diff --git a/Sources/Core/AST/AST+Walk.swift b/Sources/Core/AST/AST+Walk.swift
index 70110bae1..6c26d9256 100644
--- a/Sources/Core/AST/AST+Walk.swift
+++ b/Sources/Core/AST/AST+Walk.swift
@@ -20,7 +20,7 @@ import Utils
 ///       }
 ///     }
 ///     var v = V()
-///     ast.modules.forEach({ ast.walk($0, notifying: &v) })
+///     for m in ast.modules { ast.walk(m, notifying: &v) }
 ///     print(v.outermostFunctions)
 ///
 /// This program prints the IDs of the outermost function declarations in `ast`.
@@ -222,7 +222,7 @@ extension AST {
   /// Visits all nodes in `roots` and their children in pre-order, notifying `o` when a node is
   /// entered or left.
   public func walk<T: NodeIDProtocol, O: ASTWalkObserver>(roots: [T], notifying o: inout O) {
-    roots.forEach({ walk(AnyNodeID($0), notifying: &o) })
+    for r in roots { walk(AnyNodeID(r), notifying: &o) }
   }
 
   /// Visits all condition items in `i` and their children in pre-order, notifying `o` when a node
@@ -806,7 +806,7 @@ extension AST {
 
   /// Visits the children of `c` in pre-order, notifying `o` when a node is entered or left.
   public func traverse<O: ASTWalkObserver>(whereClause c: WhereClause, notifying o: inout O) {
-    c.constraints.forEach({ traverse(constraintExpr: $0.value, notifying: &o) })
+    for k in c.constraints { traverse(constraintExpr: k.value, notifying: &o) }
   }
 
   /// Visits the children of `c` in pre-order, notifying `o` when a node is entered or left.
@@ -816,7 +816,7 @@ extension AST {
     switch c {
     case .conformance(let lhs, let traits):
       walk(lhs, notifying: &o)
-      traits.forEach({ walk($0, notifying: &o) })
+      for t in traits { walk(t, notifying: &o) }
     case .equality(let lhs, let rhs):
       walk(lhs, notifying: &o)
       walk(rhs, notifying: &o)
diff --git a/Sources/Core/Types/LambdaType.swift b/Sources/Core/Types/LambdaType.swift
index caa7fb749..f882c9681 100644
--- a/Sources/Core/Types/LambdaType.swift
+++ b/Sources/Core/Types/LambdaType.swift
@@ -30,7 +30,7 @@ public struct LambdaType: TypeProtocol {
     self.output = output
 
     var fs = environment.flags
-    inputs.forEach({ fs.merge($0.type.flags) })
+    for i in inputs { fs.merge(i.type.flags) }
     fs.merge(output.flags)
     flags = fs
   }
diff --git a/Sources/Core/Types/MethodType.swift b/Sources/Core/Types/MethodType.swift
index bbdc58620..f964455c6 100644
--- a/Sources/Core/Types/MethodType.swift
+++ b/Sources/Core/Types/MethodType.swift
@@ -30,7 +30,7 @@ public struct MethodType: TypeProtocol {
     self.output = output
 
     var fs = receiver.flags
-    inputs.forEach({ fs.merge($0.type.flags) })
+    for i in inputs { fs.merge(i.type.flags) }
     fs.merge(output.flags)
     flags = fs
   }
diff --git a/Sources/Core/Types/SubscriptImplType.swift b/Sources/Core/Types/SubscriptImplType.swift
index 123f56fdd..fafecb69b 100644
--- a/Sources/Core/Types/SubscriptImplType.swift
+++ b/Sources/Core/Types/SubscriptImplType.swift
@@ -33,7 +33,7 @@ public struct SubscriptImplType: TypeProtocol {
     self.output = output
 
     var fs = environment.flags
-    inputs.forEach({ fs.merge($0.type.flags) })
+    for i in inputs { fs.merge(i.type.flags) }
     fs.merge(output.flags)
     flags = fs
   }
diff --git a/Sources/Core/Types/SubscriptType.swift b/Sources/Core/Types/SubscriptType.swift
index 657537f14..52165df69 100644
--- a/Sources/Core/Types/SubscriptType.swift
+++ b/Sources/Core/Types/SubscriptType.swift
@@ -35,7 +35,7 @@ public struct SubscriptType: TypeProtocol {
     self.output = output
 
     var fs = environment.flags
-    inputs.forEach({ fs.merge($0.type.flags) })
+    for i in inputs { fs.merge(i.type.flags) }
     fs.merge(output.flags)
     flags = fs
   }
diff --git a/Sources/FrontEnd/TypeChecking/TypeChecker.swift b/Sources/FrontEnd/TypeChecking/TypeChecker.swift
index aa186f22b..3aac1b0e4 100644
--- a/Sources/FrontEnd/TypeChecking/TypeChecker.swift
+++ b/Sources/FrontEnd/TypeChecking/TypeChecker.swift
@@ -364,7 +364,7 @@ struct TypeChecker {
 
   /// Type checks the sources in `batch`.
   mutating func check<S: Sequence<TranslationUnit.ID>>(_ batch: S) {
-    batch.forEach({ check($0) })
+    for u in batch { check(u) }
   }
 
   /// Type checks `u` and all declarations nested in `d`.
@@ -386,7 +386,7 @@ struct TypeChecker {
   private mutating func check<S: Sequence<T>, T: DeclID>(
     _ batch: S, ignoringSharedCache ignoreSharedCache: Bool = false
   ) {
-    batch.forEach({ check($0, ignoringSharedCache: ignoreSharedCache) })
+    for d in batch { check(d, ignoringSharedCache: ignoreSharedCache) }
   }
 
   /// Type checks `d` and all declarations nested in `d`.
@@ -565,7 +565,7 @@ struct TypeChecker {
 
   /// Type checks `d` and all declarations nested in `d`.
   private mutating func _check(_ d: ModuleDecl.ID) {
-    program[d].sources.forEach({ check($0) })
+    check(program[d].sources)
   }
 
   /// Type checks `d` and all declarations nested in `d`.
@@ -785,7 +785,7 @@ struct TypeChecker {
 
   /// Type checks `s`.
   private mutating func check(_ s: BraceStmt.ID) {
-    program[s].stmts.forEach({ check($0) })
+    for t in program[s].stmts { check(t) }
   }
 
   /// Type checks `s`.
@@ -1097,7 +1097,7 @@ struct TypeChecker {
 
     /// Appends each variant of `c` to `candidates` that is has type `t`.
     func appendDefinitions(of d: MethodDecl.ID, matching t: AnyType, in s: inout [AnyDeclID]) {
-      program[d].impls.forEach({ appendIfDefinition($0, matching: t, in: &s) })
+      for v in program[d].impls { appendIfDefinition(v, matching: t, in: &s) }
     }
 
     /// Appends `d` to `s` iff it's a a definition with type `t`.