SortedRange.opSlice() should be return scope

[WalterBright]

Progress to -dip1000 compatibility.

[dlang-bot]

Thanks for your pull request, @WalterBright!

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Testing this PR locally

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dub fetch digger
dub run digger -- build "master + phobos#6866"

[WalterBright]

buildkite/phobos fails with:

generated/linux/release/64/unittest/test_runner rt.typeinfo.ti_wchar
--
  | generated/linux/release/64/unittest/test_runner etc.linux.memoryerror
  | 0.000s PASS release64 etc.linux.memoryerror
  | done
  | make[1]: Leaving directory '/var/lib/buildkite-agent/builds/ci-agent-8331da0f-b683-48a8-9b92-9391bb7a3ec9-7/dlang/phobos/druntime'
  | 🚨 Error: The command exited with status 2

What does that mean?

[WalterBright]

Blocking dlang/dmd#9220

[wilzbach]

No, the error is:

generated/linux/release/64/benchmark.o:benchmark/runbench.d:function _D3std5range__T11SortedRangeTAkVAyaa6_61203c3d2062ZQBk__T10lowerBoundVEQCrQCq12SearchPolicyi3TyiZQBoMFNaNbNiNfyiZSQEiQEh__TQEeTQDuVQDva6_61203c3d2062ZQFf: error: undefined reference to '_D3std5range__T11SortedRangeTAkVAyaa6_61203c3d2062ZQBk7opSliceMFNaNbNiNfmmZSQCwQCv__TQCsTQCiVQCja6_61203c3d2062ZQDt'
--
  | collect2: error: ld returned 1 exit status
  | Error: linker exited with status 1

We went throw this numerous times. scope breaks the binary compatibility and thus the linking fails.
The problem here is that on Buildkite the druntime benchmark target (which is not run on the auto-tester) doesn't recompile Phobos.

tl;dr: it's safe to merge (in terms of this particular Buildkite error)

[wilzbach]

Shouldn't the compiler be able to infer the safety here automatically?

[aG0aep6G]

What's more, Range.opSlice might be unsafe. This can't be @trusted.

[aG0aep6G]

What's more, Range.opSlice might be unsafe. This can't be @trusted.

#6869

[jondegenhardt]

I read this PR earlier and was confused by this as well. The opSlice being called could easily be exported by a range that I wrote, not even a range in Phobos.

[WalterBright]

It's not always able to, often due to circular references. Phobos is full of circular templates.

[jondegenhardt]

Consider adding comments to the code with an explanation of why it's needed. If appropriate, the circumstances under which it could be removed. If it's a long explanation, add it to the PR and reference it from the code. Otherwise future readers of the code will be left wondering.