Merge pull request #74122 from Azoy/dict-builder-condition

[stdlib] Guard for empty singleton in dictionary builder

Author: Azoy

stdlib/public/core/DictionaryBuilder.swift

@@ -102,6 +102,17 @@ extension _NativeDictionary {
     ) -> Int
   ) {
     self.init(capacity: capacity)
+
+    // If the capacity is 0, then our storage is the empty singleton. Those are
+    // read only, so we shouldn't attempt to write to them.
+    if capacity == 0 {
+      let c = initializer(
+        UnsafeMutableBufferPointer(start: nil, count: 0), 
+        UnsafeMutableBufferPointer(start: nil, count: 0))
+      _precondition(c == 0)
+      return
+    }
+
     let initializedCount = initializer(
       UnsafeMutableBufferPointer(start: _keys, count: capacity),
       UnsafeMutableBufferPointer(start: _values, count: capacity))

validation-test/stdlib/Dictionary.swift

@@ -5735,8 +5735,10 @@ DictionaryTestSuite.test("BulkLoadingInitializer.Unique") {
       _unsafeUninitializedCapacity: c,
       allowingDuplicates: false
     ) { keys, values in
-      let k = keys.baseAddress!
-      let v = values.baseAddress!
+      guard let k = keys.baseAddress, let v = values.baseAddress else {
+        return 0
+      }
+
       for i in 0 ..< c {
         (k + i).initialize(to: TestKeyTy(i))
         (v + i).initialize(to: TestEquatableValueTy(i))
@@ -5762,8 +5764,10 @@ DictionaryTestSuite.test("BulkLoadingInitializer.Nonunique") {
       _unsafeUninitializedCapacity: c,
       allowingDuplicates: true
     ) { keys, values in
-      let k = keys.baseAddress!
-      let v = values.baseAddress!
+      guard let k = keys.baseAddress, let v = values.baseAddress else {
+        return 0
+      }
+
       for i in 0 ..< c {
         (k + i).initialize(to: TestKeyTy(i / 2))
         (v + i).initialize(to: TestEquatableValueTy(i / 2))