Make mutable CompressedTuple::get() constexpr.

This change makes the mutable overloads of CompressedTuple::get() constexpr.
This is consistent with std::get(std::tuple), which is constexpr since C++14.

PiperOrigin-RevId: 648603141
Change-Id: Icbd61809f7a06723cf581dbed5488b7bae998cc9

absl/container/internal/compressed_tuple.h

@@ -89,9 +89,9 @@ struct Storage {
   explicit constexpr Storage(absl::in_place_t, V&& v)
       : value(std::forward<V>(v)) {}
   constexpr const T& get() const& { return value; }
-  T& get() & { return value; }
+  constexpr T& get() & { return value; }
   constexpr const T&& get() const&& { return std::move(*this).value; }
-  T&& get() && { return std::move(*this).value; }
+  constexpr T&& get() && { return std::move(*this).value; }
 };
 
 template <typename T, size_t I>
@@ -102,9 +102,9 @@ struct ABSL_INTERNAL_COMPRESSED_TUPLE_DECLSPEC Storage<T, I, true> : T {
   explicit constexpr Storage(absl::in_place_t, V&& v) : T(std::forward<V>(v)) {}
 
   constexpr const T& get() const& { return *this; }
-  T& get() & { return *this; }
+  constexpr T& get() & { return *this; }
   constexpr const T&& get() const&& { return std::move(*this); }
-  T&& get() && { return std::move(*this); }
+  constexpr T&& get() && { return std::move(*this); }
 };
 
 template <typename D, typename I, bool ShouldAnyUseBase>
@@ -237,7 +237,7 @@ class ABSL_INTERNAL_COMPRESSED_TUPLE_DECLSPEC CompressedTuple
                                              std::forward<Vs>(base)...) {}
 
   template <int I>
-  ElemT<I>& get() & {
+  constexpr ElemT<I>& get() & {
     return StorageT<I>::get();
   }
 
@@ -247,7 +247,7 @@ class ABSL_INTERNAL_COMPRESSED_TUPLE_DECLSPEC CompressedTuple
   }
 
   template <int I>
-  ElemT<I>&& get() && {
+  constexpr ElemT<I>&& get() && {
     return std::move(*this).StorageT<I>::get();
   }
 

absl/container/internal/compressed_tuple_test.cc

@@ -31,14 +31,22 @@
 
 // These are declared at global scope purely so that error messages
 // are smaller and easier to understand.
-enum class CallType { kConstRef, kConstMove };
+enum class CallType { kMutableRef, kConstRef, kMutableMove, kConstMove };
 
 template <int>
 struct Empty {
+  constexpr CallType value() & { return CallType::kMutableRef; }
   constexpr CallType value() const& { return CallType::kConstRef; }
+  constexpr CallType value() && { return CallType::kMutableMove; }
   constexpr CallType value() const&& { return CallType::kConstMove; }
 };
 
+// Unconditionally return an lvalue reference to `t`.
+template <typename T>
+constexpr T& AsLValue(T&& t) {
+  return t;
+}
+
 template <typename T>
 struct NotEmpty {
   T value;
@@ -375,8 +383,24 @@ TEST(CompressedTupleTest, Constexpr) {
     constexpr int value() const { return v; }
     int v;
   };
-  constexpr CompressedTuple<int, double, CompressedTuple<int>, Empty<0>> x(
-      7, 1.25, CompressedTuple<int>(5), {});
+
+  using Tuple = CompressedTuple<int, double, CompressedTuple<int>, Empty<0>>;
+
+  constexpr int r0 =
+      AsLValue(Tuple(1, 0.75, CompressedTuple<int>(9), {})).get<0>();
+  constexpr double r1 =
+      AsLValue(Tuple(1, 0.75, CompressedTuple<int>(9), {})).get<1>();
+  constexpr int r2 =
+      AsLValue(Tuple(1, 0.75, CompressedTuple<int>(9), {})).get<2>().get<0>();
+  constexpr CallType r3 =
+      AsLValue(Tuple(1, 0.75, CompressedTuple<int>(9), {})).get<3>().value();
+
+  EXPECT_EQ(r0, 1);
+  EXPECT_EQ(r1, 0.75);
+  EXPECT_EQ(r2, 9);
+  EXPECT_EQ(r3, CallType::kMutableRef);
+
+  constexpr Tuple x(7, 1.25, CompressedTuple<int>(5), {});
   constexpr int x0 = x.get<0>();
   constexpr double x1 = x.get<1>();
   constexpr int x2 = x.get<2>().get<0>();
@@ -387,6 +411,18 @@ TEST(CompressedTupleTest, Constexpr) {
   EXPECT_EQ(x2, 5);
   EXPECT_EQ(x3, CallType::kConstRef);
 
+  constexpr int m0 = Tuple(5, 0.25, CompressedTuple<int>(3), {}).get<0>();
+  constexpr double m1 = Tuple(5, 0.25, CompressedTuple<int>(3), {}).get<1>();
+  constexpr int m2 =
+      Tuple(5, 0.25, CompressedTuple<int>(3), {}).get<2>().get<0>();
+  constexpr CallType m3 =
+      Tuple(5, 0.25, CompressedTuple<int>(3), {}).get<3>().value();
+
+  EXPECT_EQ(m0, 5);
+  EXPECT_EQ(m1, 0.25);
+  EXPECT_EQ(m2, 3);
+  EXPECT_EQ(m3, CallType::kMutableMove);
+
   constexpr CompressedTuple<Empty<0>, TrivialStruct, int> trivial = {};
   constexpr CallType trivial0 = trivial.get<0>().value();
   constexpr int trivial1 = trivial.get<1>().value();