Add strictly checked numeric types

base/numerics/safe_conversions.h

Index: base/numerics/safe_conversions.h
diff --git a/base/numerics/safe_conversions.h b/base/numerics/safe_conversions.h
index d9b77f76d50700bd0686de12ae22390d341776ad..5dd51919595b95d56bae3525fa5a184f5393fc0e 100644
--- a/base/numerics/safe_conversions.h
+++ b/base/numerics/safe_conversions.h
@@ -58,6 +58,68 @@ inline Dst saturated_cast(Src value) {
   return static_cast<Dst>(value);
 }
 
+// strict_cast<> is analogous to static_cast<> for numeric types, except that
+// it will cause a compile failure if the destination type is not large enough
+// to contain any value in the source type. It performs no runtime checking.
+template <typename Dst, typename Src>
+inline Dst strict_cast(Src value) {
+  static_assert(std::numeric_limits<Src>::is_specialized,
+                "Argument must be numeric.");
+  static_assert(std::numeric_limits<Dst>::is_specialized,
+                "Result must be numeric.");
+  static_assert((internal::StaticDstRangeRelationToSrcRange<Dst, Src>::value ==
+                 internal::NUMERIC_RANGE_CONTAINED),
+                "The numeric conversion is out of range for this type. You "
+                "should probably use one of the following conversion "
+                "mechanisms on the value you want to pass:\n"
+                "- base::checked_cast\n"
+                "- base::saturated_cast\n"
+                "- base::CheckedNumeric");
+
+  return static_cast<Dst>(value);
+}
+
+// StrictNumeric implements compile time range checking between numeric types by
+// wrapping assignment operations in a strict_cast. This class is intended to be
+// used for function arguments and return types, to ensure the destination type
+// can always contain the source type. This is essentially the same as enforcing
+// -Wconversion in gcc and C4302 warnings on MSVC, but it can be applied
+// incrementally at API boundaries, making it easier to convert code so that it
+// compiles cleanly with truncation warnings enabled.
+// This template should introduce no runtime overhead, but it also provides no
+// runtime checking of any of the associated mathematical operations. Use
+// CheckedNumeric for runtime range checks of tha actual value being assigned.
+template <typename T>
+class StrictNumeric {
+ public:
+  typedef T type;
+
+  StrictNumeric() : value_(0) {}
+
+  // Copy constructor.
+  template <typename Src>
+  StrictNumeric(const StrictNumeric<Src>& rhs)
+      : value_(strict_cast<T>(rhs.value_)) {}
+
+  // This is not an explicit constructor because we implicitly upgrade regular
+  // numerics to StrictNumerics to make them easier to use.
+  template <typename Src>
+  StrictNumeric(Src value)
+      : value_(strict_cast<T>(value)) {}
+
+  // The numeric cast operator basically handles all the magic.
+  template <typename Dst>
+  operator Dst() const {
+    return strict_cast<Dst>(value_);
+  }
+
+ private:
+  T value_;
+};
+
+// Explicitly make a shorter size_t typedef for convenience.
+typedef StrictNumeric<size_t> SizeT;
+
 }  // namespace base
 
 #endif  // BASE_NUMERICS_SAFE_CONVERSIONS_H_