Fix std::hash compilation error for gsl::not_null<std::shared_ptr<T>>

[Copilot]

The specialization of std::hash for gsl::not_null<std::shared_ptr<T>> was failing to compile due to incorrect type deduction in the not_null_hash template.

Problem

The issue occurred because the template parameter U was being deduced from decltype(std::declval<const T&>().get()), which for gsl::not_null<std::shared_ptr<int>> would resolve to const std::shared_ptr<int>& instead of std::shared_ptr<int>. This caused std::hash<const std::shared_ptr<int>&> to not be default constructible, triggering the deleted constructor specialization.

// This would fail to compile before the fix
std::hash<gsl::not_null<std::shared_ptr<int>>> hasher{};

Solution

Changed the not_null_hash template to use typename T::element_type instead of deducing the type from .get():

// Before
template <class T, class U = decltype(std::declval<const T&>().get()), ...>

// After  
template <class T, class U = typename T::element_type, ...>

This directly uses the element_type member typedef from gsl::not_null<T> (which is defined as using element_type = T;), avoiding const/reference qualifiers that were causing the hash specialization to fail.

Testing

• Added test case for std::hash<gsl::not_null<std::shared_ptr<T>>> in the existing TestStdHash test
• Verified the fix works with both C++14 and C++20 builds
• All existing tests continue to pass (86/86 tests pass)
• Confirmed compatibility with other pointer types (raw pointers, unique_ptr, etc.)

Fixes #1209.

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[Copilot]

Pull Request Overview

This PR fixes the compilation error related to the specialization of std::hash for gsl::not_null<std::shared_ptr> by changing the template deduction from decltype(.get()) to use typename T::element_type.

• Update not_null_hash template in include/gsl/pointers to correctly deduce the type.
• Add new tests in tests/notnull_tests.cpp to verify correct behavior of std::hash with not_null.
• Ensure compatibility with both C++14 and C++20 standards.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
tests/notnull_tests.cpp	Added test cases to validate correct hashing behavior for not_null<std::shared_ptr>.
include/gsl/pointers	Updated the not_null_hash template to use typename T::element_type for proper type deduction.

[carsonRadtke]

Can you add a comment here saying that "T is conceptually a pointer so we don't have to worry about it being a reference and violating std::hash requirements".

[Copilot]

Added the requested comment in commit 87b3085. The comment explains that T is conceptually a pointer so we don't have to worry about it being a reference and violating std::hash requirements.