Error if using return inside a constant's value (#6347)

spec/compiler/semantic/const_spec.cr

@@ -360,4 +360,17 @@ describe "Semantic: const" do
       ),
       "A is not a type, it's a constant"
   end
+
+  it "errors if using return inside constant value (#5391)" do
+    assert_error %(
+      class Foo
+        A = begin
+          return if 1 == 2
+        end
+      end
+
+      Foo::A
+      ),
+      "can't return from constant"
+  end
 end

src/compiler/crystal/semantic/main_visitor.cr

@@ -86,6 +86,7 @@ module Crystal
     # ```
     property last_block_kind : Symbol?
     property? inside_ensure : Bool = false
+    property? inside_constant = false
 
     @unreachable = false
     @is_initialize = false
@@ -193,6 +194,7 @@ module Crystal
           const_def = Def.new("const", [] of Arg)
           type_visitor = MainVisitor.new(@program, meta_vars, const_def)
           type_visitor.current_type = type.namespace
+          type_visitor.inside_constant = true
           type.value.accept type_visitor
 
           type.vars = const_def.vars
@@ -1727,6 +1729,10 @@ module Crystal
         node.raise "can't return from ensure"
       end
 
+      if inside_constant?
+        node.raise "can't return from constant"
+      end
+
       typed_def = @typed_def || node.raise("can't return from top level")
 
       if typed_def.captured_block?