permit NamedTuple{<:Any, Union{}} to be created

This is a NamedTuple where all of the fields exist but have type
Union{}. It can be useful for initializing a typejoin in some cases, and
unlike Tuple{Union{}} there are no problems with covariance that should
be preventing it from existing. A few parameters of some constructors were
adjusted for performance and correctness (mostly the former).

Really just an edge case, since nobody should reasonably need a type
that cannot be constructed, but it does seem to mildly improve the
quality of some type queries such as these:

```
julia> fieldtype(NamedTuple{(:a, :b), T} where T<:Union{}, :a)
was: ERROR: TypeError: in fieldtype, expected DataType, got Type{Union{}}
now: Union{}

julia> fieldtypes(NamedTuple{(:a, :b), T} where T<:Union{})
was: ERROR: TypeError: in fieldtype, expected DataType, got Type{Union{}}
now: (Union{}, Union{})
```

Note that `NamedTuple{(), Union{}}` is disallowed still, as that
`Union{}` implies that the generating `Tuple` had least one field.

base/boot.jl

@@ -736,8 +736,8 @@ eval(Core, :(NamedTuple{names}(args::Tuple) where {names} =
 
 using .Intrinsics: sle_int, add_int
 
-eval(Core, :(NamedTuple{names,T}(args::T) where {names, T <: Tuple} =
-             $(Expr(:splatnew, :(NamedTuple{names,T}), :args))))
+eval(Core, :((NT::Type{NamedTuple{names,T}})(args::T) where {names, T <: Tuple} =
+             $(Expr(:splatnew, :NT, :args))))
 
 # constructors for built-in types
 

base/compiler/abstractinterpretation.jl

@@ -2695,7 +2695,7 @@ function refine_partial_type(@nospecialize t)
         # if the first/second parameter of `NamedTuple` is known to be empty,
         # the second/first argument should also be empty tuple type,
         # so refine it here
-        return Const(NamedTuple())
+        return Const((;))
     end
     return t
 end

base/namedtuple.jl

@@ -112,24 +112,26 @@ Core.NamedTuple
 
 if nameof(@__MODULE__) === :Base
 
-@eval function NamedTuple{names,T}(args::Tuple) where {names, T <: Tuple}
+@eval function (NT::Type{NamedTuple{names,T}})(args::Tuple) where {names, T <: Tuple}
     if length(args) != length(names::Tuple)
         throw(ArgumentError("Wrong number of arguments to named tuple constructor."))
     end
     # Note T(args) might not return something of type T; e.g.
     # Tuple{Type{Float64}}((Float64,)) returns a Tuple{DataType}
-    $(Expr(:splatnew, :(NamedTuple{names,T}), :(T(args))))
+    $(Expr(:splatnew, :NT, :(T(args))))
 end
 
-function NamedTuple{names, T}(nt::NamedTuple) where {names, T <: Tuple}
+@eval (NT::Type{NamedTuple{(),Union{}}})(args::Tuple{}) = $(Expr(:new, :NT))
+
+function (NT::Type{NamedTuple{names, T}})(nt::NamedTuple) where {names, T <: Tuple}
     if @generated
-        Expr(:new, :(NamedTuple{names, T}),
-             Any[ :(let Tn = fieldtype(T, $n),
+        Expr(:new, :NT,
+             Any[ :(let Tn = fieldtype(NT, $n),
                       ntn = getfield(nt, $(QuoteNode(names[n])))
                       ntn isa Tn ? ntn : convert(Tn, ntn)
                   end) for n in 1:length(names) ]...)
     else
-        NamedTuple{names, T}(map(Fix1(getfield, nt), names))
+        NT(map(Fix1(getfield, nt), names))
     end
 end
 
@@ -145,14 +147,11 @@ function NamedTuple{names}(nt::NamedTuple) where {names}
     end
 end
 
-NamedTuple{names, T}(itr) where {names, T <: Tuple} = NamedTuple{names, T}(T(itr))
-NamedTuple{names}(itr) where {names} = NamedTuple{names}(Tuple(itr))
+(NT::Type{NamedTuple{names, T}})(itr) where {names, T <: Tuple} = NT(T(itr))
+(NT::Type{NamedTuple{names}})(itr) where {names} = NT(Tuple(itr))
 
 NamedTuple(itr) = (; itr...)
 
