LWG4072. std::optional comparisons: constrain harder

[steve-downey]

See https://cplusplus.github.io/LWG/issue4072

[coveralls]

Pull Request Test Coverage Report for Build 19601990650

Details

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 99.31%

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Totals
Change from base Build 19595770379:	0.0%
Covered Lines:	2446
Relevant Lines:	2463

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💛 - Coveralls