Fix #2048.

CHANGELOG.md

@@ -15,6 +15,9 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 ### Fixed
 
+* Incorrect type checking of `let` binding with explicit size
+  quantification, where size appears in type of body (#2048).
+
 ## [0.25.9]
 
 ### Added

src/Language/Futhark/TypeChecker/Terms.hs

@@ -631,7 +631,7 @@ checkExp (AppExp (LetPat sizes pat e body loc) _) = do
                 let f x = if x == v then Just (ExpSubst e') else Nothing
                 pure (applySubst f body_t, [])
           _ ->
-            unscopeType loc (patNames pat') body_t
+            unscopeType loc (map sizeName sizes' <> patNames pat') body_t
 
       pure $
         AppExp

tests/issue2048.fut

@@ -0,0 +1,9 @@
+-- ==
+-- input { [1,2,3] }
+-- output { 3i64 }
+
+def f [n] (xs: [n]i32) =
+  let [m] (ys: [m]i32) = filter (>0) xs
+  in ys
+
+entry main xs = length (f xs)