Fix #2210.

CHANGELOG.md

@@ -21,6 +21,9 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 * Handling of module-dependent size expressions in type abbreviations
   (#2209).
 
+* A `let`-bound size would mistakenly be in scope of the bound
+  expression (#2210).
+
 ## [0.25.26]
 
 ### Fixed

src/Language/Futhark/TypeChecker/Names.hs

@@ -338,8 +338,8 @@ resolveAppExp (Match e cases loc) =
       resolvePat p $ \p' -> CasePat p' <$> resolveExp body <*> pure cloc
 resolveAppExp (LetPat sizes p e1 e2 loc) = do
   checkForDuplicateNames (map sizeBinderToParam sizes) [p]
+  e1' <- resolveExp e1
   resolveSizes sizes $ \sizes' -> do
-    e1' <- resolveExp e1
     resolvePat p $ \p' -> do
       e2' <- resolveExp e2
       pure $ LetPat sizes' p' e1' e2' loc

tests/shapes/letshape12.fut

@@ -0,0 +1,7 @@
+-- #2210
+-- ==
+-- error: Unknown name "n"
+
+def f : [42]f32 =
+  let [n] turtle: f32 -> [n]f32 = \(x: f32) -> replicate n x
+  in turtle 42