fixes #5142

compiler/ccgtypes.nim

@@ -116,16 +116,18 @@ proc mangleName(m: BModule; s: PSym): Rope =
       add(result, m.idOrSig(s))
     s.loc.r = result
 
-proc typeName(typ: PType): Rope =
-  result = if typ.sym != nil and typ.kind in {tyObject, tyEnum}:
-             typ.sym.name.s.mangle.rope
-           else:
-             ~"TY"
 
 const
   irrelevantForBackend = {tyGenericBody, tyGenericInst, tyGenericInvocation,
                           tyDistinct, tyRange, tyStatic, tyAlias}
 
+proc typeName(typ: PType): Rope =
+  let typ = typ.skipTypes(irrelevantForBackend)
+  result = if typ.sym != nil and typ.kind in {tyObject, tyEnum}:
+             typ.sym.name.s.mangle.rope
+           else:
+             ~"TY"
+
 proc getTypeName(m: BModule; typ: PType; sig: SigHash): Rope =
   var t = typ
   while true:

tests/cpp/tembarrassing_generic_failure.nim

@@ -0,0 +1,8 @@
+discard """
+  cmd: "nim cpp --threads:on $file"
+"""
+
+# bug #5142
+
+var ci: Channel[int]
+ci.open