diff --git a/NEWS.md b/NEWS.md
index ac218f591e380..751386438661e 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -438,6 +438,11 @@ Deprecated or removed
     been deprecated due to inconsistency with linear algebra. Use `.+` and `.-` for these operations
     instead.
 
+  * `isleaftype` is deprecated in favor of a simpler predicate `isconcrete`. Concrete types are
+    those that might equal `typeof(x)` for some `x`; `isleaftype` includes some types for which
+    this is not true. If you are certain you need the old behavior, it is temporarily available
+    as `Base._isleaftype` ([#17086]).
+
 Command-line option changes
 ---------------------------
 
diff --git a/base/broadcast.jl b/base/broadcast.jl
index 2ec5d451512bd..09f2b2c605aad 100644
--- a/base/broadcast.jl
+++ b/base/broadcast.jl
@@ -309,7 +309,7 @@ _nullable_eltype(f, A, As...) =
     T = _broadcast_eltype(f, A, Bs...)
     shape = broadcast_indices(A, Bs...)
     iter = CartesianRange(shape)
-    if isleaftype(T)
+    if Base._isleaftype(T)
         return broadcast_t(f, T, shape, iter, A, Bs...)
     end
     if isempty(iter)
@@ -320,8 +320,8 @@ end
 @inline function broadcast_c(f, ::Type{Nullable}, a...)
     nonnull = all(hasvalue, a)
     S = _nullable_eltype(f, a...)
-    if isleaftype(S) && null_safe_op(f, maptoTuple(_unsafe_get_eltype,
-                                                   a...).types...)
+    if Base._isleaftype(S) && null_safe_op(f, maptoTuple(_unsafe_get_eltype,
+                                                         a...).types...)
         Nullable{S}(f(map(unsafe_get, a)...), nonnull)
     else
         if nonnull
diff --git a/base/complex.jl b/base/complex.jl
index 07a7cbe317423..277ab796de933 100644
--- a/base/complex.jl
+++ b/base/complex.jl
@@ -958,7 +958,7 @@ big(z::Complex{T}) where {T<:Real} = Complex{big(T)}(z)
 complex(A::AbstractArray{<:Complex}) = A
 
 function complex(A::AbstractArray{T}) where T
-    if !isleaftype(T)
+    if !isconcrete(T)
         error("`complex` not defined on abstractly-typed arrays; please convert to a more specific type")
     end
     convert(AbstractArray{typeof(complex(zero(T)))}, A)
diff --git a/base/deprecated.jl b/base/deprecated.jl
index 9cd35d490acc8..a36db544d9c34 100644
--- a/base/deprecated.jl
+++ b/base/deprecated.jl
@@ -1766,6 +1766,9 @@ import .Iterators.enumerate
 # issue #5794
 @deprecate map(f, d::T) where {T<:Associative}  T( f(p) for p in pairs(d) )
 
+# issue #17086
+@deprecate isleaftype isconcrete
+
 # PR #22932
 @deprecate +(a::Number, b::AbstractArray) broadcast(+, a, b)
 @deprecate +(a::AbstractArray, b::Number) broadcast(+, a, b)
diff --git a/base/dict.jl b/base/dict.jl
index 10c326b1358a1..24aaa18c9dbae 100644
--- a/base/dict.jl
+++ b/base/dict.jl
@@ -34,7 +34,7 @@ function show(io::IO, t::Associative{K,V}) where V where K
     if isempty(t)
         print(io, typeof(t), "()")
     else
-        if isleaftype(K) && isleaftype(V)
+        if _isleaftype(K) && _isleaftype(V)
             print(io, typeof(t).name)
         else
             print(io, typeof(t))
@@ -161,7 +161,7 @@ associative_with_eltype(DT_apply, ::Type) = DT_apply(Any, Any)()
 associative_with_eltype(DT_apply::F, kv, t) where {F} = grow_to!(associative_with_eltype(DT_apply, _default_eltype(typeof(kv))), kv)
 function associative_with_eltype(DT_apply::F, kv::Generator, t) where F
     T = _default_eltype(typeof(kv))
-    if T <: Union{Pair, Tuple{Any, Any}} && isleaftype(T)
+    if T <: Union{Pair, Tuple{Any, Any}} && _isleaftype(T)
         return associative_with_eltype(DT_apply, kv, T)
     end
     return grow_to!(associative_with_eltype(DT_apply, T), kv)
diff --git a/base/exports.jl b/base/exports.jl
index 0b359c200e64a..b15e1709383a0 100644
--- a/base/exports.jl
+++ b/base/exports.jl
@@ -931,7 +931,7 @@ export
     fieldname,
     fieldnames,
     fieldcount,
-    isleaftype,
+    isconcrete,
     oftype,
     promote,
     promote_rule,
diff --git a/base/float.jl b/base/float.jl
index 8be322ba4ea6f..ddd3d194353a2 100644
--- a/base/float.jl
+++ b/base/float.jl
@@ -877,7 +877,7 @@ Base.iszero(x::Float16) = reinterpret(UInt16, x) & ~sign_mask(Float16) == 0x0000
 float(A::AbstractArray{<:AbstractFloat}) = A
 
