LSC: Mark static const class/struct members as constexpr. This change fixes declarations that have initial values but are technically not definitions by marking them constexpr (which counts as a definition). This enables, among other things, the modified constants to be passed into functions and function templates that accept arguments by reference. Without this change, such functions would cause linker errors.

This change was approved as part of go/static-const-lsc.

Tested:
    tap_presubmit
    Some tests failed; test failures are believed to be unrelated to this CL
PiperOrigin-RevId: 307464964

Author: Googler

app/memory/atomic.h

@@ -35,27 +35,27 @@ namespace compat {
 // in length.`
 template <typename T>
 struct CanBeAtomic {
-  static const bool value = false;
+  static constexpr bool value = false;
 };
 
 template <>
 struct CanBeAtomic<int32_t> {
-  static const bool value = true;
+  static constexpr bool value = true;
 };
 
 template <>
 struct CanBeAtomic<uint32_t> {
-  static const bool value = true;
+  static constexpr bool value = true;
 };
 
 template <>
 struct CanBeAtomic<int64_t> {
-  static const bool value = true;
+  static constexpr bool value = true;
 };
 
 template <>
 struct CanBeAtomic<uint64_t> {
-  static const bool value = true;
+  static constexpr bool value = true;
 };
 
 // Provides a minimal atomic counter, required to implement SharedPtr.

app/meta/type_traits.h

@@ -42,27 +42,27 @@ struct remove_reference<T&&> {
 
 template <typename T>
 struct is_array {
-  static const bool value = false;
+  static constexpr bool value = false;
 };
 
 template <typename T>
 struct is_array<T[]> {
-  static const bool value = true;
+  static constexpr bool value = true;
 };
 
 template <typename T, std::size_t N>
 struct is_array<T[N]> {
-  static const bool value = true;
+  static constexpr bool value = true;
 };
 
 template <typename T>
 struct is_lvalue_reference {
-  static const bool value = false;
+  static constexpr bool value = false;
 };
 
 template <typename T>
 struct is_lvalue_reference<T&> {
-  static const bool value = true;
+  static constexpr bool value = true;
 };
 
 // NOLINTNEXTLINE - allow namespace overridden

app/src/include/firebase/variant.h

@@ -1139,7 +1139,7 @@ class Variant {
     char small_string[sizeof(BlobValue)];
   } value_;
 
-  static const size_t kMaxSmallStringSize = sizeof(Value::small_string);
+  static constexpr size_t kMaxSmallStringSize = sizeof(Value::small_string);
 
   friend class firebase::internal::VariantInternal;
 };

app/src/reference_counted_future_impl.h

@@ -100,18 +100,18 @@ class ReferenceCountedFutureImpl : public detail::FutureApiInterface {
  public:
   /// This handle will never be returned by @ref Alloc, so you can use it
   /// to signify an uninitialized or invalid value.
-  static const FutureHandle kInvalidHandle = kInvalidFutureHandle;
+  static constexpr FutureHandle kInvalidHandle = kInvalidFutureHandle;
 
   /// Returned by @ref GetFutureError when the passed in handle is invalid.
-  static const int kErrorFutureIsNoLongerValid = -1;
+  static constexpr int kErrorFutureIsNoLongerValid = -1;
 
   /// Returned by @ref GetFutureErrorMessage when the passed in handle is
   /// invalid.
   static const char kErrorMessageFutureIsNoLongerValid[];
 
   /// Pass into @ref Alloc for `fn_idx` when you don't want to update any
   /// function.
-  static const int kNoFunctionIndex = -1;
+  static constexpr int kNoFunctionIndex = -1;
 
   explicit ReferenceCountedFutureImpl(size_t last_result_count)
       : next_future_handle_(kInvalidHandle + 1),

database/src/desktop/push_child_name_generator.h

@@ -32,10 +32,11 @@ class PushChildNameGenerator {
 
  private:
   static const char* const kPushChars;
-  static const int kNumPushChars = 64;
-  static const int kNumTimestampChars = 8;
-  static const int kNumRandomChars = 12;
-  static const int kGeneratedNameLength = kNumTimestampChars + kNumRandomChars;
+  static constexpr int kNumPushChars = 64;
+  static constexpr int kNumTimestampChars = 8;
+  static constexpr int kNumRandomChars = 12;
+  static constexpr int kGeneratedNameLength =
+      kNumTimestampChars + kNumRandomChars;
 
   // For random number generation.
   std::random_device random_device_;