Regression on master when documenting type alias

[yuyichao]

This happens somewhere between 7dfa7d1 and c2f42a6 7dfa7d1...c2f42a6

Reduced case from SIMD.jl

julia> """
       """
       ContiguousSubArray{T,N,P,I,L} = SubArray{T,N,P,I,L}
ERROR: UndefVarError: `T` not defined in `Main`
Suggestion: check for spelling errors or missing imports.
Stacktrace:
 [1] top-level scope
   @ REPL[7]:1

julia> ContiguousSubArray{T,N,P,I,L} = SubArray{T,N,P,I,L}
SubArray

Some packages that this seems to affect includes

ChainRules, YaoBlocks and SIMD.

[yuyichao]

I guess this is probably caused by #59882

[yuyichao]

This this seems to be due to the expression being document cannot be evaluated as a local expression (or on the RHS of assignment), i.e.

julia> a = (ContiguousSubArray{T} = SubArray{T})
ERROR: UndefVarError: `T` not defined in `Main`
Suggestion: check for spelling errors or missing imports.
Stacktrace:
 [1] top-level scope
   @ REPL[5]:1

From the old expansion

julia> @macroexpand Core.@doc "" ContiguousSubArray{T} = SubArray{T}
quote
    ContiguousSubArray{T} = SubArray{T}
    (Base.Docs.doc!)(Main, (Base.Docs.Binding)(Main, :ContiguousSubArray), (Base.Docs.docstr)((Core.svec)(""), (Dict{Symbol, Any})(:path => "REPL[1]", :linenumber => 1, :module => Main)), Union{})
end

and the new one

julia> @macroexpand Core.@doc "" ContiguousSubArray{T} = SubArray{T}
quote
    var"#1#val" = (ContiguousSubArray{T} = SubArray{T})
    (Base.Docs.doc!)(Main, (Base.Docs.Binding)(Main, :ContiguousSubArray), (Base.Docs.docstr)((Core.svec)(""), (Dict{Symbol, Any})(:path => "REPL[1]", :linenumber => 1, :module => Main)), Union{})
    var"#1#val"
end