 function float(A::AbstractArray{T}) where T
-    if !isleaftype(T)
+    if !isconcrete(T)
         error("`float` not defined on abstractly-typed arrays; please convert to a more specific type")
     end
     convert(AbstractArray{typeof(float(zero(T)))}, A)
diff --git a/base/inference.jl b/base/inference.jl
index 198a5aaaca788..e35858709bbec 100644
--- a/base/inference.jl
+++ b/base/inference.jl
@@ -63,6 +63,8 @@ const NF = NotFound()
 const LineNum = Int
 const VarTable = Array{Any,1}
 
+const isleaftype = _isleaftype
+
 # The type of a variable load is either a value or an UndefVarError
 mutable struct VarState
     typ
diff --git a/base/interactiveutil.jl b/base/interactiveutil.jl
index 8a780d3304102..790906dea361a 100644
--- a/base/interactiveutil.jl
+++ b/base/interactiveutil.jl
@@ -555,7 +555,7 @@ Evaluates the arguments to the function or macro call, determines their types, a
 function type_close_enough(@nospecialize(x), @nospecialize(t))
     x == t && return true
     return (isa(x,DataType) && isa(t,DataType) && x.name === t.name &&
-            !isleaftype(t) && x <: t) ||
+            !_isleaftype(t) && x <: t) ||
            (isa(x,Union) && isa(t,DataType) && (type_close_enough(x.a, t) || type_close_enough(x.b, t)))
 end
 
diff --git a/base/iterators.jl b/base/iterators.jl
index 9a401adfb2e00..5a29cc9d77db1 100644
--- a/base/iterators.jl
+++ b/base/iterators.jl
@@ -733,17 +733,14 @@ iteratoreltype(::Type{Flatten{I}}) where {I} = _flatteneltype(I, iteratoreltype(
 _flatteneltype(I, ::HasEltype) = iteratoreltype(eltype(I))
 _flatteneltype(I, et) = EltypeUnknown()
 
-flatten_iteratorsize(::Union{HasShape, HasLength}, b::Type{<:Tuple}) = isleaftype(b) ? HasLength() : SizeUnknown()
-flatten_iteratorsize(::Union{HasShape, HasLength}, b::Type{<:Number}) = HasLength()
+flatten_iteratorsize(::Union{HasShape, HasLength}, ::Type{<:NTuple{N,Any}}) where {N} = HasLength()
+flatten_iteratorsize(::Union{HasShape, HasLength}, ::Type{<:Tuple}) = SizeUnknown()
+flatten_iteratorsize(::Union{HasShape, HasLength}, ::Type{<:Number}) = HasLength()
 flatten_iteratorsize(a, b) = SizeUnknown()
 
 iteratorsize(::Type{Flatten{I}}) where {I} = flatten_iteratorsize(iteratorsize(I), eltype(I))
 
