explicitly convert to `InternalSizeType` in F14

Summary:
For F14Vector containers, we assume that the size could fit in `uint32_t`,
i.e., the `InternalSizeType`. This diff fixes where we implicitly convert
(shorten) a `size_t` to `InternalSizeType`.

Reviewed By: nbronson

fbshipit-source-id: e970071dbde6394262a555facc8e2a24c6007f68

folly/container/detail/F14Policy.h

@@ -1115,7 +1115,8 @@ class VectorContainerPolicy : public BasePolicy<
   template <typename... Args>
   void constructValueAtItem(std::size_t size, Item* itemAddr, Args&&... args) {
     Alloc& a = this->alloc();
-    *itemAddr = size;
+    FOLLY_SAFE_DCHECK(size < std::numeric_limits<InternalSizeType>::max(), "");
+    *itemAddr = static_cast<InternalSizeType>(size);
     auto dst = std::addressof(values_[size]);
     // TODO(T31574848): clean up assume-s used to optimize placement new
     assume(dst != nullptr);

folly/container/detail/F14Table.h

@@ -1320,7 +1320,7 @@ class F14Table : public Policy {
     return size() == 0;
   }
 
-  std::size_t size() const noexcept {
+  typename Policy::InternalSizeType size() const noexcept {
     return sizeAndPackedBegin_.size_;
   }
 
@@ -1810,7 +1810,13 @@ class F14Table : public Policy {
         rawAllocation);
     chunks_ =
         initializeChunks(rawAllocation, newChunkCount, newMaxSizeWithoutRehash);
-    chunkMask_ = newChunkCount - 1;
+
+    FOLLY_SAFE_DCHECK(
+        newChunkCount <
+            std::numeric_limits<typename Policy::InternalSizeType>::max(),
+        "");
+    chunkMask_ =
+        static_cast<typename Policy::InternalSizeType>(newChunkCount - 1);
 
     bool success = false;
     SCOPE_EXIT {
@@ -1829,7 +1835,12 @@ class F14Table : public Policy {
         finishedAllocSize =
             chunkAllocSize(newChunkCount, newMaxSizeWithoutRehash);
         chunks_ = origChunks;
-        chunkMask_ = origChunkCount - 1;
+        FOLLY_SAFE_DCHECK(
+            origChunkCount <
+                std::numeric_limits<typename Policy::InternalSizeType>::max(),
+            "");
+        chunkMask_ =
+            static_cast<typename Policy::InternalSizeType>(origChunkCount - 1);
       }
 
       this->afterRehash(