improve isdefined precision for 0 field types

Compiler/src/tfuncs.jl

@@ -450,6 +450,10 @@ end
                     return Const(true)
                 end
             end
+        # datatype_fieldcount is what `fieldcount` uses internally
+        # and returns nothing (!==0) for non-definite field counts.
+        elseif datatype_fieldcount(a1) === 0
+            return Const(false)
         end
     elseif isa(a1, Union)
         # Results can only be `Const` or `Bool`

Compiler/test/inference.jl

@@ -1210,6 +1210,7 @@ let isdefined_tfunc(@nospecialize xs...) =
     @test isdefined_tfunc(Union{UnionIsdefinedA,UnionIsdefinedB}, Const(:x)) === Const(true)
     @test isdefined_tfunc(Union{UnionIsdefinedA,UnionIsdefinedB}, Const(:y)) === Const(false)
     @test isdefined_tfunc(Union{UnionIsdefinedA,Nothing}, Const(:x)) === Bool
+    @test isdefined_tfunc(Nothing, Any) === Const(false)
 end
 
 # https://github.com/aviatesk/JET.jl/issues/379

base/runtime_internals.jl

@@ -1116,7 +1116,7 @@ function datatype_fieldcount(t::DataType)
             return length(names)
         end
         if types isa DataType && types <: Tuple
-            return fieldcount(types)
+            return datatype_fieldcount(types)
         end
         return nothing
     elseif isabstracttype(t)