-function flatten_length(f, ::Type{T}) where {T<:Tuple}
-    if !isleaftype(T)
-        throw(ArgumentError(
-            "Cannot compute length of a tuple-type which is not a leaf-type"))
-    end
+function flatten_length(f, T::Type{<:NTuple{N,Any}}) where {N}
     fieldcount(T)*length(f.it)
 end
 flatten_length(f, ::Type{<:Number}) = length(f.it)
diff --git a/base/methodshow.jl b/base/methodshow.jl
index 286980c197be0..109d22b8868e4 100644
--- a/base/methodshow.jl
+++ b/base/methodshow.jl
@@ -108,7 +108,7 @@ function show(io::IO, m::Method; kwtype::Nullable{DataType}=Nullable{DataType}()
                 # TODO: more accurate test? (tn.name === "#" name)
             ft0 === typeof(getfield(ft.name.module, ft.name.mt.name))
         print(io, ft.name.mt.name)
-    elseif isa(ft, DataType) && ft.name === Type.body.name && isleaftype(ft)
+    elseif isa(ft, DataType) && ft.name === Type.body.name
         f = ft.parameters[1]
         if isa(f, DataType) && isempty(f.parameters)
             print(io, f)
@@ -235,7 +235,7 @@ function show(io::IO, ::MIME"text/html", m::Method; kwtype::Nullable{DataType}=N
             isdefined(ft.name.module, ft.name.mt.name) &&
             ft0 === typeof(getfield(ft.name.module, ft.name.mt.name))
         print(io, ft.name.mt.name)
-    elseif isa(ft, DataType) && ft.name === Type.body.name && isleaftype(ft)
+    elseif isa(ft, DataType) && ft.name === Type.body.name
         f = ft.parameters[1]
         if isa(f, DataType) && isempty(f.parameters)
             print(io, f)
diff --git a/base/nullable.jl b/base/nullable.jl
index 64e0069999c8c..953deb33e30de 100644
--- a/base/nullable.jl
+++ b/base/nullable.jl
@@ -339,7 +339,7 @@ end
 """
 Return the given type if it is concrete, and `Union{}` otherwise.
 """
-nullable_returntype(::Type{T}) where {T} = isleaftype(T) ? T : Union{}
+nullable_returntype(::Type{T}) where {T} = _isleaftype(T) ? T : Union{}
 
 """
     map(f, x::Nullable)
@@ -365,7 +365,7 @@ Nullable{Bool}()
 """
 function map(f, x::Nullable{T}) where T
     S = promote_op(f, T)
-    if isleaftype(S) && null_safe_op(f, T)
+    if _isleaftype(S) && null_safe_op(f, T)
         Nullable(f(unsafe_get(x)), !isnull(x))
     else
         if isnull(x)
diff --git a/base/promotion.jl b/base/promotion.jl
index 57c18a2e00293..928f8a38808f4 100644
--- a/base/promotion.jl
+++ b/base/promotion.jl
@@ -346,13 +346,13 @@ promote_op(::Any...) = (@_inline_meta; Any)
 function promote_op(f, ::Type{S}) where S
     @_inline_meta
     T = _return_type(f, Tuple{_default_type(S)})
-    isleaftype(S) && return isleaftype(T) ? T : Any
+    _isleaftype(S) && return _isleaftype(T) ? T : Any
     return typejoin(S, T)
 end
 function promote_op(f, ::Type{R}, ::Type{S}) where {R,S}
     @_inline_meta
     T = _return_type(f, Tuple{_default_type(R), _default_type(S)})
-    isleaftype(R) && isleaftype(S) && return isleaftype(T) ? T : Any
+    _isleaftype(R) && _isleaftype(S) && return _isleaftype(T) ? T : Any
     return typejoin(R, S, T)
 end
 
diff --git a/base/reflection.jl b/base/reflection.jl
index 41b67dff32964..c1ab5459de132 100644
--- a/base/reflection.jl
+++ b/base/reflection.jl
@@ -287,28 +287,41 @@ isbits(t::DataType) = (@_pure_meta; !t.mutable & (t.layout != C_NULL) && datatyp
 isbits(t::Type) = (@_pure_meta; false)
 isbits(x) = (@_pure_meta; isbits(typeof(x)))
 
+_isleaftype(@nospecialize(t)) = (@_pure_meta; isa(t, DataType) && t.isleaftype)
+
 """
-    isleaftype(T)
+    isconcrete(T)
 
-Determine whether `T`'s only subtypes are itself and `Union{}`. This means `T` is
-a concrete type that can have instances.
+Determine whether `T` is a concrete type, meaning it can have direct instances
+(values `x` such that `typeof(x) === T`).
 
