[sap] Use C++20 'requires' syntax for double-only functions

[jwnimmer-tri]

This change is 

[amcastro-tri]

Thank you so much @jwnimmer-tri! this is definitely a useful C++20 fature, making the intetion very clear.

Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: 1 unresolved discussion, needs platform reviewer assigned, needs at least two assigned reviewers

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multibody/contact_solvers/sap/sap_solver.h line 306 at r1 (raw file):

  // @pre context is not nullptr.
  // @pre context was created via a call to model.MakeContext().
  SapSolverStatus SolveWithGuessImpl(const SapModel<T>& model,

btw, I like this function name change!

[amcastro-tri]

adding myself +@amcastro-tri for feature review

Reviewable status: 1 unresolved discussion, needs platform reviewer assigned, needs at least two assigned reviewers

[SeanCurtis-TRI]

Reviewable status: 1 unresolved discussion, needs platform reviewer assigned, needs at least two assigned reviewers

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multibody/contact_solvers/sap/sap_solver.cc line 218 at r1 (raw file):

                                                 systems::Context<T>* context)
  requires std::is_same_v<T, double>
{  // NOLINT(whitespace/braces)

BTW What's the story with this linting directive? Is this going to have to be part of every incantation that uses requires?

[jwnimmer-tri]

Reviewable status: 1 unresolved discussion, needs platform reviewer assigned, needs at least two assigned reviewers

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multibody/contact_solvers/sap/sap_solver.cc line 218 at r1 (raw file):

Previously, SeanCurtis-TRI (Sean Curtis) wrote…

BTW What's the story with this linting directive? Is this going to have to be part of every incantation that uses requires?

As of today, yes. The medium-term goal is to either:

(1) Change our linters so that when enable_clang_format_lint = True, the whitespace/* rules in CPPLINT are disarmed, or

(2) Upgrade to a newer cpplint that understands requires.

[jwnimmer-tri]

FYI We'll give this until my afternoon time for any platform reviewers to raise objections (per my slack post), but then we can merge it.

[SeanCurtis-TRI]

Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: complete! all discussions resolved, LGTM from assignees amcastro-tri,SeanCurtis-TRI(platform)