consider proc return type as weak reference in codegen (#24894)

fixes #7706

(cherry picked from commit 9c2593444a)

compiler/ccgtypes.nim

@@ -597,7 +597,7 @@ proc genProcParams(m: BModule; t: PType, rettype, params: var Rope,
   if t.returnType == nil or isInvalidReturnType(m.config, t):
     rettype = "void"
   else:
-    rettype = getTypeDescAux(m, t.returnType, check, dkResult)
+    rettype = getTypeDescWeak(m, t.returnType, check, dkResult)
   for i in 1..<t.n.len:
     if t.n[i].kind != nkSym: internalError(m.config, t.n.info, "genProcParams")
     var param = t.n[i].sym

tests/proc/trecursivereturntype.nim

@@ -0,0 +1,15 @@
+# issue #7706
+
+type
+  Op = enum
+    Halt
+    Inc
+    Dec
+
+type
+  InstrNext = proc (val: var int, code: seq[Op], pc: var int, stop: var bool): OpH {.inline, nimcall.}
+
+  OpH = object
+    handler: InstrNext
+
+var a: OpH