pw_vector: Add constexpr max_size() for fixed-sized vector

This change introduces the max_size() function for the
fixed-sized vector implementation. By declaring it as
static constexpr we can use the max_size of a
fixed-size vector to size another vector at compile time.
If only the generic vector gets passed to a function, the
generic max_size function gets called, that won't return
the constexpr max size value.

Tested: Added ConstexprMaxSize test case

Change-Id: I45ea288d756641e108103c4e0ce958759876e32a
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/122891
Reviewed-by: Wyatt Hepler <[email protected]>
Commit-Queue: Christoph Klee <[email protected]>

pw_containers/public/pw_containers/vector.h

@@ -114,6 +114,8 @@ class Vector : public Vector<T, vector_impl::kGeneric> {
   Vector(std::initializer_list<T> list)
       : Vector<T, vector_impl::kGeneric>(kMaxSize, list) {}
 
+  static constexpr size_t max_size() { return kMaxSize; }
+
   // Construct from std::string_view when T is char.
   template <typename U = T,
             typename = std::enable_if_t<std::is_same_v<U, char>>>

pw_containers/vector_test.cc

@@ -488,6 +488,19 @@ TEST(Vector, Generic) {
   }
 }
 
+TEST(Vector, ConstexprMaxSize) {
+  Vector<int, 10> vector;
+  Vector<int, vector.max_size()> vector2;
+
+  EXPECT_EQ(vector.max_size(), vector2.max_size());
+
+  // The following code would fail with the following compiler error:
+  // "non-type template argument is not a constant expression"
+  // Reason: the generic_vector doesn't return a constexpr max_size value.
+  // Vector<int>& generic_vector(vector);
+  // Vector<int, generic_vector.max_size()> vector3;
+}
+
 // Test that Vector<T> is trivially destructible when its type is.
 static_assert(std::is_trivially_destructible_v<Vector<int>>);
 static_assert(std::is_trivially_destructible_v<Vector<int, 4>>);