diff --git a/base/reflection.jl b/base/reflection.jl
index 83f3967b98dbe..97f1ed14c6729 100644
--- a/base/reflection.jl
+++ b/base/reflection.jl
@@ -374,15 +374,18 @@ LLT_ALIGN(x, sz) = (x + sz - 1) & -sz
 # amount of total space taken by T when stored in a container
 function aligned_sizeof(@nospecialize T::Type)
     @_foldable_meta
-    if isbitsunion(T)
-        _, sz, al = uniontype_layout(T)
-        return LLT_ALIGN(sz, al)
+    if isa(T, Union)
+        if allocatedinline(T)
+            # NOTE this check is equivalent to `isbitsunion(T)`, we can improve type
+            # inference in the second branch with the outer `isa(T, Union)` check
+            _, sz, al = uniontype_layout(T)
+            return LLT_ALIGN(sz, al)
+        end
     elseif allocatedinline(T)
         al = datatype_alignment(T)
         return LLT_ALIGN(Core.sizeof(T), al)
-    else
-        return Core.sizeof(Ptr{Cvoid})
     end
+    return Core.sizeof(Ptr{Cvoid})
 end
 
 gc_alignment(sz::Integer) = Int(ccall(:jl_alignment, Cint, (Csize_t,), sz))