don't use binding of auto for inferred return type

fixes nim-lang#23200

compiler/semstmts.nim

@@ -1819,7 +1819,15 @@ proc semInferredLambda(c: PContext, pt: TIdTable, n: PNode): PNode =
   #incl(s.flags, sfFromGeneric)
   #s.owner = original
 
+  let originalReturnType = n.typ[0]
   n = replaceTypesInBody(c, pt, n, original)
+  if originalReturnType != nil and originalReturnType.kind == tyAnything:
+    # the return type should not be set to a previous binding of `auto`
+    # this could alternatively be done by changing tyAnything to
+    # differentiate based on which place it's inferring the type of
+    # (i.e. a unique instance of tyAnything for every reference to `auto`),
+    # but for now this is sufficient as we only care to infer return types
+    n.typ[0] = originalReturnType
   result = n
   s.ast = result
   n[namePos].sym = s

tests/overload/t23200.nim

@@ -0,0 +1,16 @@
+import std/[sugar, sequtils]
+
+proc dosomething(iter: int -> (iterator: int)) =
+  discard
+
+proc dosomething(iter: int -> seq[int]) =
+  discard
+
+proc makeSeq(x: int): seq[int] =
+  @[x]
+
+# Works fine with 1.6.12 and 1.6.14
+dosomething(makeSeq)
+
+# Works with 1.6.12, fails with 1.6.14
+dosomething((y) => makeSeq(y))

tests/types/t15836_2.nim

@@ -1,8 +1,3 @@
-
-discard """
-  action: "compile"
-  disabled: true
-"""
 import std/sugar
 
 type Tensor[T] = object