don't use previous bindings of auto for routine return types (#23207)

fixes #23200, fixes #18866

#21065 made it so `auto` proc return types remained as `tyAnything` and
not turned to `tyUntyped`. This had the side effect that anything
previously bound to `tyAnything` in the proc type match was then bound
to the proc return type, which is wrong since we don't know the proc
return type even if we know the expected parameter types (`tyUntyped`
also [does not care about its previous bindings in
`typeRel`](https://github.com/nim-lang/Nim/blob/ab4278d2179639f19967431a7aa1be858046f7a7/compiler/sigmatch.nim#L1059-L1061)
maybe for this reason).

Now we mark `tyAnything` return types for routines as `tfRetType` [as
done for other meta return
types](https://github.com/nim-lang/Nim/blob/18b5fb256d4647efa6a64df451d37129d36e96f3/compiler/semtypes.nim#L1451),
and ignore bindings to `tyAnything` + `tfRetType` types in `semtypinst`.
On top of this, we reset the type relation in `paramTypesMatch` only
after creating the instantiation (instead of trusting
`isInferred`/`isInferredConvertible` before creating the instantiation),
using the same mechanism that `isBothMetaConvertible` uses.

This fixes the issues as well as making the disabled t15386_2 test
introduced in #21065 work. As seen in the changes for the other tests,
the error messages give an obscure `proc (a: GenericParam): auto` now,
but it does give the correct error that the overload doesn't match
instead of matching the overload pre-emptively and expecting a specific
return type.

tsugar had to be changed due to #16906, which is the problem where
`void` is not inferred in the case where `result` was never touched.

compiler/semtypes.nim

@@ -1436,7 +1436,8 @@ proc semProcTypeNode(c: PContext, n, genericParams: PNode,
             "' is only valid for macros and templates")
       # 'auto' as a return type does not imply a generic:
       elif r.kind == tyAnything:
-        discard
+        r = copyType(r, c.idgen, r.owner)
+        r.flags.incl tfRetType
       elif r.kind == tyStatic:
         # type allowed should forbid this type
         discard

compiler/semtypinst.nim

@@ -315,6 +315,9 @@ proc replaceTypeVarsS(cl: var TReplTypeVars, s: PSym, t: PType): PSym =
     result.ast = replaceTypeVarsN(cl, s.ast)
 
 proc lookupTypeVar(cl: var TReplTypeVars, t: PType): PType =
+  if tfRetType in t.flags and t.kind == tyAnything:
+    # don't bind `auto` return type to a previous binding of `auto`
+    return nil
   result = cl.typeMap.lookup(t)
   if result == nil:
     if cl.allowMetaTypes or tfRetType in t.flags: return
@@ -551,7 +554,9 @@ proc replaceTypeVarsTAux(cl: var TReplTypeVars, t: PType): PType =
   result = t
   if t == nil: return
 
-  if t.kind in {tyStatic, tyGenericParam, tyConcept} + tyTypeClasses:
+  const lookupMetas = {tyStatic, tyGenericParam, tyConcept} + tyTypeClasses - {tyAnything}
+  if t.kind in lookupMetas or
+      (t.kind == tyAnything and tfRetType notin t.flags):
     let lookup = cl.typeMap.lookup(t)
     if lookup != nil: return lookup
 

compiler/sigmatch.nim

@@ -2120,6 +2120,7 @@ template matchesVoidProc(t: PType): bool =
 
 proc paramTypesMatchAux(m: var TCandidate, f, a: PType,
                         argSemantized, argOrig: PNode): PNode =
+  result = nil
   var
     fMaybeStatic = f.skipTypes({tyDistinct})
     arg = argSemantized
@@ -2182,28 +2183,33 @@ proc paramTypesMatchAux(m: var TCandidate, f, a: PType,
     else:
       return argSemantized # argOrig
 
-  # If r == isBothMetaConvertible then we rerun typeRel.
-  # bothMetaCounter is for safety to avoid any infinite loop,
-  #  I don't have any example when it is needed.
-  # lastBindingsLenth is used to check whether m.bindings remains the same,
-  #  because in that case there is no point in continuing.
-  var bothMetaCounter = 0
-  var lastBindingsLength = -1
-  while r == isBothMetaConvertible and
-      lastBindingsLength != m.bindings.counter and
-      bothMetaCounter < 100:
-    lastBindingsLength = m.bindings.counter
-    inc(bothMetaCounter)
-    if arg.kind in {nkProcDef, nkFuncDef, nkIteratorDef} + nkLambdaKinds:
-      result = c.semInferredLambda(c, m.bindings, arg)
-    elif arg.kind != nkSym:
-      return nil
-    else:
-      let inferred = c.semGenerateInstance(c, arg.sym, m.bindings, arg.info)
-      result = newSymNode(inferred, arg.info)
-    inc(m.convMatches)
-    arg = result
-    r = typeRel(m, f, arg.typ)
+  block instantiateGenericRoutine:
+    # In the case where the matched value is a generic proc, we need to
+    # fully instantiate it and then rerun typeRel to make sure it matches.
+    # instantiationCounter is for safety to avoid any infinite loop,
+    #  I don't have any example when it is needed.
+    # lastBindingCount is used to check whether m.bindings remains the same,
+    #  because in that case there is no point in continuing.
+    var instantiationCounter = 0
+    var lastBindingCount = -1
+    while r in {isBothMetaConvertible, isInferred, isInferredConvertible} and
+        lastBindingCount != m.bindings.counter and
+        instantiationCounter < 100:
+      lastBindingCount = m.bindings.counter
+      inc(instantiationCounter)
+      if arg.kind in {nkProcDef, nkFuncDef, nkIteratorDef} + nkLambdaKinds:
+        result = c.semInferredLambda(c, m.bindings, arg)
+      elif arg.kind != nkSym:
+        return nil
+      elif arg.sym.kind in {skMacro, skTemplate}:
+        return nil
+      else:
+        if arg.sym.ast == nil:
+          return nil
+        let inferred = c.semGenerateInstance(c, arg.sym, m.bindings, arg.info)
+        result = newSymNode(inferred, arg.info)
+      arg = result
+      r = typeRel(m, f, arg.typ)
 
