fix cat invalidations

This patch removes the invalidation on cat and thus reduces the OneHotArrays loading time from 4.5s to 0.5s (the normal status)

src/blockmap.jl

@@ -505,13 +505,28 @@ for k in 1:8 # is 8 sufficient?
     mapargs = ntuple(n ->:($(Symbol(:A, n))), Val(k-1))
     # yields (:LinearMap(A1), :LinearMap(A2), ..., :LinearMap(A(k-1)))
 
-    @eval function Base.cat($(Is...), $L, As::MapOrVecOrMat...; dims::Dims{2})
-        if dims == (1,2)
-            return BlockDiagonalMap(convert_to_lmaps($(mapargs...))...,
-                                    $(Symbol(:A, k)),
-                                    convert_to_lmaps(As...)...)
-        else
-            throw(ArgumentError("dims keyword in cat of LinearMaps must be (1,2)"))
+    @static if VERSION >= v"1.8"
+        # Dispatching on `cat` makes compiler hard to infer types and causes invalidations
+        # after https://github.com/JuliaLang/julia/pull/45028
+        # Here we instead dispatch on _cat
+        @eval function Base._cat(dims, $(Is...), $L, As...)
+            if dims == (1,2)
+                return BlockDiagonalMap(convert_to_lmaps($(mapargs...))...,
+                                        $(Symbol(:A, k)),
+                                        convert_to_lmaps(As...)...)
+            else
+                throw(ArgumentError("dims keyword in cat of LinearMaps must be (1,2)"))
+            end
+        end
+    else
+        @eval function Base.cat($(Is...), $L, As...; dims::Dims{2})
+            if dims == (1,2)
+                return BlockDiagonalMap(convert_to_lmaps($(mapargs...))...,
+                                        $(Symbol(:A, k)),
+                                        convert_to_lmaps(As...)...)
+            else
+                throw(ArgumentError("dims keyword in cat of LinearMaps must be (1,2)"))
+            end
         end
     end
 end