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 <>
(cherry picked from commit 2a8c759df0)

compiler/sigmatch.nim

@@ -1453,8 +1453,12 @@ 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:

compiler/types.nim

@@ -1184,6 +1184,13 @@ 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
 

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()