skip tyAlias in generic alias checks [backport:2.0] (#24417)

fixes #24415

Since #23978 (which is in 2.0), all generic types that alias to another
type now insert a `tyAlias` layer in their value. However the
`skipGenericAlias` etc code which `sigmatch` uses is not updated for
this, so `tyAlias` is now skipped in these.

The relevant code in sigmatch is:
https://github.com/nim-lang/Nim/blob/67ad1ae1598b08039c971812dc172dd48624b7b0/compiler/sigmatch.nim#L1668-L1673

This behavior is also suspicious IMO, not skipping a structural
`tyGenericInst` alias can be useful for code like #10220, but this is
currently arbitrarily decided based on "depth" and whether the alias is
to another `tyGenericInst` type or not. Maybe in the future we could
enforce the use of a nominal type.

compiler/types.nim

@@ -1201,17 +1201,19 @@ proc sameChildrenAux(a, b: PType, c: var TSameTypeClosure): bool =
     if not result: return
 
 proc isGenericAlias*(t: PType): bool =
-  return t.kind == tyGenericInst and t.skipModifier.kind == tyGenericInst
+  return t.kind == tyGenericInst and t.skipModifier.skipTypes({tyAlias}).kind == tyGenericInst
 
 proc genericAliasDepth*(t: PType): int =
   result = 0
-  var it = t
+  var it = t.skipTypes({tyAlias})
   while it.isGenericAlias:
-    it = it.skipModifier
+    it = it.skipModifier.skipTypes({tyAlias})
     inc result
 
 proc skipGenericAlias*(t: PType): PType =
-  return if t.isGenericAlias: t.skipModifier else: t
+  result = t.skipTypes({tyAlias})
+  if result.isGenericAlias:
+    result = result.skipModifier.skipTypes({tyAlias})
 
 proc sameFlags*(a, b: PType): bool {.inline.} =
   result = eqTypeFlags*a.flags == eqTypeFlags*b.flags

tests/overload/tgenericalias.nim

@@ -11,3 +11,12 @@ block: # issue #13799
   proc works2(): Y[int] = t(X[int, int])
   proc works3(): Y[int] = t(Y[int])
   proc broken(): Y[int] = s(Y[int])
+
+block: # issue #24415
+  type GVec2[T] = object
+    x, y: T
+  type Uniform[T] = T
+  proc foo(v: Uniform[GVec2[float32]]): float32 =
+    result = v.x
+  let f = GVec2[float32](x: 1.0f, y: 2.0f)
+  doAssert foo(f) == 1.0f