Fix: rebind while condition variables after while body

spec/compiler/semantic/while_spec.cr

@@ -169,4 +169,32 @@ describe "Semantic: while" do
       a
       )) { nilable int32 }
   end
+
+  it "rebinds condition variable after while body (#6158)" do
+    assert_type(%(
+      class Foo
+        @parent : self?
+
+        def parent
+          @parent
+        end
+      end
+
+      class Bar
+        def initialize(@parent : Foo)
+        end
+
+        def parent
+          @parent
+        end
+      end
+
+      a = Foo.new
+      b = Bar.new(a)
+      while b = b.parent
+        break if 1 == 1
+      end
+      b
+      )) { nilable types["Foo"] }
+  end
 end

src/compiler/crystal/semantic/main_visitor.cr

@@ -2081,6 +2081,22 @@ module Crystal
         node.body.accept self
       end
 
+      # After while's body, bind variables *before* the condition to the
+      # ones after the body, because the loop will repeat.
+      #
+      # For example:
+      #
+      #    x = exp
+      #    # x starts with the type of exp
+      #    while x = x.method
+      #      # but after the loop, the x above (in x.method)
+      #      # should now also get the type of x.method, recursively
+      #    end
+      before_cond_vars.each do |name, before_cond_var|
+        var = @vars[name]?
+        before_cond_var.bind_to(var) if var && !var.same?(before_cond_var)
+      end
+
       cond = node.cond.single_expression
 
       endless_while = cond.true_literal?