 # Examples
 ```jldoctest
-julia> isleaftype(Complex)
+julia> isconcrete(Complex)
 false
 
-julia> isleaftype(Complex{Float32})
+julia> isconcrete(Complex{Float32})
 true
 
-julia> isleaftype(Vector{Complex})
+julia> isconcrete(Vector{Complex})
 true
 
-julia> isleaftype(Vector{Complex{Float32}})
+julia> isconcrete(Vector{Complex{Float32}})
 true
+
+julia> isconcrete(Union{})
+false
+
+julia> isconcrete(Union{Int,String})
+false
 ```
 """
-isleaftype(@nospecialize(t)) = (@_pure_meta; isa(t, DataType) && t.isleaftype)
+function isconcrete(@nospecialize(t))
+    @_pure_meta
+    return (isa(t, DataType) && !t.abstract &&
+            !t.hasfreetypevars &&
+            (t.name !== Tuple.name || all(isconcrete, t.parameters)))
+end
 
 """
     Base.isabstract(T)
@@ -791,7 +804,7 @@ function _dump_function_linfo(linfo::Core.MethodInstance, world::UInt, native::B
     end
 
     # TODO: use jl_is_cacheable_sig instead of isleaftype
-    isleaftype(linfo.specTypes) || (str = "; WARNING: This code may not match what actually runs.\n" * str)
+    _isleaftype(linfo.specTypes) || (str = "; WARNING: This code may not match what actually runs.\n" * str)
     return str
 end
 
@@ -822,7 +835,7 @@ code_native(::IO, ::Any, ::Symbol) = error("illegal code_native call") # resolve
 
 # give a decent error message if we try to instantiate a staged function on non-leaf types
 function func_for_method_checked(m::Method, @nospecialize types)
-    if isdefined(m,:generator) && !isdefined(m,:source) && !isleaftype(types)
+    if isdefined(m,:generator) && !isdefined(m,:source) && !_isleaftype(types)
         error("cannot call @generated function `", m, "` ",
               "with abstract argument types: ", types)
     end
diff --git a/base/refpointer.jl b/base/refpointer.jl
index 9ad3be1d89dbf..6c98b4b3aa1fd 100644
--- a/base/refpointer.jl
+++ b/base/refpointer.jl
@@ -55,7 +55,7 @@ convert(::Type{Ref{T}}, x) where {T} = RefValue{T}(x)
 function unsafe_convert(P::Type{Ptr{T}}, b::RefValue{T}) where T
     if isbits(T) || isbitsunion(T)
         return convert(P, pointer_from_objref(b))
-    elseif isleaftype(T)
+    elseif _isleaftype(T)
         return convert(P, pointer_from_objref(b.x))
     else
         # If the slot is not leaf type, it could be either isbits or not.
diff --git a/base/replutil.jl b/base/replutil.jl
index a59905d86af45..5fef42c044118 100644
--- a/base/replutil.jl
+++ b/base/replutil.jl
@@ -468,7 +468,7 @@ function show_method_candidates(io::IO, ex::MethodError, kwargs::Vector=Any[])
     # pool MethodErrors for these two functions.
     if f === convert && !isempty(arg_types_param)
         at1 = arg_types_param[1]
-        if isa(at1,DataType) && (at1::DataType).name === Type.body.name && isleaftype(at1)
+        if isa(at1,DataType) && (at1::DataType).name === Type.body.name && !Core.Inference.has_free_typevars(at1)
             push!(funcs, (at1.parameters[1], arg_types_param[2:end]))
         end
     end
diff --git a/base/set.jl b/base/set.jl
index 916d9746a1435..d9dea0a0ebc36 100644
--- a/base/set.jl
+++ b/base/set.jl
@@ -18,7 +18,7 @@ for sets of arbitrary objects.
 Set(itr) = Set{eltype(itr)}(itr)
 function Set(g::Generator)
     T = _default_eltype(typeof(g))
-    (isleaftype(T) || T === Union{}) || return grow_to!(Set{T}(), g)
+    (_isleaftype(T) || T === Union{}) || return grow_to!(Set{T}(), g)
     return Set{T}(g)
 end
 
