[ruby/prism] Fix constant path full name when parent is not a

constant
(https://github.com/ruby/prism/pull/1742)

* Raise if constant path parts contains nodes that can't be used to build full name

* Fix typo in constant path error documentation

Co-authored-by: Tim Morgan <tim@timmorgan.org>

---------

https://github.com/ruby/prism/commit/d73a053262

Co-authored-by: Tim Morgan <tim@timmorgan.org>

2 files changed, 39 insertions, 0 deletions

diff --git a/lib/prism/node_ext.rb b/lib/prism/node_ext.rb
index 4febc615c1..5fc13eb06d 100644
--- a/lib/prism/node_ext.rb
+++ b/lib/prism/node_ext.rb
@@ -74,6 +74,14 @@ module Prism
   end
 
   class ConstantPathNode < Node
+    # An error class raised when dynamic parts are found while computing a
+    # constant path's full name. For example:
+    # Foo::Bar::Baz -> does not raise because all parts of the constant path are
+    # simple constants
+    # var::Bar::Baz -> raises because the first part of the constant path is a
+    # local variable
+    class DynamicPartsInConstantPathError < StandardError; end
+
     # Returns the list of parts for the full name of this constant path.
     # For example: [:Foo, :Bar]
     def full_name_parts
@@ -85,6 +93,10 @@ module Prism
         current = current.parent
       end
 
+      unless current.is_a?(ConstantReadNode)
+        raise DynamicPartsInConstantPathError, "Constant path contains dynamic parts. Cannot compute full name"
+      end
+
       parts.unshift(current&.name || :"")
     end
 
diff --git a/test/prism/constant_path_node_test.rb b/test/prism/constant_path_node_test.rb
index 5bb9830687..9842be0c49 100644
--- a/test/prism/constant_path_node_test.rb
+++ b/test/prism/constant_path_node_test.rb
@@ -15,6 +15,33 @@ module Prism
       assert_equal("Foo::Bar::Baz::Qux", constant_path.full_name)
     end
 
+    def test_full_name_for_constant_path_with_self
+      source = <<~RUBY
+        self:: # comment
+          Bar::Baz::
+            Qux
+      RUBY
+
+      constant_path = Prism.parse(source).value.statements.body.first
+      assert_raise(ConstantPathNode::DynamicPartsInConstantPathError) do
+        constant_path.full_name
+      end
+    end
+
+    def test_full_name_for_constant_path_with_variable
+      source = <<~RUBY
+        foo:: # comment
+          Bar::Baz::
+            Qux
+      RUBY
+
+      constant_path = Prism.parse(source).value.statements.body.first
+
+      assert_raise(ConstantPathNode::DynamicPartsInConstantPathError) do
+        constant_path.full_name
+      end
+    end
+
     def test_full_name_for_constant_path_target
       source = <<~RUBY
         Foo:: # comment