fixes #4756 (#5557)

compiler/sigmatch.nim

@@ -1086,8 +1086,12 @@ proc typeRel(c: var TCandidate, f, aOrig: PType, doBind = true): TTypeRelation =
   of tyBuiltInTypeClass:
     considerPreviousT:
       let targetKind = f.sons[0].kind
-      if targetKind == a.skipTypes({tyRange, tyGenericInst, tyBuiltInTypeClass, tyAlias}).kind or
+      let effectiveArgType = a.skipTypes({tyRange, tyGenericInst,
-         (targetKind in {tyProc, tyPointer} and a.kind == tyNil):
+                                          tyBuiltInTypeClass, tyAlias})
+      let typeClassMatches = targetKind == effectiveArgType.kind and
+                             not effectiveArgType.isEmptyContainer
+      if typeClassMatches or
+        (targetKind in {tyProc, tyPointer} and effectiveArgType.kind == tyNil):
         put(c, f, a)
         return isGeneric
       else:

tests/errmsgs/t4756.nim

@@ -0,0 +1,16 @@
+discard """
+errormsg: "type mismatch: got (string, arr: seq[empty])"
+line: 15
+"""
+
+# https://github.com/nim-lang/Nim/issues/4756
+
+type
+  Test* = ref object
+    name*: string
+
+proc newTest(name: string, arr: seq): Test =
+    result = Test(name: name)
+
+let test = newTest("test", arr = @[])
+