Use valid identifiers for sandbox module names (#1540)

CHANGELOG.md

@@ -10,6 +10,10 @@
 
 * ![Bugfix][badge-bugfix] A bad `repo` argument to `deploydocs` containing a protocol now throws an error instead of being misinterpreted. ([#1531][github-1531], [#1533][github-1533])
 
+## Version `v0.26.3`
+
+* ![Maintenance][badge-maintenance] The internal naming of the temporary modules used to run doctests changed to accommodate upcoming printing changes in Julia. ([JuliaLang/julia#39841][julia-39841], [#1540][github-1540])
+
 ## Version `v0.26.2`
 
 * ![Enhancement][badge-enhancement] `doctest()` no longer throws an error if cleaning up the temporary directory fails for some reason. ([#1513][github-1513], [#1526][github-1526])
@@ -760,8 +764,10 @@
 [github-1529]: https://github.com/JuliaDocs/Documenter.jl/pull/1529
 [github-1531]: https://github.com/JuliaDocs/Documenter.jl/issues/1531
 [github-1533]: https://github.com/JuliaDocs/Documenter.jl/pull/1533
+[github-1540]: https://github.com/JuliaDocs/Documenter.jl/pull/1540
 
 [julia-38079]: https://github.com/JuliaLang/julia/issues/38079
+[julia-39841]: https://github.com/JuliaLang/julia/pull/39841
 
 [documenterlatex]: https://github.com/JuliaDocs/DocumenterLaTeX.jl
 [documentermarkdown]: https://github.com/JuliaDocs/DocumenterMarkdown.jl

src/DocTests.jl

@@ -124,7 +124,11 @@ function doctest(ctx::DocTestContext, block_immutable::Markdown2.CodeBlock)
     if startswith(lang, "jldoctest")
         # Define new module or reuse an old one from this page if we have a named doctest.
         name = match(r"jldoctest[ ]?(.*)$", split(lang, ';', limit = 2)[1])[1]
-        sym = isempty(name) ? gensym("doctest-") : Symbol("doctest-", name)
+        if isempty(name)
+            sym = Symbol("__doctest__", lstrip(string(gensym()), '#'))
+        else
+            sym = Symbol("__doctest__named__", name)
+        end
         sandbox = get!(() -> Expanders.get_new_sandbox(sym), ctx.meta, sym)
 
         # Normalise line endings.