fix subtype match of generic object types

compiler/sigmatch.nim

@@ -642,12 +642,22 @@ proc genericParamPut(c: var TCandidate; last, fGenericOrigin: PType) =
       if x == nil:
         put(c, fGenericOrigin[i], last[i])
 
+proc isGenericObjectOf(f, a: PType): bool =
+  ## checks if `f` is an unparametrized generic type
+  ## that `a` is an instance of
+  if not (f.sym != nil and f.sym.typ.kind == tyGenericBody):
+    # covers the case where `f` is the last child (body) of the `tyGenericBody`
+    return false
+  let aRoot = genericRoot(a)
+  # use sym equality to check if the `tyGenericBody` types are equal
+  result = aRoot != nil and f.sym == aRoot.sym
+
 proc isObjectSubtype(c: var TCandidate; a, f, fGenericOrigin: PType): int =
   var t = a
   assert t.kind == tyObject
   var depth = 0
   var last = a
-  while t != nil and not sameObjectTypes(f, t):
+  while t != nil and not (sameObjectTypes(f, t) or isGenericObjectOf(f, t)):
     if t.kind != tyObject:  # avoid entering generic params etc
       return -1
     t = t.baseClass

compiler/types.nim

@@ -1938,6 +1938,9 @@ proc isCharArrayPtr*(t: PType; allowPointerToChar: bool): bool =
   else:
     result = false
 
+proc isRefPtrObject*(t: PType): bool =
+  t.kind in {tyRef, tyPtr} and tfRefsAnonObj in t.flags
+
 proc nominalRoot*(t: PType): PType =
   ## the "name" type of a given instance of a nominal type,
   ## i.e. the type directly associated with the symbol where the root
@@ -1970,7 +1973,7 @@ proc nominalRoot*(t: PType): PType =
       result = result.skipModifier[0]
     let val = result.skipModifier
     if val.kind in {tyDistinct, tyEnum, tyObject} or
-        (val.kind in {tyRef, tyPtr} and tfRefsAnonObj in val.flags):
+        isRefPtrObject(val):
       # atomic nominal types, this generic body is attached to them
       discard
     else:
@@ -2000,3 +2003,20 @@ proc nominalRoot*(t: PType): PType =
     # skips all typeclasses
     # is this correct for `concept`?
     result = nil
+
+proc genericRoot*(t: PType): PType =
+  ## gets the root generic type (`tyGenericBody`) from `t`,
+  ## if `t` is a generic type or the body of a generic instantiation
+  case t.kind
+  of tyGenericBody:
+    result = t
+  of tyGenericInst, tyGenericInvocation:
+    result = t.genericHead
+  else:
+    if t.typeInst != nil:
+      result = t.typeInst.genericHead
+    elif t.sym != nil and t.sym.typ.kind == tyGenericBody:
+      # can happen if `t` is the last child (body) of the generic body
+      result = t.sym.typ
+    else:
+      result = nil

tests/objects/tgenericsubtype.nim

@@ -0,0 +1,18 @@
+block: # has generic field
+  type
+    Foo[T] = object of RootObj
+      x: T
+    Bar = object of Foo[int]
+
+  proc foo(x: typedesc[Foo]) = discard
+
+  foo(Bar)
+
+block: # no generic field
+  type
+    Foo[T] = object of RootObj
+    Bar = object of Foo[int]
+
+  proc foo(x: typedesc[Foo]) = discard
+
+  foo(Bar)