Disallow more circulation once we set lb==ub.

close #26487.
This should be valid as we never set `X<:Y<:X` (assuming `Y` is outer var).

Author: N5N3

src/subtype.c

@@ -3249,7 +3249,13 @@ static jl_value_t *intersect(jl_value_t *x, jl_value_t *y, jl_stenv_t *e, int pa
                     return jl_bottom_type;
                 }
                 int ccheck;
-                if (yub == xub ||
+                if (xlb == xub && ylb == yub && jl_has_typevar(xlb, y) && jl_has_typevar(ylb, x)) {
+                    // specical case for e.g.
+                    // 1) Val{Y}<:X<:Val{Y} && Val{X}<:Y<:Val{X}
+                    // 2) Y<:X<:Y && Val{X}<:Y<:Val{X} => Val{Y}<:Y<:Val{Y}
+                    ccheck = 0;
+                }
+                else if (yub == xub ||
                     (subtype_by_bounds(xlb, yub, e) && subtype_by_bounds(ylb, xub, e))) {
                     ccheck = 1;
                 }

test/subtype.jl

@@ -2357,6 +2357,11 @@ end
                Tuple{Type{Vector{Union{T, R}}}, Matrix{Union{T, R}}} where {R<:Real, T<:Real},
                Union{})
 
+#issue 26487
+@testintersect(Tuple{Type{Tuple{T,Val{T}}}, Val{T}} where T,
+               Tuple{Type{Tuple{Val{T},T}}, Val{T}} where T,
+               Union{})
+
 @testset "known subtype/intersect issue" begin
     #issue 45874
     # Causes a hang due to jl_critical_error calling back into malloc...
@@ -2384,9 +2389,6 @@ end
     #issue 33137
     @test_broken (Tuple{Q,Int} where Q<:Int) <: Tuple{T,T} where T
 
-    #issue 26487
-    @test_broken typeintersect(Tuple{Type{Tuple{T,Val{T}}}, Val{T}} where T, Tuple{Type{Tuple{Val{T},T}}, Val{T}} where T) <: Any
-
     # issue 24333
     @test_broken (Type{Union{Ref,Cvoid}} <: Type{Union{T,Cvoid}} where T)