From 484ce822c4ef0d3cccf083ce5aa3b98cf5e30c5d Mon Sep 17 00:00:00 2001
From: N5N3 <2642243996@qq.com>
Date: Wed, 11 Jan 2023 15:10:47 +0800
Subject: [PATCH] Disallow more circulation once we set `lb`==`ub`.

close #26487.
This should be valid as we never set `X<:Y<:X` (assuming `Y` is the outer var).
---
 src/subtype.c   | 10 +++++++++-
 test/subtype.jl |  8 +++++---
 2 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/src/subtype.c b/src/subtype.c
index 59055e6bc168e..8189e9701e452 100644
--- a/src/subtype.c
+++ b/src/subtype.c
@@ -3249,7 +3249,15 @@ 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, (jl_tvar_t *)y) && 
+                    jl_has_typevar(ylb, (jl_tvar_t *)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;
                 }
diff --git a/test/subtype.jl b/test/subtype.jl
index 3517e8fdc906c..7236d3438a692 100644
--- a/test/subtype.jl
+++ b/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)