allow nested combinations of (named)tuples, symbols, and bits as type parameters

NEWS.md

@@ -9,6 +9,7 @@ New language features
   handled via `Base.split_rest`. ([#42902])
 * Character literals now support the same syntax allowed in string literals; i.e. the syntax can
   represent invalid UTF-8 sequences as allowed by the `Char` type ([#44989]).
+* Nested combinations of tuples and named tuples of symbols are now allowed as type parameters ([#46300]).
 
 Language changes
 ----------------

base/compiler/tfuncs.jl

@@ -1351,18 +1351,7 @@ end
 add_tfunc(fieldtype, 2, 3, fieldtype_tfunc, 0)
 
 # Like `valid_tparam`, but in the type domain.
-function valid_tparam_type(T::DataType)
-    T === Symbol && return true
-    isbitstype(T) && return true
-    if T <: Tuple
-        isconcretetype(T) || return false
-        for P in T.parameters
-            (P === Symbol || isbitstype(P)) || return false
-        end
-        return true
-    end
-    return false
-end
+valid_tparam_type(T::DataType) = valid_typeof_tparam(T)
 valid_tparam_type(U::Union) = valid_tparam_type(U.a) && valid_tparam_type(U.b)
 valid_tparam_type(U::UnionAll) = valid_tparam_type(unwrap_unionall(U))
 

base/compiler/typeutils.jl

@@ -102,17 +102,26 @@ function valid_as_lattice(@nospecialize(x))
     return false
 end
 
-# test if non-Type, non-TypeVar `x` can be used to parameterize a type
-function valid_tparam(@nospecialize(x))
-    if isa(x, Tuple)
-        for t in x
-            isa(t, Symbol) || isbits(t) || return false
+function valid_typeof_tparam(@nospecialize(t))
+    if t === Symbol || isbitstype(t)
+        return true
+    end
+    isconcretetype(t) || return false
+    if t <: NamedTuple
+        t = t.parameters[2]
+    end
+    if t <: Tuple
+        for p in t.parameters
+            valid_typeof_tparam(p) || return false
         end
         return true
     end
-    return isa(x, Symbol) || isbits(x)
+    return false
 end
 
+# test if non-Type, non-TypeVar `x` can be used to parameterize a type
+valid_tparam(@nospecialize(x)) = valid_typeof_tparam(typeof(x))
+
 function compatible_vatuple(a::DataType, b::DataType)
     vaa = a.parameters[end]
     vab = a.parameters[end]

src/builtins.c

@@ -1258,20 +1258,28 @@ JL_CALLABLE(jl_f_set_binding_type)
 
 // apply_type -----------------------------------------------------------------
 
-int jl_valid_type_param(jl_value_t *v)
+static int is_nestable_type_param(jl_value_t *t)
 {
-    if (jl_is_tuple(v)) {
+    if (jl_is_namedtuple_type(t))
+        t = jl_tparam1(t);
+    if (jl_is_tuple_type(t)) {
         // NOTE: tuples of symbols are not currently bits types, but have been
         // allowed as type parameters. this is a bit ugly.
-        jl_value_t *tt = jl_typeof(v);
-        size_t i, l = jl_nparams(tt);
-        for(i=0; i < l; i++) {
-            jl_value_t *pi = jl_tparam(tt,i);
-            if (!(pi == (jl_value_t*)jl_symbol_type || jl_isbits(pi)))
+        size_t i, l = jl_nparams(t);
+        for (i = 0; i < l; i++) {
+            jl_value_t *pi = jl_tparam(t, i);
+            if (!(pi == (jl_value_t*)jl_symbol_type || jl_isbits(pi) || is_nestable_type_param(pi)))
                 return 0;
         }
         return 1;
     }
+    return 0;
+}
+
+int jl_valid_type_param(jl_value_t *v)
+{
+    if (jl_is_tuple(v) || jl_is_namedtuple(v))
+        return is_nestable_type_param(jl_typeof(v));
     if (jl_is_vararg(v))
         return 0;
     // TODO: maybe more things

test/core.jl

@@ -1987,9 +1987,8 @@ mutable struct TupleParam{P}
     x::Bool
 end
 
-function tupledispatch(a::TupleParam{(1,:a)})
-    a.x
-end
+tupledispatch(a::TupleParam{(1,:a)}) = a.x
+tupledispatch(a::TupleParam{(1,(:a,))}) = 42
 
 # tuples can be used as type params
 let t1 = TupleParam{(1,:a)}(true),
@@ -2001,6 +2000,10 @@ let t1 = TupleParam{(1,:a)}(true),
     # dispatch works properly
     @test tupledispatch(t1) == true
     @test_throws MethodError tupledispatch(t2)
+
+    @test tupledispatch(TupleParam{(1,(:a,))}(true)) === 42
+    @test_throws TypeError TupleParam{NamedTuple{(:a,), Tuple{Any}}((1,))}
+    @test_throws TypeError Val{NamedTuple{(:a,), Tuple{NamedTuple{<:Any,Tuple{Int}}}}(((x=2,),))}
 end
 
 # issue #5254