   case r
   of isConvertible:
@@ -2230,23 +2236,15 @@ proc paramTypesMatchAux(m: var TCandidate, f, a: PType,
       result = arg
     else:
       result = implicitConv(nkHiddenStdConv, f, arg, m, c)
-  of isInferred, isInferredConvertible:
-    if arg.kind in {nkProcDef, nkFuncDef, nkIteratorDef} + nkLambdaKinds:
-      result = c.semInferredLambda(c, m.bindings, arg)
-    elif arg.kind != nkSym:
-      return nil
-    elif arg.sym.kind in {skMacro, skTemplate}:
-      return nil
-    else:
-      if arg.sym.ast == nil:
-        return nil
-      let inferred = c.semGenerateInstance(c, arg.sym, m.bindings, arg.info)
-      result = newSymNode(inferred, arg.info)
-    if r == isInferredConvertible:
-      inc(m.convMatches)
-      result = implicitConv(nkHiddenStdConv, f, result, m, c)
-    else:
-      inc(m.genericMatches)
+  of isInferred:
+    # result should be set in above while loop:
+    assert result != nil
+    inc(m.genericMatches)
+  of isInferredConvertible:
+    # result should be set in above while loop:
+    assert result != nil
+    inc(m.convMatches)
+    result = implicitConv(nkHiddenStdConv, f, result, m, c)
   of isGeneric:
     inc(m.genericMatches)
     if arg.typ == nil:
@@ -2260,8 +2258,10 @@ proc paramTypesMatchAux(m: var TCandidate, f, a: PType,
     else:
       result = arg
   of isBothMetaConvertible:
-    # This is the result for the 101th time.
-    result = nil
+    # result should be set in above while loop:
+    assert result != nil
+    inc(m.convMatches)
+    result = arg
   of isFromIntLit:
     # too lazy to introduce another ``*matches`` field, so we conflate
     # ``isIntConv`` and ``isIntLit`` here:

tests/concepts/tconcepts_issues.nim

@@ -510,7 +510,7 @@ proc depthOf*[V](orderType: typedesc[BreadthOrder], tree: AnyTree[V], root, goal
   if root == goal:
     return 0
   var order = init[LevelNode[V]](orderType)
-  order.expand(tree, root, (leaf) => (1, leaf))
+  order.expand(tree, root, (leaf) => (1.uint, leaf))
   while order.hasNext():
     let depthNode: LevelNode[V] = order.popNext()
     if depthNode.node == goal:

tests/errmsgs/t16654.nim

@@ -1,6 +1,6 @@
 discard """
   cmd: "nim check $options $file"
-  errormsg: "type mismatch: got <int> but expected 'float'"
+  errormsg: "type mismatch: got <int literal(1), proc (r: GenericParam): auto>"
 """
 
 when true: # bug #16654

tests/misc/t12869.nim

@@ -1,5 +1,5 @@
 discard """
-  errormsg: "type mismatch: got <bool> but expected 'int'"
+  errormsg: "type mismatch: got <openArray[int], proc (x: GenericParam, y: GenericParam): auto, SortOrder>"
   line: 12
 """
 

tests/proc/tinferlambdareturn.nim

@@ -0,0 +1,36 @@
+import std/[sugar, sequtils]
+
+block: # issue #23200
+  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))
+  dosomething(proc (y: auto): auto = makeSeq(y))
+  proc foo(y: auto): auto = makeSeq(y)
+  dosomething(foo)
+
+block: # issue #18866
+  proc somefn[T](list: openarray[T], op: proc (v: T): float) =
+    discard op(list[0])
+
+  type TimeD = object
+    year:  Natural
+    month: 1..12
+    day:   1..31
+
+  doAssert not compiles(@[TimeD()].somefn(proc (v: auto): auto =
+    v
+  ))
+  @[TimeD()].somefn(proc (v: auto): auto =
+    v.year.float
+  )
+  proc foo(v: auto): auto = v
+  doAssert not compiles(@[TimeD()].somefn(foo))
+  proc bar(v: auto): auto = v.year.float
+  @[TimeD()].somefn(bar)

tests/stdlib/tsugar.nim

@@ -295,7 +295,8 @@ template main() =
       for i in 0..5:
         xs.add(i)
 
-      xs.apply(d => ys.add(d))
+      xs.apply(proc (d: auto) = ys.add(d))
+      # ^ can be turned into d => ys.add(d) when we can infer void return type, #16906
       doAssert ys == @[0, 1, 2, 3, 4, 5]
 
     test()

tests/types/t15836.nim

@@ -1,5 +1,5 @@
 discard """
-  errormsg: "type mismatch: got <string> but expected 'int'"
+  errormsg: "type mismatch: got <int literal(1), proc (a: GenericParam): auto>"
   line: 11
 """ 
 

tests/types/t15836_2.nim

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