diff --git a/base/compiler/typelimits.jl b/base/compiler/typelimits.jl
index 09372b2dc9613..436c24d85d8d0 100644
--- a/base/compiler/typelimits.jl
+++ b/base/compiler/typelimits.jl
@@ -277,22 +277,9 @@ function type_more_complex(@nospecialize(t), @nospecialize(c), sources::SimpleVe
             else
                 tupledepth = 0
             end
-            isgenerator = (t.name.name === :Generator && t.name.module === _topmod(t.name.module))
             for i = 1:length(tP)
                 tPi = tP[i]
                 cPi = cP[i + ntail]
-                if isgenerator
-                    let tPi = unwrap_unionall(tPi),
-                        cPi = unwrap_unionall(cPi)
-                        if isa(tPi, DataType) && isa(cPi, DataType) &&
-                            !isabstracttype(tPi) && !isabstracttype(cPi) &&
-                                sym_isless(cPi.name.name, tPi.name.name)
-                            # allow collect on (anonymous) Generators to nest, provided that their functions are appropriately ordered
-                            # TODO: is there a better way?
-                            continue
-                        end
-                    end
-                end
                 type_more_complex(tPi, cPi, sources, depth + 1, tupledepth, 0) && return true
             end
             return false
diff --git a/base/compiler/utilities.jl b/base/compiler/utilities.jl
index 7c3b19ead54cc..4ad66c2d8ed01 100644
--- a/base/compiler/utilities.jl
+++ b/base/compiler/utilities.jl
@@ -66,8 +66,6 @@ end
 is_meta_expr_head(head::Symbol) = head === :boundscheck || head === :meta || head === :loopinfo
 is_meta_expr(@nospecialize x) = isa(x, Expr) && is_meta_expr_head(x.head)
 
-sym_isless(a::Symbol, b::Symbol) = ccall(:strcmp, Int32, (Ptr{UInt8}, Ptr{UInt8}), a, b) < 0
-
 function is_self_quoting(@nospecialize(x))
     return isa(x,Number) || isa(x,AbstractString) || isa(x,Tuple) || isa(x,Type) ||
         isa(x,Char) || x === nothing || isa(x,Function)
diff --git a/test/compiler/inference.jl b/test/compiler/inference.jl
index dc46bc2495c7f..cfd44967a5081 100644
--- a/test/compiler/inference.jl
+++ b/test/compiler/inference.jl
@@ -96,6 +96,16 @@ end
 @test !Core.Compiler.type_more_complex(Tuple{Vararg{Tuple{}}}, Tuple{Vararg{Tuple}}, Core.svec(), 0, 0, 0)
 @test  Core.Compiler.type_more_complex(Tuple{Vararg{Tuple}}, Tuple{Vararg{Tuple{}}}, Core.svec(), 0, 0, 0)
 
+# issue #51694
+@test Core.Compiler.type_more_complex(
+       Base.Generator{Base.Iterators.Flatten{Array{Bool, 1}}, typeof(identity)},
+       Base.Generator{Array{Bool, 1}, typeof(identity)},
+       Core.svec(), 0, 0, 0)
+@test Core.Compiler.type_more_complex(
+       Base.Generator{Base.Iterators.Flatten{Base.Generator{Array{Bool, 1}, typeof(identity)}}, typeof(identity)},
+       Base.Generator{Array{Bool, 1}, typeof(identity)},
+       Core.svec(), 0, 0, 0)
+
 let # 40336
     t = Type{Type{Int}}
     c = Type{Int}