fix crash with tyBuiltInTypeClass matching itself

fixes #24449

compiler/sigmatch.nim

@@ -1878,9 +1878,12 @@ proc typeRel(c: var TCandidate, f, aOrig: PType,
       let target = f.genericHead
       let targetKind = target.kind
       var effectiveArgType = reduceToBase(a)
+      # the skipped child of tyBuiltInTypeClass can be structured differently,
+      # newConstraint constructs them with no children
+      let typeClassArg = effectiveArgType.kind == tyBuiltInTypeClass
       effectiveArgType = effectiveArgType.skipTypes({tyBuiltInTypeClass})
       if targetKind == effectiveArgType.kind:
-        if effectiveArgType.isEmptyContainer:
+        if not typeClassArg and effectiveArgType.isEmptyContainer:
           return isNone
         if targetKind == tyProc:
           if target.flags * {tfIterator} != effectiveArgType.flags * {tfIterator}:

tests/overload/tvartypeclass.nim

@@ -1,11 +1,16 @@
-# issue #13302
+block: # issue #13302
+  proc foo(x: object): int = x.i*2
+  proc foo(x: var object) = x.i*=2
+  type Foo = object
+    i: int
+  let x = Foo(i: 3)
+  var y = Foo(i: 4)
+  doAssert foo(x) == 6
+  foo(y)
+  doAssert y.i == 8
 
-proc foo(x: object): int = x.i*2
-proc foo(x: var object) = x.i*=2
-type Foo = object
-  i: int
-let x = Foo(i: 3)
-var y = Foo(i: 4)
-doAssert foo(x) == 6
-foo(y)
-doAssert y.i == 8
+block: # issue #24449
+  proc p(x: var seq)= discard
+  proc p(x: seq)= discard
+  var s : seq[int]
+  p(s)