-# avoids invalidating Union{}(...)
-NamedTuple{names, Union{}}(itr::Tuple) where {names} = throw(MethodError(NamedTuple{names, Union{}}, (itr,)))
-
 end # if Base
 
 # Like NamedTuple{names, T} as a constructor, but omits the additional

src/builtins.c

@@ -1219,6 +1219,8 @@ static jl_value_t *get_fieldtype(jl_value_t *t, jl_value_t *f, int dothrow)
             tt = ((jl_tvar_t*)tt)->ub;
         if (tt == (jl_value_t*)jl_any_type)
             return (jl_value_t*)jl_any_type;
+        if (tt == (jl_value_t*)jl_bottom_type)
+            return (jl_value_t*)jl_bottom_type;
         JL_GC_PUSH1(&f);
         if (jl_is_symbol(f))
             f = jl_box_long(field_index+1);

src/jltypes.c

@@ -2141,15 +2141,20 @@ static jl_value_t *inst_datatype_inner(jl_datatype_t *dt, jl_svec_t *p, jl_value
                         jl_errorf("duplicate field name in NamedTuple: \"%s\" is not unique", jl_symbol_name((jl_sym_t*)ni));
                 }
             }
-            if (!jl_is_datatype(values_tt))
-                jl_error("NamedTuple field type must be a tuple type");
-            if (jl_is_va_tuple((jl_datatype_t*)values_tt) || jl_nparams(values_tt) != nf)
-                jl_error("NamedTuple names and field types must have matching lengths");
-            ndt->types = ((jl_datatype_t*)values_tt)->parameters;
+            if (values_tt == jl_bottom_type && nf > 0) {
+                ndt->types = jl_svec_fill(nf, jl_bottom_type);
+            }
+            else {
+                if (!jl_is_datatype(values_tt))
+                    jl_error("NamedTuple field type must be a tuple datatype");
+                if (jl_is_va_tuple((jl_datatype_t*)values_tt) || jl_nparams(values_tt) != nf)
+                    jl_error("NamedTuple names and field types must have matching lengths");
+                ndt->types = ((jl_datatype_t*)values_tt)->parameters;
+            }
             jl_gc_wb(ndt, ndt->types);
         }
         else {
-            ndt->types = jl_emptysvec; // XXX: this is essentially always false
+            ndt->types = jl_emptysvec; // XXX: this is essentially always incorrect
         }
     }
     else if (tn == jl_genericmemoryref_typename || tn == jl_genericmemory_typename) {

test/namedtuple.jl

@@ -437,3 +437,18 @@ let c = (a=1, b=2),
     d = (b=3, c=(d=1,))
     @test @inferred(mergewith51009((x,y)->y, c, d)) === (a = 1, b = 3, c = (d = 1,))
 end
+
+@test_throws ErrorException NamedTuple{(), Union{}}
+for NT in (NamedTuple{(:a, :b), Union{}}, NamedTuple{(:a, :b), T} where T<:Union{})
+    @test fieldtype(NT, 1) == Union{}
+    @test fieldtype(NT, :b) == Union{}
+    @test_throws ErrorException fieldtype(NT, :c)
+    @test_throws BoundsError fieldtype(NT, 0)
+    @test_throws BoundsError fieldtype(NT, 3)
+    @test Base.return_types((Type{NT},)) do NT; fieldtype(NT, :a); end == Any[Type{Union{}}]
+    @test fieldtype(NamedTuple{<:Any, Union{}}, 1) == Union{}
+end
+let NT = NamedTuple{<:Any, Union{}}
+    @test fieldtype(NT, 100) == Union{}
+    @test only(Base.return_types((Type{NT},)) do NT; fieldtype(NT, 100); end) >: Type{Union{}}
+end