fix function η with subsingleton types

golden-tests/tests/eta-singleton/eta-sing.quox

@@ -0,0 +1,33 @@
+-- inspired by https://github.com/agda/agda/issues/2556
+
+postulate0 A : ★
+
+def0 ZZ : ★ = 0 ≡ 0 : ℕ
+
+def reflZ : ZZ = δ _ ⇒ 0
+
+
+namespace erased {
+  def0 ZZA : ★ = 0.ZZ → A
+
+  def propeq : (x : ZZA) → x ≡ (λ _ ⇒ x reflZ) : ZZA =
+    λ x ⇒ δ _ ⇒ x
+
+  def defeq : 0.(P : ZZA → ★) → 0.(x : ZZA) → P (λ _ ⇒ x reflZ) → P x =
+    λ P x p ⇒ p
+}
+
+namespace unrestricted {
+  def0 ZZA : ★ = ω.ZZ → A
+
+  def defeq : 0.(P : ZZA → ★) → 0.(x : ZZA) → P (λ _ ⇒ x reflZ) → P x =
+    λ P x p ⇒ p
+}
+
+namespace linear {
+  def0 ZZA : ★ = 1.ZZ → A
+
+  #[fail]
+  def defeq : 0.(P : ZZA → ★) → 0.(x : ZZA) → P (λ _ ⇒ x reflZ) → P x =
+    λ P x p ⇒ p
+}