Proposal: Investigate C++20

[BrianMichell]

Motivation

• Keeping up with modern code and standards to reduce overall technical debt.
• More recent versions of Tensorstore explicitly support C++20.

Expected changes for C++20 support

GCC supports the majority of the C++20 Language Features, but for full (supported) language features we will need to bump to GCC13.

Clang appears to be lagging behind GCC in C++20 support with a notable lack of Modules support. All other sections claim at least partial support. For full (supported) language features we will need to bump to Clang 19.

Staying modern

GCC notes the ISO C++23 standard as evolving and will not attempt to maintain backward compatibility. As such it does not seem sensible to include in this investigation.

Known C++20 features that are lacking or partial

GCC

• Translation-unit-local entities
• Reverse operator handling
• Migrating minor modules maladies
• No Private Module Fragment,
  Parser-level Global Module Entity Merging,
  Global Module Implications of extern "C/C++",
  or Partition-specific Definition Visibility

Clang

• Inconsistencies with non-type template parameters
• Relaxing redefinition restrictions for re-exportation robustness
• Prevent import declarations in global module fragment of a module unit being implicitly imported
• Modules dependency discovery
• Merging of multiple definitions for unnamed unscoped enumerations
• Translation-unit-local entities
• Meaningful exports
• Linkage for modular constants
• Full support for Windows coroutines
• Class template argument deduction for alias templates

What must happen

This is intended to be a living document as requirements are brought to light

• Basic compilation with GCC13 and Clang19.
• Validation that defined API behavior remains constant across compilers and C++ versions.
• Regression testing between C++ versions.
• Support for multiple C++ versions in CMake and API as needed.
• Updates to documentation for supported and tested versions.
• CI/CD pipelines updates for supported versions.
• A clear and public versioning strategy to reduce technical debt including supported compiler deprecation.
• Collect user/community feedback from users.