allow matching at 0 where appropriate

(for pairs, and for enums with 0 or 1 constructors)

lib/Quox/Typechecker.idr

@@ -305,8 +305,9 @@ parameters {auto _ : IsQty q} {auto _ : CanTC q m}
     }
 
   infer' ctx sg (CasePair pi pair ret body) = do
-    -- if 1 ≤ π
-    expectCompatQ one pi
+    -- no check for 1 ≤ π, since pairs have a single constructor.
+    -- e.g. at 0 the components are also 0 in the body
+    --
     -- if Ψ | Γ ⊢ σ · pair ⇒ (x : A) × B ⊳ Σ₁
     pairres <- inferC ctx sg pair
     -- if Ψ | Γ, p : (x : A) × B ⊢₀ ret ⇐ Type
@@ -327,11 +328,11 @@ parameters {auto _ : IsQty q} {auto _ : CanTC q m}
     }
 
   infer' ctx sg (CaseEnum pi t ret arms) {d, n} = do
-    -- if 1 ≤ π
-    expectCompatQ one pi
     -- if Ψ | Γ ⊢ σ · t ⇒ {ts} ⊳ Σ₁
     tres <- inferC ctx sg t
     ttags <- expectEnum !ask ctx tres.type
+    -- if 1 ≤ π, OR there is only zero or one option
+    unless (length (SortedSet.toList ttags) <= 1) $ expectCompatQ one pi
     -- if Ψ | Γ, x : {ts} ⊢₀ A ⇐ Type
     checkTypeC (extendTy zero ret.name tres.type ctx) ret.term Nothing
     -- if for each "a ⇒ s" in arms,