Fix generic forward declarations; fixes #4104; fixes #4908 (#5566)

tests/generics/tforwardgeneric.nim

@@ -1,7 +1,6 @@
 discard """
-  output: "1.1000000000000001e+00 11"
+  output: "1.1 11\n42\n0"
   ccodecheck: "!@'ClEnv'"
-  disabled: "true"
 """
 
 proc p[T](a, b: T): T
@@ -12,3 +11,18 @@ proc p[T](a, b: T): T =
   let c = b
   result = a + b + c
 
+# https://github.com/nim-lang/Nim/issues/4908
+proc foo(t: typedesc[int]): int
+proc bar(): int = foo(int)
+proc foo(t: typedesc[int]): int =
+  return 0
+
+# https://github.com/nim-lang/Nim/issues/4104
+proc print[T](t: T) # Error: implementation of 'print.print(t: int)' expected
+print 42 # moving this line after the implementation fixes the error,
+         # but such behaviour makes forward declaration pointless
+proc print[T](t: T) =
+  echo t
+
+echo bar()
+