Fix ineffective if warnings in discarded case branches

lib/puppet/pops/validation/checker4_0.rb

@@ -144,6 +144,20 @@ def ends_with_idem(o)
     end
   end
 
+  # Returns an idempotent expression whose value is discarded when +o+ is used as a statement.
+  # A case expression can have side effects before its branch result, so inspect each branch tail
+  # even when the case expression as a whole is not idempotent.
+  def discarded_value_expression(o)
+    return o if idem(o)
+    return nil unless o.is_a?(Model::CaseExpression)
+
+    o.options.each do |option|
+      violator = ends_with_idem(option.then_expr)
+      return violator if violator
+    end
+    nil
+  end
+
   #---ASSIGNMENT CHECKS
 
   def assign_VariableExpression(o, via_index)
@@ -269,8 +283,9 @@ def check_BlockExpression(o)
       acceptor.accept(Issues::RESOURCE_WITHOUT_TITLE, o, :name => o.statements[0].value)
     else
       o.statements[0..-2].each do |statement|
-        if idem(statement)
-          acceptor.accept(Issues::IDEM_EXPRESSION_NOT_LAST, statement)
+        violator = discarded_value_expression(statement)
+        if violator
+          acceptor.accept(Issues::IDEM_EXPRESSION_NOT_LAST, violator)
           break # only flag the first
         end
       end

spec/unit/pops/validator/validator_spec.rb

@@ -499,6 +499,38 @@ def with_environment(environment, env_params = {})
   end
 
   context 'for non productive expressions' do
+    it 'flags an ineffective conditional at the end of a discarded case branch' do
+      source = <<~'PUPPET'
+        case 'Ubuntu' {
+          'Ubuntu': {
+            $packages = if true { ['base'] } else { [] }
+            if true {
+              $packages << 'extra'
+            }
+          }
+        }
+        $x = 1
+      PUPPET
+
+      expect(validate(parse(source))).to have_issue(Puppet::Pops::Issues::IDEM_EXPRESSION_NOT_LAST)
+    end
+
+    it 'allows a final value when the case result is used' do
+      source = <<~PUPPET
+        class example {
+          $packages = case $os_name {
+            'Ubuntu': {
+              $base = ['base']
+              if $virtual { $base + ['vm'] } else { $base }
+            }
+            default: { [] }
+          }
+          notice($packages)
+        }
+      PUPPET
+
+      expect(validate(parse(source))).not_to have_issue(Puppet::Pops::Issues::IDEM_EXPRESSION_NOT_LAST)
+    end
     [ '1',
       '3.14',
       "'a'",