diff --git a/glean.cabal.in b/glean.cabal.in
index 510885bdf..bedd3f379 100644
--- a/glean.cabal.in
+++ b/glean.cabal.in
@@ -56,7 +56,7 @@ common fb-haskell
        ghc-options: -O2
 
 common fb-cpp
-  cxx-options: -std=c++17 -Wno-nullability-completeness -fno-omit-frame-pointer
+  cxx-options: -std=c++20 -Wno-nullability-completeness -fno-omit-frame-pointer
   if !flag(clang)
      cxx-options: -fcoroutines
   if flag(asan)
@@ -202,7 +202,7 @@ common deps
         hinotify ^>= 0.4.1
 
 common hsc2hs-cpp
-    hsc2hs-options: --cc=g++ --lflag=-lstdc++ --cflag=-D__HSC2HS__=1 --cflag=-std=c++17
+    hsc2hs-options: --cc=g++ --lflag=-lstdc++ --cflag=-D__HSC2HS__=1 --cflag=-std=c++20
 
 common thrift-server
     if flag(fbthrift)
diff --git a/glean/db/Glean/Query/Reorder.hs b/glean/db/Glean/Query/Reorder.hs
index 4d6b17573..75837d89e 100644
--- a/glean/db/Glean/Query/Reorder.hs
+++ b/glean/db/Glean/Query/Reorder.hs
@@ -929,7 +929,9 @@ toCgStatement stmt = case stmt of
     lhs' <- fixVars IsPat lhs
     return [CgStatement lhs' gen']
   FlatAllStatement v e g -> do
-    cg <- withScopeFor (scopeVars g <> vars e) $ do
+    -- withinNegation enforces that bindings within the all are local
+    -- to this scope and not visible outside.
+    cg <- withinNegation $ withScopeFor (scopeVars g <> vars e) $ do
       stmts <- reorderGroup g
       e' <- fixVars IsExpr e
       return [CgAllStatement v e' stmts]
diff --git a/glean/db/Glean/Query/Vars.hs b/glean/db/Glean/Query/Vars.hs
index 79671c59b..7065497d3 100644
--- a/glean/db/Glean/Query/Vars.hs
+++ b/glean/db/Glean/Query/Vars.hs
@@ -29,6 +29,7 @@ import Data.IntSet (IntSet)
 import qualified Data.IntMap as IntMap
 import Data.IntMap (IntMap)
 import Data.List.NonEmpty (NonEmpty)
+import Data.Maybe
 
 import Glean.Display
 import Glean.Query.Codegen.Types
@@ -185,29 +186,28 @@ reWildGenerator used gen = case gen of
   PrimCall op args ty ->
     PrimCall op (map (reWild used) args) ty
 
-reWildStatement :: VarMap -> CgStatement -> CgStatement
+reWildStatement :: VarMap -> CgStatement -> Maybe CgStatement
 reWildStatement used (CgStatement lhs rhs) =
-  CgStatement (reWild used lhs) (reWildGenerator used rhs)
-reWildStatement used s@(CgAllStatement (Var ty n x) expr stmts) =
+  Just $ CgStatement (reWild used lhs) (reWildGenerator used rhs)
+reWildStatement used (CgAllStatement (Var ty n x) expr stmts) =
   case IntMap.lookup n used of
-    Nothing -> error $
-      "reWildStatement: var " <> show n <>
-      " not in scope in " <> show (displayVerbose s) <>
-      "\nVarMap: " <> show used
+    Nothing ->
+      -- the variable isn't used, so the statement has no effect
+      Nothing
     Just new ->
-      CgAllStatement
+      Just $ CgAllStatement
         (Var ty new x) (reWild used expr)
-        (map (reWildStatement used) stmts)
+        (mapMaybe (reWildStatement used) stmts)
 reWildStatement used (CgNegation stmts) =
-  CgNegation (map (reWildStatement used) stmts)
+  Just $ CgNegation (mapMaybe (reWildStatement used) stmts)
 reWildStatement used (CgDisjunction stmtss) =
-  CgDisjunction (map (map (reWildStatement used)) stmtss)
+  Just $ CgDisjunction (map (mapMaybe (reWildStatement used)) stmtss)
 reWildStatement used (CgConditional cond then_ else_) =
-  CgConditional
-    (map (reWildStatement used) cond)
-    (map (reWildStatement used) then_)
-    (map (reWildStatement used) else_)
+  Just $ CgConditional
+    (mapMaybe (reWildStatement used) cond)
+    (mapMaybe (reWildStatement used) then_)
+    (mapMaybe (reWildStatement used) else_)
 
 reWildQuery :: VarMap -> CgQuery -> CgQuery
 reWildQuery used (CgQuery head stmts) =
-  CgQuery (reWild used head) (map (reWildStatement used) stmts)
+  CgQuery (reWild used head) (mapMaybe (reWildStatement used) stmts)
diff --git a/glean/lang/clang/glean-clang.cabal b/glean/lang/clang/glean-clang.cabal
index 6abeab69f..d24339c32 100644
--- a/glean/lang/clang/glean-clang.cabal
+++ b/glean/lang/clang/glean-clang.cabal
@@ -103,7 +103,7 @@ common fb-haskell
       ghc-options: -O2
 
 common fb-cpp
-  cxx-options: -DOSS=1 -std=c++17 -Wno-nullability-completeness
+  cxx-options: -DOSS=1 -std=c++20 -Wno-nullability-completeness
   if arch(x86_64)
       cxx-options: -DGLEAN_X86_64 -march=haswell
   if flag(opt)
@@ -142,11 +142,7 @@ executable clang-index
         gflags,
         atomic,
         re2
-    cxx-options: -fexceptions -DOSS=1 -std=c++17
-    if arch(x86_64)
-      cxx-options: -DGLEAN_X86_64 -march=haswell
-    if flag(opt)
-      cxx-options: -O3
+    cxx-options: -fexceptions
 
 executable clang-derive
     import: fb-haskell, fb-cpp, deps, exe
diff --git a/glean/test/tests/Angle/SetTest.hs b/glean/test/tests/Angle/SetTest.hs
index 742f8c670..9c2e1de41 100644
--- a/glean/test/tests/Angle/SetTest.hs
+++ b/glean/test/tests/Angle/SetTest.hs
@@ -36,7 +36,7 @@ import Glean.Typed.Binary
 import Glean.Types
 
 import qualified Data.HashMap.Strict as HashMap
-import Data.Set
+import Data.Set as Set
 
 import Test.HUnit
 import TestRunner
@@ -157,6 +157,15 @@ setSemanticsTest dbTestCase = TestList
       [set] <- runQuery_ env repo $ angleData @(Set Glean.Test.Predicate)
         [s| all (glean.test.Predicate _) |]
       assertEqual "angle - set matching" 4 (size set)
+  , TestLabel "unused all" $ dbTestCase $ \env repo -> do
+      r <- runQuery_ env repo $ angleData @Nat
+        [s| X where _ = all (X = 1); X = 0 |]
+      assertEqual "unused all" r [Nat 0]
+  , TestLabel "local constraint" $ dbTestCase $ \env repo -> do
+      r <- runQuery_ env repo $ angleData @(Set Nat)
+        [s| all (X where X = 1) where X = 1|2 |]
+      assertEqual "local constraint"
+        (sort r) [Set.empty, Set.fromList [Nat 1]]
   ]
 
 setLimitTest :: Test