From 3992e138a22d4c6db5444b8ed5773c9a72f2c7cf Mon Sep 17 00:00:00 2001
From: apaz-cli <aarpazdera@gmail.com>
Date: Thu, 27 Oct 2022 18:08:23 -0500
Subject: [PATCH] Improve error message for `which(fn, types)`

Error messages for `MethodError` are much more helpful
in determining why the method was not successfully dispatched
than simply "No unique matching method found."

Fixes #47322
---
 base/reflection.jl | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/base/reflection.jl b/base/reflection.jl
index 3c5887ec8dfb6e..f85df3d3b43911 100644
--- a/base/reflection.jl
+++ b/base/reflection.jl
@@ -1505,7 +1505,12 @@ See also: [`parentmodule`](@ref), and `@which` and `@edit` in [`InteractiveUtils
 function which(@nospecialize(f), @nospecialize(t))
     t = to_tuple_type(t)
     tt = signature_type(f, t)
-    return which(tt)
+    world = get_world_counter()
+    match, _ = Core.Compiler._findsup(tt, nothing, world)
+    if match === nothing
+        throw(MethodError(f, t, world))
+    end
+    return match.method
 end
 
 """