fixes #13378 [backport] (#13392)

compiler/sigmatch.nim

@@ -1942,6 +1942,8 @@ proc localConvMatch(c: PContext, m: var TCandidate, f, a: PType,
 
   if result != nil:
     if result.typ == nil: return nil
+    # bug #13378, ensure we produce a real generic instantiation:
+    result = c.semExpr(c, call)
     # resulting type must be consistent with the other arguments:
     var r = typeRel(m, f[0], result.typ)
     if r < isGeneric: return nil

tests/generics/tgeneric0.nim

@@ -4,6 +4,7 @@ discard """
 0
 float32
 float32
+(name: "Resource 1", readers: ..., writers: ...)
 '''
 """
 
@@ -124,3 +125,18 @@ block tgeneric5:
     echo T
 
   bar(0.0'f32)
+
+# bug #13378
+
+type
+  Resource = ref object of RootObj
+    name: string
+    readers, writers: seq[RenderTask]
+
+  RenderTask = ref object
+    name: string
+
+var res = Resource(name: "Resource 1")
+
+(proc (r: typeof(res)) =
+   echo r[])(res)