From 2a8c759df0e8a952f7cbea8539e3fb7aa234795a Mon Sep 17 00:00:00 2001
From: SirOlaf <34164198+SirOlaf@users.noreply.github.com>
Date: Fri, 8 Sep 2023 06:50:39 +0200
Subject: [PATCH] Fix #21742: Check generic alias depth before skip (#22443)

Close #21742

Checking if there's any side-effects and if just changing typeRel is
adequate for this issue before trying to look into related ones.

`skipBoth` is also not that great, it can lead to code that works
sometimes but fails when the proc is instantiated with branching
aliases. This is mostly an issue with error clarity though.

---------

Co-authored-by: SirOlaf <unknown>
Co-authored-by: SirOlaf <>
---
 compiler/sigmatch.nim     |  8 ++++++--
 compiler/types.nim        |  7 +++++++
 tests/generics/t21742.nim | 10 ++++++++++
 3 files changed, 23 insertions(+), 2 deletions(-)
 create mode 100644 tests/generics/t21742.nim

diff --git a/compiler/sigmatch.nim b/compiler/sigmatch.nim
index bb99463b64..1cf29dcfd4 100644
--- a/compiler/sigmatch.nim
+++ b/compiler/sigmatch.nim
@@ -1456,8 +1456,12 @@ proc typeRel(c: var TCandidate, f, aOrig: PType,
     let origF = f
     var f = if prev == nil: f else: prev
 
-    let roota = a.skipGenericAlias
-    let rootf = f.skipGenericAlias
+    let deptha = a.genericAliasDepth()
+    let depthf = f.genericAliasDepth()
+    let skipBoth = deptha == depthf and (a.len > 0 and f.len > 0 and a.base != f.base)
+
+    let roota = if skipBoth or deptha > depthf: a.skipGenericAlias else: a
+    let rootf = if skipBoth or depthf > deptha: f.skipGenericAlias else: f
 
     if a.kind == tyGenericInst:
       if roota.base == rootf.base:
diff --git a/compiler/types.nim b/compiler/types.nim
index 4dc9d36f5f..f5e965df98 100644
--- a/compiler/types.nim
+++ b/compiler/types.nim
@@ -1175,6 +1175,13 @@ proc sameChildrenAux(a, b: PType, c: var TSameTypeClosure): bool =
 proc isGenericAlias*(t: PType): bool =
   return t.kind == tyGenericInst and t.lastSon.kind == tyGenericInst
 
+proc genericAliasDepth*(t: PType): int =
+  result = 0
+  var it = t
+  while it.isGenericAlias:
+    it = it.lastSon
+    inc result
+
 proc skipGenericAlias*(t: PType): PType =
   return if t.isGenericAlias: t.lastSon else: t
 
diff --git a/tests/generics/t21742.nim b/tests/generics/t21742.nim
new file mode 100644
index 0000000000..c49c8ee97c
--- /dev/null
+++ b/tests/generics/t21742.nim
@@ -0,0 +1,10 @@
+type
+  Foo[T] = object
+    x:T
+  Bar[T,R] = Foo[T]
+  Baz = Bar[int,float]
+
+proc qux[T,R](x: Bar[T,R]) = discard
+
+var b:Baz
+b.qux()
\ No newline at end of file