diff --git a/base/array.jl b/base/array.jl
index f8de513cc60e8d..e6d64c313c16df 100644
--- a/base/array.jl
+++ b/base/array.jl
@@ -211,11 +211,9 @@ julia> Base.bitsunionsize(Union{Float64, UInt8, Int128})
 ```
 """
 function bitsunionsize(u::Union)
-    sz = Ref{Csize_t}(0)
-    algn = Ref{Csize_t}(0)
-    isunboxed = ccall(:jl_islayout_inline, Cint, (Any, Ptr{Csize_t}, Ptr{Csize_t}), u, sz, algn)
-    @assert isunboxed != Cint(0)
-    return sz[]
+    isinline, sz, _ = uniontype_layout(u)
+    @assert isinline
+    return sz
 end
 
 length(a::Array) = arraylen(a)
diff --git a/base/compiler/tfuncs.jl b/base/compiler/tfuncs.jl
index a6007e0cfac62b..ec41e46ffa96fc 100644
--- a/base/compiler/tfuncs.jl
+++ b/base/compiler/tfuncs.jl
@@ -302,16 +302,34 @@ function sizeof_nothrow(@nospecialize(x))
         end
     elseif isa(x, Conditional)
         return true
-    else
-        x = widenconst(x)
     end
     if isa(x, Union)
         return sizeof_nothrow(x.a) && sizeof_nothrow(x.b)
     end
-    isconstType(x) && (x = x.parameters[1]) # since sizeof(typeof(x)) == sizeof(x)
-    x === DataType && return false
-    return isconcretetype(x) || isprimitivetype(x)
+    t, exact = instanceof_tfunc(x)
+    if !exact
+        # Could always be bottom at runtime, which throws
+        return false
+    end
+    if t !== Bottom
+        t === DataType && return true
+        x = t
+        x = unwrap_unionall(x)
+        if isa(x, Union)
+            isinline, sz, _ = uniontype_layout(x)
+            return isinline
+        end
+        isa(x, DataType) || return false
+        x.layout == C_NULL && return false
+        (datatype_nfields(x) == 0 && !datatype_pointerfree(x)) && return false
+        return true
+    else
+        x = widenconst(x)
+        x === DataType && return false
+        return isconcretetype(x) || isprimitivetype(x)
+    end
 end
+
 function _const_sizeof(@nospecialize(x))
     # Constant Vector does not have constant size
     isa(x, Vector) && return Int
@@ -330,12 +348,27 @@ function sizeof_tfunc(@nospecialize(x),)
     isa(x, Const) && return _const_sizeof(x.val)
     isa(x, Conditional) && return _const_sizeof(Bool)
     isconstType(x) && return _const_sizeof(x.parameters[1])
-    x = widenconst(x)
     if isa(x, Union)
         return tmerge(sizeof_tfunc(x.a), sizeof_tfunc(x.b))
     end
-    x !== DataType && isconcretetype(x) && return _const_sizeof(x)
-    isprimitivetype(x) && return _const_sizeof(x)
+    # Core.sizeof operates on either a type or a value. First check which
+    # case we're in.
+    t, exact = instanceof_tfunc(x)
+    if t !== Bottom
+        # The value corresponding to `x` at runtime could be a type.
+        # Normalize the query to ask about that type.
+        x = unwrap_unionall(t)
+        if isa(x, Union)
+            isinline, sz, _ = uniontype_layout(x)
+            return isinline ? Const(Int(sz)) : (exact ? Bottom : Int)
+        end
+        isa(x, DataType) || return Int
+        (isconcretetype(x) || isprimitivetype(x)) && return _const_sizeof(x)
+    else
+        x = widenconst(x)
+        x !== DataType && isconcretetype(x) && return _const_sizeof(x)
+        isprimitivetype(x) && return _const_sizeof(x)
+    end
     return Int
 end
 add_tfunc(Core.sizeof, 1, 1, sizeof_tfunc, 1)
diff --git a/base/reflection.jl b/base/reflection.jl
index dc0d753eb41758..927a7d229cf55c 100644
--- a/base/reflection.jl
+++ b/base/reflection.jl
@@ -347,15 +347,19 @@ function datatype_alignment(dt::DataType)
     return Int(alignment)
 end
 
+function uniontype_layout(T::Type)
+    sz = RefValue{Csize_t}(0)
+    algn = RefValue{Csize_t}(0)
+    isinline = ccall(:jl_islayout_inline, Cint, (Any, Ptr{Csize_t}, Ptr{Csize_t}), T, sz, algn) != 0
+    (isinline, sz[], algn[])
+end
+
 # amount of total space taken by T when stored in a container
 function aligned_sizeof(T::Type)
     @_pure_meta
     if isbitsunion(T)
-        sz = Ref{Csize_t}(0)
-        algn = Ref{Csize_t}(0)
-        ccall(:jl_islayout_inline, Cint, (Any, Ptr{Csize_t}, Ptr{Csize_t}), T, sz, algn)
-        al = algn[]
-        return (sz[] + al - 1) & -al
+        _, sz, al = uniontype_layout(T)
+        return (sz + al - 1) & -al
     elseif allocatedinline(T)
         al = datatype_alignment(T)
         return (Core.sizeof(T) + al - 1) & -al
@@ -381,6 +385,19 @@ function datatype_haspadding(dt::DataType)
     return flags & 1 == 1
 end
 
+"""
+    Base.datatype_nfields(dt::DataType) -> Bool
+
+Return the number of fields known to this datatype's layout.
