fixes #12033 (#12039)

compiler/semstmts.nim

@@ -1904,6 +1904,10 @@ proc semProcAux(c: PContext, n: PNode, kind: TSymKind,
         discard
       popProcCon(c)
   else:
+    if s.kind in {skProc, skFunc} and s.typ[0] != nil and s.typ[0].kind == tyUntyped:
+      # `auto` is represented as `tyUntyped` at this point in compilation.
+      localError(c.config, n[paramsPos][0].info, "return type 'auto' cannot be used in forward declarations")
+
     if s.kind == skMethod: semMethodPrototype(c, s, n)
     if proto != nil: localError(c.config, n.info, errImplOfXexpected % proto.name.s)
     if {sfImportc, sfBorrow, sfError} * s.flags == {} and s.magic == mNone:

tests/proc/tillegalreturntype.nim

@@ -1,8 +1,11 @@
 discard """
   cmd: "nim check $file"
   errmsg: ""
-  nimout: '''tillegalreturntype.nim(8, 11) Error: return type 'typed' is only valid for macros and templates
+  nimout: '''
-tillegalreturntype.nim(11, 11) Error: return type 'untyped' is only valid for macros and templates'''
+tillegalreturntype.nim(11, 11) Error: return type 'typed' is only valid for macros and templates
+tillegalreturntype.nim(14, 11) Error: return type 'untyped' is only valid for macros and templates
+tillegalreturntype.nim(17, 41) Error: return type 'auto' cannot be used in forward declarations
+'''
 """
 
 proc x(): typed =
@@ -10,3 +13,9 @@ proc x(): typed =
 
 proc y(): untyped =
   discard
+
+proc test_proc[T, U](arg1: T, arg2: U): auto
+
+proc test_proc[T, U](arg1: T, arg2: U): auto =
+    echo "Proc has been called"
+    return arg1 / arg2