@@ -266,7 +266,7 @@ function unique(itr)
         return out
     end
     x, i = next(itr, i)
-    if !isleaftype(T) && iteratoreltype(itr) == EltypeUnknown()
+    if !_isleaftype(T) && iteratoreltype(itr) == EltypeUnknown()
         S = typeof(x)
         return _unique_from(itr, S[x], Set{S}((x,)), i)
     end
diff --git a/base/show.jl b/base/show.jl
index 7d2ac38960156..4bd1e9204c944 100644
--- a/base/show.jl
+++ b/base/show.jl
@@ -253,7 +253,7 @@ function show_type_name(io::IO, tn::TypeName)
         globname_str = string(globname)
         if ('#' ∉ globname_str && '@' ∉ globname_str && isdefined(tn, :module) &&
             isbindingresolved(tn.module, globname) && isdefined(tn.module, globname) &&
-            isa(getfield(tn.module, globname), tn.wrapper) && isleaftype(tn.wrapper))
+            isa(getfield(tn.module, globname), tn.wrapper) && _isleaftype(tn.wrapper))
             globfunc = true
         end
     end
@@ -617,7 +617,7 @@ function show_expr_type(io::IO, @nospecialize(ty), emph::Bool)
     elseif ty === Core.IntrinsicFunction
         print(io, "::I")
     else
-        if emph && (!isleaftype(ty) || ty == Core.Box)
+        if emph && (!_isleaftype(ty) || ty == Core.Box)
             emphasize(io, "::$ty")
         else
             print(io, "::$ty")
@@ -1171,7 +1171,7 @@ function show_tuple_as_call(io::IO, name::Symbol, sig::Type)
                 isdefined(uw.name.module, uw.name.mt.name) &&
                 ft == typeof(getfield(uw.name.module, uw.name.mt.name))
             print(io, uw.name.mt.name)
-        elseif isa(ft, DataType) && ft.name === Type.body.name && isleaftype(ft)
+        elseif isa(ft, DataType) && ft.name === Type.body.name && !Core.Inference.has_free_typevars(ft)
             f = ft.parameters[1]
             print(io, f)
         else
@@ -1913,7 +1913,7 @@ function array_eltype_show_how(X)
         str = string(e)
     end
     # Types hard-coded here are those which are created by default for a given syntax
-    (isleaftype(e),
+    (_isleaftype(e),
      (!isempty(X) && (e===Float64 || e===Int || e===Char || e===String) ? "" : str))
 end
 
diff --git a/doc/src/stdlib/base.md b/doc/src/stdlib/base.md
index a6afa955fe6ea..2eaf02425e1e7 100644
--- a/doc/src/stdlib/base.md
+++ b/doc/src/stdlib/base.md
@@ -106,7 +106,7 @@ Base.fieldoffset
 Core.fieldtype
 Base.isimmutable
 Base.isbits
-Base.isleaftype
+Base.isconcrete
 Base.typejoin
 Base.typeintersect
 Base.instances
diff --git a/test/arrayops.jl b/test/arrayops.jl
index 1457684b6a0ec..2f1b4309bc9b2 100644
--- a/test/arrayops.jl
+++ b/test/arrayops.jl
@@ -1916,7 +1916,7 @@ let A = zeros(Int, 2, 2), B = zeros(Float64, 2, 2)
     for f in [f1, f2, f3, f4, f5, f6, f7, f8, f9, f10, f11, f12, f13, f14, f15, f16,
               f17, f18, f19, f20, f21, f22, f23, f24, f25, f26, f27, f28, f29, f30,
               f31, f32, f33, f34, f35, f36, f37, f38, f39, f40, f41, f42]
-        @test isleaftype(Base.return_types(f, ())[1])
+        @test Base._isleaftype(Base.return_types(f, ())[1])
     end
 end
 
