Prerequisites

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Description

During iota-reduction, terms of structures that live in Prop do not get eta-expanded. This was introduced in #1033 in order to avoid constructing primitive projections for terms that shouldn't have them (i.e existentials). However, the criteria applied is too strong, and blocks the reduction of subsingletons that could get eta-expanded otherwise. This, among other things, break both the transitivity and congruence of definitional equality.

Context

This issue has been reported by a few different users on Zulip: here, here and here

Steps to Reproduce

variable (P Q : Prop) (A : Type) (f : P → Q → A) (x : P ∧ Q)

example : And.rec f ⟨x.1,x.2⟩ = f (And.left x) (And.right x) := rfl
example : (And.rec f ⟨x.1,x.2⟩ : A) = And.rec f x  := rfl

Expected behavior: both examples work

Actual behavior: The first one works, the second fails.

Versions

4.5.0-rc1
Web Browser

Additional Information

Impact

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