morespecific: add rule for Type{Union{}}

Make Type{Union{}} in method definitions always the most specific type
(normally these would end up being ambiguous). This ensures we do not
invalidate them, nor need to consider ambiguities that might arise from
intersections with them.

NEWS.md

@@ -8,6 +8,12 @@ Language changes
 ----------------
 
 * When a task forks a child, the parent task's task-local RNG (random number generator) is no longer affected. The seeding of child based on the parent task also takes a more disciplined approach to collision resistance, using a design based on the SplitMix and DotMix splittable RNG schemes ([#49110]).
+* A new morespecific rule for methods resolves ambiguities containing Union{} in favor of
+  the method defined explicitly to handle the Union{} argument. This makes it possible to
+  define methods to explicitly handle Union{} without the ambiguities that commonly would
+  result previously. This also lets the runtime optimize certain method lookups in a way
+  that significantly improves load and inference times for heavily overloaded methods that
+  dispatch on Types (such as traits and constructors).
 
 Compiler/Runtime improvements
 -----------------------------

base/essentials.jl

@@ -310,13 +310,8 @@ See also: [`round`](@ref), [`trunc`](@ref), [`oftype`](@ref), [`reinterpret`](@r
 """
 function convert end
 
-# make convert(::Type{<:Union{}}, x::T) intentionally ambiguous for all T
-# so it will never get called or invalidated by loading packages
-# with carefully chosen types that won't have any other convert methods defined
-convert(T::Type{<:Core.IntrinsicFunction}, x) = throw(MethodError(convert, (T, x)))
-convert(T::Type{<:Nothing}, x) = throw(MethodError(convert, (Nothing, x)))
-convert(::Type{T}, x::T) where {T<:Core.IntrinsicFunction} = x
-convert(::Type{T}, x::T) where {T<:Nothing} = x
+# ensure this is never ambiguous, and therefore fast for lookup
+convert(T::Type{Union{}}, x) = throw(MethodError(convert, (T, x)))
 
 convert(::Type{Type}, x::Type) = x # the ssair optimizer is strongly dependent on this method existing to avoid over-specialization
                                    # in the absence of inlining-enabled

src/subtype.c

@@ -4460,6 +4460,21 @@ static int num_occurs(jl_tvar_t *v, jl_typeenv_t *env)
     return 0;
 }
 
+int tuple_cmp_typeofbottom(jl_datatype_t *a, jl_datatype_t *b)
+{
+    size_t i, la = jl_nparams(a), lb = jl_nparams(b);
+    for (i = 0; i < la || i < lb; i++) {
+        jl_value_t *pa = i < la ? jl_tparam(a, i) : NULL;
+        jl_value_t *pb = i < lb ? jl_tparam(b, i) : NULL;
+        int xa = pa == (jl_value_t*)jl_typeofbottom_type || pa == (jl_value_t*)jl_typeofbottom_type->super;
+        int xb = pb == (jl_value_t*)jl_typeofbottom_type || pb == (jl_value_t*)jl_typeofbottom_type->super;
+        if (xa != xb)
+            return xa - xb;
+    }
+    return 0;
+}
+
+
 #define HANDLE_UNIONALL_A                                               \
     jl_unionall_t *ua = (jl_unionall_t*)a;                              \
     jl_typeenv_t newenv = { ua->var, 0x0, env };                        \
@@ -4478,6 +4493,13 @@ static int type_morespecific_(jl_value_t *a, jl_value_t *b, jl_value_t *a0, jl_v
         return 0;
 
     if (jl_is_tuple_type(a) && jl_is_tuple_type(b)) {
+        // compare whether a and b have Type{Union{}} included,
+        // which makes them instantly the most specific, regardless of all else,
+        // for whichever is left most (the left-to-right behavior here ensures
+        // we do not need to keep track of conflicts with multiple methods).
+        int msp = tuple_cmp_typeofbottom((jl_datatype_t*)a, (jl_datatype_t*)b);
+        if (msp)
+            return msp > 0;
         // When one is JL_VARARG_BOUND and the other has fixed length,
         // allow the argument length to fix the tvar
         jl_vararg_kind_t akind = jl_va_tuple_kind((jl_datatype_t*)a);

test/specificity.jl

@@ -311,3 +311,8 @@ let A = Tuple{Type{SubString{S}},AbstractString} where S<:AbstractString,
     @test  args_morespecific(B, C)
     @test  args_morespecific(A, C)
 end
+
+@test args_morespecific(Tuple{Type{Union{}}, Any}, Tuple{Any, Type{Union{}}})
+@test args_morespecific(Tuple{typeof(Union{}), Any}, Tuple{Any, Type{Union{}}})
+@test args_morespecific(Tuple{Type{Union{}}, Type{Union{}}, Any}, Tuple{Type{Union{}}, Any, Type{Union{}}})
+@test args_morespecific(Tuple{Type{Union{}}, Type{Union{}}, Any, Type{Union{}}}, Tuple{Type{Union{}}, Any, Type{Union{}}, Type{Union{}}})