diff --git a/test/core.jl b/test/core.jl
index c7e40f706445d..65e456d176e9e 100644
--- a/test/core.jl
+++ b/test/core.jl
@@ -169,7 +169,7 @@ let elT = T22624.body.body.body.types[1].parameters[1]
     elT2 = elT.body.types[1].parameters[1]
     @test elT2 == T22624{Int64, Int64, C} where C
     @test elT2.body.types[1].parameters[1] === elT2
-    @test isleaftype(elT2.body.types[1])
+    @test Base._isleaftype(elT2.body.types[1])
 end
 
 # issue #3890
@@ -4284,7 +4284,7 @@ let a = Val{Val{TypeVar(:_, Int)}},
 
     @test !isdefined(a, :instance)
     @test  isdefined(b, :instance)
-    @test isleaftype(b)
+    @test Base._isleaftype(b)
 end
 
 # A return type widened to Type{Union{T,Void}} should not confuse
diff --git a/test/inference.jl b/test/inference.jl
index a599922235824..218e8ebd6fe7f 100644
--- a/test/inference.jl
+++ b/test/inference.jl
@@ -186,6 +186,7 @@ function find_tvar10930(arg)
 end
 @test find_tvar10930(Vararg{Int}) === 1
 
+const isleaftype = Base._isleaftype
 
 # issue #12474
 @generated function f12474(::Any)
diff --git a/test/reflection.jl b/test/reflection.jl
index dc0af73e23987..752dda5fc51c7 100644
--- a/test/reflection.jl
+++ b/test/reflection.jl
@@ -139,11 +139,35 @@ end # module WarnType
 @test isbits(Tuple{Vararg{Any, 0}})
 
 # issue #16670
-@test isleaftype(Tuple{Int, Vararg{Int, 2}})
-@test !isleaftype(Tuple{Integer, Vararg{Int, 2}})
-@test !isleaftype(Tuple{Int, Vararg{Int}})
-@test isleaftype(Type{Tuple{Integer, Vararg{Int}}})
-@test isleaftype(Type{Vector})
+@test Base._isleaftype(Tuple{Int, Vararg{Int, 2}})
+@test !Base._isleaftype(Tuple{Integer, Vararg{Int, 2}})
+@test !Base._isleaftype(Tuple{Int, Vararg{Int}})
+@test Base._isleaftype(Type{Tuple{Integer, Vararg{Int}}})
+@test Base._isleaftype(Type{Vector})
+@test isconcrete(Int)
+@test isconcrete(Vector{Int})
+@test isconcrete(Tuple{Int, Vararg{Int, 2}})
+@test !isconcrete(Tuple{Any})
+@test !isconcrete(Tuple{Integer, Vararg{Int, 2}})
+@test !isconcrete(Tuple{Int, Vararg{Int}})
+@test !isconcrete(Type{Tuple{Integer, Vararg{Int}}})
+@test !isconcrete(Type{Vector})
+@test !isconcrete(Type{Int})
+@test !isconcrete(Tuple{Type{Int}})
+@test isconcrete(DataType)
+@test isconcrete(Union)
+@test !isconcrete(Union{})
+@test !isconcrete(Tuple{Union{}})
+@test !isconcrete(Complex)
+@test !isconcrete(Complex.body)
+@test !isconcrete(AbstractArray{Int,1})
+struct AlwaysHasLayout{T}
+    x
+end
+@test !isconcrete(AlwaysHasLayout) && !isconcrete(AlwaysHasLayout.body)
+@test isconcrete(AlwaysHasLayout{Any})
+@test isconcrete(Ptr{Void})
+@test !isconcrete(Ptr) && !isconcrete(Ptr.body)
 
 # issue #10165
 i10165(::Type) = 0