(No equational lemmas)/(Mistaken as field notation) when using a `namespace` inside a `section` with universe polymorphism

Prerequisites

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    • Specifically, check out the wishlist, open RFCs,
      or feature requests.
  • Reduced the issue to a self-contained, reproducible test case.

Description

Allowing a namespace inside a section can cause compile issues.

Steps to Reproduce

1. Compile the following code:

   section
     universes u
   
     inductive my_unit : Type u
     | foo
   
     namespace my_unit
       def one : ℕ := 1
     end my_unit
   
     lemma test : my_unit.one = 1 := by refl
   end
   

Expected behavior: Code compiles, in particular, my_unit.one isn't treated as field notation.

Actual behavior: Obtain error message:

invalid field notation, type is not of the form (C ...) where C is a constant
  my_unit
has type
  Type u

Reproduces how often: 100%

Versions

• Lean (version 3.32.1, commit 35b3a9c4e2d3, Release) on cygwin.

Additional Information

If you rephrase the code, you can also get another issue which I think is related:

section
  universes u

  inductive my_unit : Type u
  | foo

  def my_unit.one (_ : my_unit) : ℕ := 1

  lemma test : my_unit.one my_unit.foo = 1 := by unfold my_unit.one
end

gives:

unfold tactic failed, my_unit.one does not have equational lemmas nor is a projection
state:
⊢ my_unit.foo.one = 1

There some small adjustments I've been able to make to get either example to compile. I think the second and third qualifies as a workaround.

• Don't use universe polymorphism.

  section
    inductive my_unit : Type
    | foo
  
    namespace my_unit
      def one : ℕ := 1
    end my_unit
  
    lemma test : my_unit.one = 1 := by refl
  end
  

• Move the namespace out of the section.

  section
    universes u
  
    inductive my_unit : Type u
    | foo
  end
  
  namespace my_unit
    universes u
    def one : ℕ := 1
    -- other code which uses `u`
  end my_unit
  
  section
    universes u
    lemma test : my_unit.one = 1 := by refl
  end
  

• Use _root_.

  section
    universes u
  
    inductive my_unit : Type u
    | foo
  
    namespace my_unit
      def one : ℕ := 1
    end my_unit
  
    lemma test : _root_.my_unit.one = 1 := by refl
  end
  

Hopefully I haven't made any mistakes or missed an issue/PR, I couldn't find anything related when searching for both "namespace" and "section".

Start by reproducing both minimal examples with Lean 3.32.1, focusing on the namespace inside a universe-polymorphic section and the resulting field-notation or equational-lemma errors. Compare the listed workarounds, including moving the namespace and using _root_. Done means the original examples compile, my_unit.one resolves as a name, and the second example can be unfolded.