+Can be called on any `isconcretetype`.
+"""
+function datatype_nfields(dt::DataType)
+    @_pure_meta
+    dt.layout == C_NULL && throw(UndefRefError())
+    return unsafe_load(convert(Ptr{DataTypeLayout}, dt.layout)).nfields
+end
+
+
 """
     Base.datatype_pointerfree(dt::DataType) -> Bool
 
diff --git a/src/codegen.cpp b/src/codegen.cpp
index f2082fdb040294..c504920fd08c78 100644
--- a/src/codegen.cpp
+++ b/src/codegen.cpp
@@ -3039,21 +3039,6 @@ static bool emit_builtin_call(jl_codectx_t &ctx, jl_cgval_t *ret, jl_value_t *f,
             *ret = mark_julia_type(ctx, ctx.builder.CreateMul(len, elsize), false, jl_long_type);
             return true;
         }
-        if (jl_is_type_type((jl_value_t*)sty) && !jl_is_typevar(jl_tparam0(sty))) {
-            sty = (jl_datatype_t*)jl_tparam0(sty);
-        }
-        if (jl_is_datatype(sty) && sty != jl_symbol_type &&
-                sty->name != jl_array_typename &&
-                sty != jl_simplevector_type && sty != jl_string_type &&
-                // exclude DataType, since each DataType has its own size, not sizeof(DataType).
-                // this is issue #8798
-                sty != jl_datatype_type) {
-            if (jl_is_concrete_type((jl_value_t*)sty) ||
-                    (jl_field_names(sty) == jl_emptysvec && jl_datatype_size(sty) > 0)) {
-                *ret = mark_julia_type(ctx, ConstantInt::get(T_size, jl_datatype_size(sty)), false, jl_long_type);
-                return true;
-            }
-        }
     }
 
     else if (f == jl_builtin_apply_type && nargs > 0) {
diff --git a/test/compiler/inference.jl b/test/compiler/inference.jl
index 772093f66bdea0..285f944d4bd8c3 100644
--- a/test/compiler/inference.jl
+++ b/test/compiler/inference.jl
@@ -1400,12 +1400,17 @@ egal_conditional_lattice3(x, y) = x === y + y ? "" : 1
 
 using Core.Compiler: PartialStruct, nfields_tfunc, sizeof_tfunc, sizeof_nothrow
 @test sizeof_tfunc(Const(Ptr)) === sizeof_tfunc(Union{Ptr, Int, Type{Ptr{Int8}}, Type{Int}}) === Const(Sys.WORD_SIZE ÷ 8)
-@test sizeof_tfunc(Type{Ptr}) === Int
+@test sizeof_tfunc(Type{Ptr}) === Const(sizeof(Ptr))
 @test sizeof_nothrow(Union{Ptr, Int, Type{Ptr{Int8}}, Type{Int}})
 @test sizeof_nothrow(Const(Ptr))
-@test !sizeof_nothrow(Type{Ptr})
-@test !sizeof_nothrow(Type{Union{Ptr{Int}, Int}})
+@test sizeof_nothrow(Type{Ptr})
+@test sizeof_nothrow(Type{Union{Ptr{Int}, Int}})
 @test !sizeof_nothrow(Const(Tuple))
+@test !sizeof_nothrow(Type{Vector{Int}})
+@test !sizeof_nothrow(Type{Union{Int, String}})
+@test sizeof_nothrow(String)
+@test !sizeof_nothrow(Type{String})
+@test sizeof_tfunc(Type{Union{Int64, Int32}}) == Const(Core.sizeof(Union{Int64, Int32}))
 let PT = PartialStruct(Tuple{Int64,UInt64}, Any[Const(10, false), UInt64])
     @test sizeof_tfunc(PT) === Const(16)
     @test nfields_tfunc(PT) === Const(2)
@@ -2734,3 +2739,8 @@ end
 
 f_generator_splat(t::Tuple) = tuple((identity(l) for l in t)...)
 @test Base.return_types(f_generator_splat, (Tuple{Symbol, Int64, Float64},)) == Any[Tuple{Symbol, Int64, Float64}]
+
+# Issue #36710 - sizeof(::UnionAll) tfunc correctness
+@test (sizeof(Ptr),) == sizeof.((Ptr,)) == sizeof.((Ptr{Cvoid},))
+@test Core.Compiler.sizeof_tfunc(UnionAll) === Int
+@test !Core.Compiler.sizeof_nothrow(UnionAll)