subtype: add repeating/separable fast path into subtype_vararg

fix JuliaLang#58115

Author: N5N3

src/subtype.c

@@ -1069,7 +1069,8 @@ static int forall_exists_equal(jl_value_t *x, jl_value_t *y, jl_stenv_t *e);
 
 static int subtype_tuple_varargs(
     jl_vararg_t *vtx, jl_vararg_t *vty,
-    size_t vx, size_t vy,
+    jl_value_t *lastx, jl_value_t *lasty,
+    size_t vx, size_t vy, size_t x_reps,
     jl_stenv_t *e, int param)
 {
     jl_value_t *xp0 = jl_unwrap_vararg(vtx); jl_value_t *xp1 = jl_unwrap_vararg_num(vtx);
@@ -1111,12 +1112,30 @@ static int subtype_tuple_varargs(
             }
         }
     }
-
-    // in Vararg{T1} <: Vararg{T2}, need to check subtype twice to
-    // simulate the possibility of multiple arguments, which is needed
-    // to implement the diagonal rule correctly.
-    if (!subtype(xp0, yp0, e, param)) return 0;
-    if (!subtype(xp0, yp0, e, 1)) return 0;
+    int x_same = vx > 1 || (lastx && obviously_egal(xp0, lastx));
+    int y_same = vy > 1 || (lasty && obviously_egal(yp0, lasty));
+    // keep track of number of consecutive identical subtyping
+    x_reps = y_same && x_same ? x_reps + 1 : 1;
+    if (x_reps > 2) {
+        // an identical type on the left doesn't need to be compared to the same
+        // element type on the right more than twice.
+    }
+    else if (x_same && e->Runions.depth == 0 && y_same &&
+        !jl_has_free_typevars(xp0) && !jl_has_free_typevars(yp0)) {
+        // fast path for repeated elements
+    }
+    else if (e->Runions.depth == 0 && !jl_has_free_typevars(xp0) && !jl_has_free_typevars(yp0)) {
+        // fast path for separable sub-formulas
+        if (!jl_subtype(xp0, yp0))
+            return 0;
+    }
+    else {
+        // in Vararg{T1} <: Vararg{T2}, need to check subtype twice to
+        // simulate the possibility of multiple arguments, which is needed
+        // to implement the diagonal rule correctly.
+        if (!subtype(xp0, yp0, e, param)) return 0;
+        if (x_reps < 2 && !subtype(xp0, yp0, e, 1)) return 0;
+    }
 
 constrain_length:
     if (!yp1) {
@@ -1246,7 +1265,8 @@ static int subtype_tuple_tail(jl_datatype_t *xd, jl_datatype_t *yd, int8_t R, jl
             return subtype_tuple_varargs(
                 (jl_vararg_t*)xi,
                 (jl_vararg_t*)yi,
-                vx, vy, e, param);
+                lastx, lasty,
+                vx, vy, x_reps, e, param);
         }
 
         if (j >= ly)

test/subtype.jl

@@ -2782,3 +2782,7 @@ let Tvar1 = TypeVar(:Tvar1), Tvar2 = TypeVar(:Tvar2)
     V2 = UnionAll(Tvar2, Union{(@eval($(Symbol(:T58129, k)){$Tvar2}) for k in 1:100)...})
     @test Set{<:V2} <: AbstractSet{<:V1}
 end
+
+#issue 58115
+@test Tuple{Tuple{Vararg{Tuple{Vararg{Tuple{Vararg{Tuple{Vararg{Tuple{Vararg{             Union{Tuple{}, Tuple{Tuple{}}}}}}}}}}}}}  , Tuple{}} <:
+      Tuple{Tuple{Vararg{Tuple{Vararg{Tuple{Vararg{Tuple{Vararg{Tuple{Vararg{Tuple{Vararg{Union{Tuple{}, Tuple{Tuple{}}}}}}}}}}}}}}}, Tuple{}}