More MSVC Warnings Fixes

Summary: This fixes up a couple more MSVC warnings I ran into when trying to update the OSS React Native Windows build to a newer version of Folly. Specifically, another instance of C4146 "unary minus operator applied to unsigned type, result still unsigned", and a couple instances of C4244 "conversion from 'type1' to 'type2', possible loss of data"

Differential Revision: D50383545

folly/ConstexprMath.h

@@ -459,7 +459,10 @@ constexpr T constexpr_mult(T const a, T const b) {
 
 namespace detail {
 
-template <typename T, typename E>
+template <
+    typename T,
+    typename E,
+    std::enable_if_t<std::is_signed<E>::value, int> = 1>
 constexpr T constexpr_ipow(T const base, E const exp) {
   if (std::is_floating_point<T>::value) {
     if (exp < E(0)) {
@@ -485,6 +488,31 @@ constexpr T constexpr_ipow(T const base, E const exp) {
   return constexpr_mult(constexpr_mult(div, div), rem);
 }
 
+template <
+    typename T,
+    typename E,
+    std::enable_if_t<std::is_unsigned<E>::value, int> = 1>
+constexpr T constexpr_ipow(T const base, E const exp) {
+  if (std::is_floating_point<T>::value) {
+    if (exp == E(0)) {
+      return T(1);
+    }
+    if (constexpr_isnan(base)) {
+      return base;
+    }
+  }
+  if (exp == E(0)) {
+    return T(1);
+  }
+  if (exp == E(1)) {
+    return base;
+  }
+  auto const hexp = constexpr_trunc(exp / E(2));
+  auto const div = constexpr_ipow(base, hexp);
+  auto const rem = hexp * E(2) == exp ? T(1) : base;
+  return constexpr_mult(constexpr_mult(div, div), rem);
+}
+
 } // namespace detail
 
 /// constexpr_exp

folly/concurrency/CacheLocality.cpp

@@ -467,7 +467,8 @@ class SimpleAllocator {
       // leave pointers that could hide leaks at shutdown, since the backing
       // slabs may not be deallocated if the instance is a leaky singleton.
       auto* base = static_cast<char*>(ptr);
-      std::fill(base + sizeof(void*), base + allocator->sz_, 0);
+      std::fill(
+          base + sizeof(void*), base + allocator->sz_, static_cast<char>(0));
     }
     allocator->freelist_ = ptr;
   }

folly/hash/Hash.h

@@ -528,7 +528,7 @@ struct integral_hasher {
       auto const u = to_unsigned(i);
       auto const hi = static_cast<uint64_t>(u >> sizeof(Int) * 4);
       auto const lo = static_cast<uint64_t>(u);
-      return hash::hash_128_to_64(hi, lo);
+      return static_cast<size_t>(hash::hash_128_to_64(hi, lo));
     }
   }
 };