[struct_pack] fix c++17 enable_if missing ::type and wrong std::string_view type in user_defined_serialization_imp

[songqing]

Why

Issue 1: In the C++17 fallback path, sp_get_needed_size is incorrectly checked as returning std::string_view, while the C++20 version clearly requires it to return std::size_t. The type is completely wrong.

Issue 2: In std::void_tstd::enable_if, std::enable_if itself is a valid type (it just doesn’t have a ::type member), so std::void_t will never fail on it. This makes the condition check completely ineffective.

The correct form should be std::enable_if_t or typename std::enable_if::type, so that when condition == false it actually triggers a SFINAE substitution failure.

What is changing

Example

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LGTM