Fix MISRA C++ 2023 rule 15.1.3 "Conversion operators and constructors that are callable with a single argument shall be explicit"

[mikir]

Zserio version and language
Zserio: latest from master
Language: C++

Describe the bug
Check and resolve SCA warning MISRACPP2023-15_1_3-a ("Conversion operators and constructors that are callable with a single argument shall be explicit") reported in Zserio on C++ runtime library.

Rule ID	severity	Message	Path	Line	Link to sonarcloud
MISRACPP2023-15_1_3-a	2	Constructor 'inplace_optional_holder' allowing conversion should be made explicit	/zserio_runtime/include/roadgraph_reliable/zserio_runtime/zserio/OptionalHolder.h	670	https://sonarcloud.io/project/issues?open=AY3k4v9hXMcagTJBbG4m&id=ndsev_zserio
MISRACPP2023-15_1_3-a	2	Constructor 'inplace_optional_holder' allowing conversion should be made explicit	/zserio_runtime/include/roadgraph_reliable/zserio_runtime/zserio/OptionalHolder.h	678	https://sonarcloud.io/project/issues?open=AY3k4v9hXMcagTJBbG4o&id=ndsev_zserio
MISRACPP2023-15_1_3-a	2	Constructor 'inplace_optional_holder' allowing conversion should be made explicit	/zserio_runtime/include/roadgraph_reliable/zserio_runtime/zserio/OptionalHolder.h	689	https://sonarcloud.io/project/issues?open=AY3k4v9hXMcagTJBbG4q&id=ndsev_zserio
MISRACPP2023-15_1_3-a	2	Constructor 'Span' allowing conversion should be made explicit	/zserio_runtime/include/roadgraph_reliable/zserio_runtime/zserio/Span.h	111	https://sonarcloud.io/project/issues?open=AY3k4v9ZXMcagTJBbG4W&id=ndsev_zserio
MISRACPP2023-15_1_3-a	2	Constructor 'Span' allowing conversion should be made explicit	/zserio_runtime/include/roadgraph_reliable/zserio_runtime/zserio/Span.h	123	https://sonarcloud.io/project/issues?open=AY3k4v9ZXMcagTJBbG4X&id=ndsev_zserio
MISRACPP2023-15_1_3-a	2	Constructor 'Span' allowing conversion should be made explicit	/zserio_runtime/include/roadgraph_reliable/zserio_runtime/zserio/Span.h	135	https://sonarcloud.io/project/issues?open=AY3k4v9ZXMcagTJBbG4Y&id=ndsev_zserio
MISRACPP2023-15_1_3-a	2	Constructor 'Span' allowing conversion should be made explicit	/zserio_runtime/include/roadgraph_reliable/zserio_runtime/zserio/Span.h	147	https://sonarcloud.io/project/issues?open=AY3k4v9ZXMcagTJBbG4Z&id=ndsev_zserio
MISRACPP2023-15_1_3-a	2	Constructor 'Span' allowing conversion should be made explicit	/zserio_runtime/include/roadgraph_reliable/zserio_runtime/zserio/Span.h	159	https://sonarcloud.io/project/issues?open=AY3k4v9ZXMcagTJBbG4a&id=ndsev_zserio
MISRACPP2023-15_1_3-a	2	Constructor 'Span' allowing conversion should be made explicit	/zserio_runtime/include/roadgraph_reliable/zserio_runtime/zserio/Span.h	171	https://sonarcloud.io/project/issues?open=AY3k4v9ZXMcagTJBbG4b&id=ndsev_zserio
MISRACPP2023-15_1_3-a	2	Constructor 'BasicStringView' allowing conversion should be made explicit	/zserio_runtime/include/roadgraph_reliable/zserio_runtime/zserio/StringView.h	54	https://sonarcloud.io/project/issues?open=AY3k4v85XMcagTJBbG3l&id=ndsev_zserio
MISRACPP2023-15_1_3-a	2	Constructor 'BasicStringView' allowing conversion should be made explicit	/zserio_runtime/include/roadgraph_reliable/zserio_runtime/zserio/StringView.h	74	https://sonarcloud.io/project/issues?open=AY3k4v85XMcagTJBbG3m&id=ndsev_zserio
MISRACPP2023-15_1_3-a	2	Constructor 'UniquePtrDeleter' allowing conversion should be made explicit	/zserio_runtime/include/roadgraph_reliable/zserio_runtime/zserio/UniquePtr.h	57	https://sonarcloud.io/project/issues?open=AY3k4v99XMcagTJBbG5C&id=ndsev_zserio
MISRACPP2023-15_1_3-a	2	Constructor 'UniquePtrDeleter' allowing conversion should be made explicit	/zserio_runtime/include/roadgraph_reliable/zserio_runtime/zserio/UniquePtr.h	68	https://sonarcloud.io/project/issues?open=AY3k4v99XMcagTJBbG5D&id=ndsev_zserio

How to reproduce
see the sonarcloud links

Expected behavior
No SCA warnings for MISRA 2023 ruleset

Additional context

[mikir]

We went through all implicit costructors in the C++ runtime library and they all have been accepted as the exceptions:

Module	Line	Command	Link
OptionalHolder.h	669	constexpr inplace_optional_holder(NullOptType) noexcept	SonarCloud
OptionalHolder.h	677	inplace_optional_holder(const T& value)	SonarCloud
OptionalHolder.h	688	inplace_optional_holder(T&& value)	SonarCloud
Span.h	112	constexpr Span(element_type (&arr)[N]) noexcept	SonarCloud
Span.h	125	constexpr Span(std::array<U, N>& arr) noexcept	SonarCloud
Span.h	138	constexpr Span(const std::array<U, N>& arr) noexcept	SonarCloud
Span.h	150	constexpr Span(std::vector<U, ALLOC>& vec)	SonarCloud
Span.h	163	constexpr Span(const std::vector<U, ALLOC>& vec)	SonarCloud
Span.h	176	constexpr Span(const Span<U, N>& span) noexcept	SonarCloud
StringView.h	54	BasicStringView(const const_pointer str) noexcept	SonarCloud
StringView.h	76	template <typename ALLOC> constexpr BasicStringView(const std::basic_string<CharT, Traits, ALLOC>& str) noexcept	SonarCloud
UniquePtr.h	57	template <typename ALLOC_U = ALLOC_T> UniquePtrDeleter(const ALLOC_U& allocator)	SonarCloud
UniquePtr.h	68	template <typename ALLOC_U> UniquePtrDeleter(const UniquePtrDeleter<ALLOC_U>& deleter)	SonarCloud

We use implicit constructor in the generated code as well:

Zserio Type	Generated Method	Description
bitmask	Value constructor	This is intended because we need to allow implicit conversion from bitmask value to bitmask.

[mohammedRafeeque]

Hello @mikir
Can resolve the remaining marked exceptional cases just prefixing "explicit" ?

Please check

[mikir]

Unfortunatelly, we cannot change these exceptions to be explicit because we really need the implicit conversions in these cases.

For example, Span<> is defined in the standard library in this way, so we cannot do anything with this.