fixes #20141; dereferencing pointer to incomplete type error with cast (#20147)

Co-authored-by: xflywind <43030857+xflywind@users.noreply.github.com>

compiler/ccgexprs.nim

@@ -2189,6 +2189,11 @@ proc genSomeCast(p: BProc, e: PNode, d: var TLoc) =
     elif etyp.kind == tyBool and srcTyp.kind in IntegralTypes:
       putIntoDest(p, d, e, "(($1) != 0)" % [rdCharLoc(a)], a.storage)
     else:
+      if etyp.kind == tyPtr:
+        # generates the definition of structs for casts like cast[ptr object](addr x)[]
+        let internalType = etyp.skipTypes({tyPtr})
+        if internalType.kind == tyObject:
+          discard getTypeDesc(p.module, internalType)
       putIntoDest(p, d, e, "(($1) ($2))" %
           [getTypeDesc(p.module, e.typ), rdCharLoc(a)], a.storage)
 

tests/ccgbugs/t20141.nim

@@ -0,0 +1,27 @@
+discard """
+  joinable: false
+"""
+
+# bug #20141
+type
+  A = object
+  B = object
+  U = proc()
+
+proc m(h: var B) = discard
+
+template n[T, U](x: U): T =
+  static: doAssert true
+  cast[ptr T](addr x)[]
+
+proc k() =
+  var res: A
+  m(n[B](res))
+
+proc w(mounter: U) = discard
+
+proc mount(proto: U) = discard
+proc v() = mount k
+
+# This is required for failure
+w(v)