Fix else clause of case-when syntax typing

lib/steep/type_construction.rb

@@ -2051,14 +2051,16 @@ def synthesize(node, hint: nil, condition: false)
 
               if els
                 branch_results << condition_constr.synthesize(els, hint: hint)
+              else
+                branch_results << Pair.new(type: AST::Builtin.nil_type, constr: condition_constr)
+              end
+
+              branch_results.reject! do |result|
+                result.type.is_a?(AST::Types::Bot)
               end
 
               types = branch_results.map(&:type)
               envs = branch_results.map {|result| result.constr.context.type_env }
-
-              unless els
-                types << AST::Builtin.nil_type
-              end
             end
 
             constr = constr.update_type_env do |env|

lib/steep/type_inference/case_when.rb

@@ -243,6 +243,8 @@ def result
           end
         end
 
+        results.reject! { _1.type.is_a?(AST::Types::Bot) }
+
         types = results.map {|result| result.type }
         envs = results.map {|result| result.env }
 

test/type_check_test.rb

@@ -2927,4 +2927,160 @@ def test_tuple_type_assertion
       YAML
     )
   end
+
+  def test_case_when__no_subject__assignment_in_when__raise_in_else
+    run_type_check_test(
+      signatures: {
+      },
+      code: {
+        "a.rb" => <<~RUBY
+          case
+          when 1
+            v = 1
+          when 2
+            v = 10
+          else
+            raise
+          end
+
+          v + 1
+        RUBY
+      },
+      expectations: <<~YAML
+        ---
+        - file: a.rb
+          diagnostics: []
+      YAML
+    )
+  end
+
+  def test_case_when__no_subject__assignment_in_when__no_else
+    run_type_check_test(
+      signatures: {
+      },
+      code: {
+        "a.rb" => <<~RUBY
+          case
+          when 1
+            v = 1
+          when 2
+            v = 10
+          else
+          end
+
+          v + 1
+        RUBY
+      },
+      expectations: <<~YAML
+        ---
+        - file: a.rb
+          diagnostics:
+          - range:
+              start:
+                line: 9
+                character: 2
+              end:
+                line: 9
+                character: 3
+            severity: ERROR
+            message: Type `(::Integer | nil)` does not have method `+`
+            code: Ruby::NoMethod
+      YAML
+    )
+  end
+
+  def test_case_when__with_subject__assignment_in_when__raise_in_else
+    run_type_check_test(
+      signatures: {
+      },
+      code: {
+        "a.rb" => <<~RUBY
+          case rand(3)
+          when 1
+            v = 1
+          when 2
+            v = 10
+          else
+            raise
+          end
+
+          v + 1
+        RUBY
+      },
+      expectations: <<~YAML
+        ---
+        - file: a.rb
+          diagnostics: []
+      YAML
+    )
+  end
+
+  def test_case_when__with_subject__assignment_in_when__empty_else
+    run_type_check_test(
+      signatures: {
+      },
+      code: {
+        "a.rb" => <<~RUBY
+          case rand(3)
+          when 1
+            v = 1
+          when 2
+            v = 10
+          else
+          end
+
+          v + 1
+        RUBY
+      },
+      expectations: <<~YAML
+        ---
+        - file: a.rb
+          diagnostics:
+          - range:
+              start:
+                line: 9
+                character: 2
+              end:
+                line: 9
+                character: 3
+            severity: ERROR
+            message: Type `(::Integer | nil)` does not have method `+`
+            code: Ruby::NoMethod
+      YAML
+    )
+  end
+
+  def test_case_when__with_subject__assignment_in_when__no_else
+    run_type_check_test(
+      signatures: {
+      },
+      code: {
+        "a.rb" => <<~RUBY
+          case rand(3)
+          when 1
+            v = 1
+          when 2
+            v = 10
+          end
+
+          v + 1
+        RUBY
+      },
+      expectations: <<~YAML
+        ---
+        - file: a.rb
+          diagnostics:
+          - range:
+              start:
+                line: 8
+                character: 2
+              end:
+                line: 8
+                character: 3
+            severity: ERROR
+            message: Type `(::Integer | nil)` does not have method `+`
+            code: Ruby::NoMethod
+      YAML
+    )
+  end
 end