Improve inference in vcat

src/abstractdataframe/abstractdataframe.jl

@@ -1547,12 +1547,25 @@ Base.vcat(dfs::AbstractDataFrame...;
                       AbstractVector{<:AbstractString}}=:setequal) =
     reduce(vcat, dfs; cols=cols)
 
-Base.reduce(::typeof(vcat),
-            dfs::Union{AbstractVector{<:AbstractDataFrame},
-                       Tuple{Vararg{AbstractDataFrame}}};
-            cols::Union{Symbol, AbstractVector{Symbol},
-                        AbstractVector{<:AbstractString}}=:setequal) =
-    _vcat([df for df in dfs if ncol(df) != 0]; cols=cols)
+if Base.VERSION >= v"1.5"
+    function Base.reduce(::typeof(vcat),
+                         dfs::Union{AbstractVector{DF},Tuple{Vararg{DF}}};
+                         cols::Union{Symbol, AbstractVector{Symbol},
+                                     AbstractVector{<:AbstractString}}=:setequal) where DF<:AbstractDataFrame
+        if @isdefined(DF) && isconcretetype(DF)
+            return _vcat(DF[df for df in dfs if ncol(df) != 0]; cols=cols)
+        else
+            return _vcat(AbstractDataFrame[df for df in dfs if ncol(df) != 0]; cols=cols)
+        end
+    end
+else
+    Base.reduce(::typeof(vcat),
+                dfs::Union{AbstractVector{<:AbstractDataFrame},
+                           Tuple{Vararg{AbstractDataFrame}}};
+                cols::Union{Symbol, AbstractVector{Symbol},
+                AbstractVector{<:AbstractString}}=:setequal) =
+        _vcat([df for df in dfs if ncol(df) != 0]; cols=cols)
+end
 
 function _vcat(dfs::AbstractVector{<:AbstractDataFrame};
                cols::Union{Symbol, AbstractVector{Symbol},
@@ -1586,13 +1599,15 @@ function _vcat(dfs::AbstractVector{<:AbstractDataFrame};
 
         if !isempty(coldiff)
             # if any DataFrames are a full superset of names, skip them
-            filter!(u -> !issetequal(u, header), uniqueheaders)
+            let header=header     # julia #15276
+                filter!(u -> !issetequal(u, header), uniqueheaders)
+            end
             estrings = map(enumerate(uniqueheaders)) do (i, head)
                 matching = findall(h -> head == h, allheaders)
                 headerdiff = setdiff(coldiff, head)
-                cols = join(headerdiff, ", ", " and ")
+                badcols = join(headerdiff, ", ", " and ")
                 args = join(matching, ", ", " and ")
-                return "column(s) $cols are missing from argument(s) $args"
+                return "column(s) $badcols are missing from argument(s) $args"
             end
             throw(ArgumentError(join(estrings, ", ", ", and ")))
         end

src/other/index.jl

@@ -422,7 +422,7 @@ Base.@propagate_inbounds SubIndex(parent::AbstractIndex, cols) =
     SubIndex(parent, parent[cols])
 
 Base.length(x::SubIndex) = length(x.cols)
-Base.names(x::SubIndex) = string.(_names(x))
+Base.names(x::SubIndex) = string.(_names(x))::Vector{String}
 _names(x::SubIndex) = view(_names(x.parent), x.cols)
 
 function Base.haskey(x::SubIndex, key::Symbol)