecker.php
@@ -87,6 +87,11 @@ class TernaryChecker
                 $context->referenced_var_ids,
                 $t_if_context->referenced_var_ids
             );
+
+            $context->unreferenced_vars = array_intersect_key(
+                $context->unreferenced_vars,
+                $t_if_context->unreferenced_vars
+            );
         }
 
         if ($negated_if_types) {
@@ -122,6 +127,11 @@ class TernaryChecker
             $t_else_context->referenced_var_ids
         );
 
+        $context->unreferenced_vars = array_intersect_key(
+            $context->unreferenced_vars,
+            $t_else_context->unreferenced_vars
+        );
+
         $lhs_type = null;
 
         if ($stmt->if) {
diff --git a/src/Psalm/Checker/StatementsChecker.php b/src/Psalm/Checker/StatementsChecker.php
index dbfb7aa32..6d7bcd09d 100644
--- a/src/Psalm/Checker/StatementsChecker.php
+++ b/src/Psalm/Checker/StatementsChecker.php
@@ -26,6 +26,7 @@ use Psalm\Issue\InvalidDocblock;
 use Psalm\Issue\InvalidGlobal;
 use Psalm\Issue\UnevaluatedCode;
 use Psalm\Issue\UnrecognizedStatement;
+use Psalm\Issue\UnusedVariable;
 use Psalm\IssueBuffer;
 use Psalm\Scope\LoopScope;
 use Psalm\StatementsSource;
@@ -516,6 +517,14 @@ class StatementsChecker extends SourceChecker implements StatementsSource
             }
         }
 
+        if ($root_scope
+            && $context->collect_references
+            && !$project_checker->find_references_to
+            && $context->check_variables
+        ) {
+            $this->checkUnreferencedVars($context);
+        }
+
         if ($project_checker->alter_code && $root_scope && $this->vars_to_initialize) {
             $file_contents = $project_checker->codebase->getFileContents($this->getFilePath());
 
@@ -531,6 +540,35 @@ class StatementsChecker extends SourceChecker implements StatementsSource
         return null;
     }
 
+    /**
+     * @param  Context  $context
+     *
+     * @return void
+     */
+    private function checkUnreferencedVars(Context $context)
+    {
+        $source = $this->getSource();
+        $function_storage = $source instanceof FunctionLikeChecker ? $source->getFunctionLikeStorage($this) : null;
+
+        foreach ($context->unreferenced_vars as $var_name => $original_location) {
+            if ($var_name === '$_') {
+                continue;
+            }
+
+            if (!$function_storage || !array_key_exists(substr($var_name, 1), $function_storage->param_types)) {
+                if (IssueBuffer::accepts(
+                    new UnusedVariable(
+                        'Variable ' . $var_name . ' is never referenced',
+                        $original_location
+                    ),
+                    $this->getSuppressedIssues()
+                )) {
+                    // fall through
+                }
+            }
+        }
+    }
+
     /**
      * @param   PhpParser\Node\Stmt\Static_ $stmt
      * @param   Context                     $context
diff --git a/src/Psalm/Context.php b/src/Psalm/Context.php
index 9a2fa03c4..dda0f2538 100644
--- a/src/Psalm/Context.php
+++ b/src/Psalm/Context.php
@@ -138,6 +138,13 @@ class Context
      */
     public $referenced_var_ids = [];
 
+    /**
+     * A list of variables that have never been referenced
+     *
+     * @var array<string, CodeLocation>
+     */
+    public $unreferenced_vars = [];
+
     /**
      * A list of variables that have been passed by reference (where we know their type)
      *
@@ -629,6 +636,10 @@ class Context
 
         if ($stripped_var[0] === '$' && $stripped_var !== '$this') {
             $this->referenced_var_ids[$var_name] = true;
+
+            if ($this->collect_references) {
+                unset($this->unreferenced_vars[$var_name]);
+            }
         }
 
         return isset($this->vars_in_scope[$var_name]);
diff --git a/src/Psalm/ReferenceConstraint.php b/src/Psalm/ReferenceConstraint.php
index 9c42a0b73..9350f0ce3 100644
--- a/src/Psalm/ReferenceConstraint.php
+++ b/src/Psalm/ReferenceConstraint.php
@@ -3,13 +3,13 @@ namespace Psalm;
 
 class ReferenceConstraint
 {
-    /** @var Type\Union */
+    /** @var Type\Union|null */
     public $type;
 
     /**
      * @param  Type\Union $type
      */
-    public function __construct(Type\Union $type)
+    public function __construct(Type\Union $type = null)
     {
         $this->type = $type;
     }
diff --git a/src/Psalm/Stubs/CoreGenericFunctions.php b/src/Psalm/Stubs/CoreGenericFunctions.php
index eccd57853..c430725aa 100644
--- a/src/Psalm/Stubs/CoreGenericFunctions.php
+++ b/src/Psalm/Stubs/CoreGenericFunctions.php
@@ -49,6 +49,18 @@ function array_slice(array $arr, int $offset, int $length = null, bool $preserve
  */
 function array_intersect(array $arr, array $arr2, array $arr3 = null, array $arr4 = null) {}
 
+/**
+ * @template TKey
+ * @template TValue
+ *
+ * @param array<TKey, TValue> $arr
+ * @param array $arr2
+ * @param array|null $arr3
+ * @param array|null $arr4
+ * @return array<TKey, TValue>
+ */
+function array_intersect_key(array $arr, array $arr2, array $arr3 = null, array $arr4 = null) {}
+
 /**
  * @template TKey
  * @template TValue
diff --git a/tests/UnusedCodeTest.php b/tests/UnusedCodeTest.php
index adc878b1a..585ed7f7f 100644
--- a/tests/UnusedCodeTest.php
+++ b/tests/UnusedCodeTest.php
@@ -247,6 +247,20 @@ class UnusedCodeTest extends TestCase
                     }',
                 'error_message' => 'UnusedVariable',
             ],
+            'unusuedVariableInBranchOfIf' => [
+                '<?php
+                    function foo(): void {
+                        if (rand(0, 1)) {
+                            $a = "foo";
+                            echo $a;
+                        }
+
+                        if (rand(0, 1)) {
+                            $a = "foo";
+                        }
+                    }',
+                'error_message' => 'UnusedVariable',
+            ],
             'unusedClass' => [
                 '<?php
                     class A { }',