fix #18886 crash on ambiguous proc cast (#20472)

* fix #18886 crash on ambiguous proc cast * follow suggestion

Author: bung87

compiler/lookups.nim

@@ -488,17 +488,24 @@ proc errorUseQualifier*(c: PContext; info: TLineInfo; s: PSym) =
  var amb: bool
  discard errorUseQualifier(c, info, s, amb)
 
-proc errorUseQualifier(c: PContext; info: TLineInfo; candidates: seq[PSym]) =
+proc errorUseQualifier(c: PContext; info: TLineInfo; candidates: seq[PSym]; prefix = "use one of") =
  var err = "ambiguous identifier: '" & candidates[0].name.s & "'"
  var i = 0
  for candidate in candidates:
- if i == 0: err.add " -- use one of the following:\n"
+ if i == 0: err.add " -- $1 the following:\n" % prefix
  else: err.add "\n"
  err.add " " & candidate.owner.name.s & "." & candidate.name.s
  err.add ": " & typeToString(candidate.typ)
  inc i
  localError(c.config, info, errGenerated, err)
 
+proc errorUseQualifier*(c: PContext; info:TLineInfo; choices: PNode) =
+ var candidates = newSeq[PSym](choices.len)
+ let prefix = if choices[0].typ.kind != tyProc: "use one of" else: "you need a helper proc to disambiguate"
+ for i, n in choices:
+ candidates[i] = n.sym
+ errorUseQualifier(c, info, candidates, prefix)
+
 proc errorUndeclaredIdentifier*(c: PContext; info: TLineInfo; name: string, extra = "") =
  var err = "undeclared identifier: '" & name & "'" & extra
  if c.recursiveDep.len > 0:

compiler/semexprs.nim

@@ -371,6 +371,8 @@ proc semCast(c: PContext, n: PNode): PNode =
  checkSonsLen(n, 2, c.config)
  let targetType = semTypeNode(c, n[0], nil)
  let castedExpr = semExprWithType(c, n[1])
+ if castedExpr.kind == nkClosedSymChoice:
+ errorUseQualifier(c, n[1].info, castedExpr)
  if tfHasMeta in targetType.flags:
  localError(c.config, n[0].info, "cannot cast to a non concrete type: '$1'" % $targetType)
  if not isCastable(c, targetType, castedExpr.typ, n.info):

tests/errmsgs/t18886.nim

@@ -0,0 +1,18 @@
+discard """
+ cmd: "nim check --hints:off $file"
+ errormsg: ""
+ nimout: '''
+t18886.nim(18, 24) Error: ambiguous identifier: 'bar' -- you need a helper proc to disambiguate the following:
+ t18886.bar: proc (i: ptr int){.noSideEffect, gcsafe, locks: 0.}
+ t18886.bar: proc (i: ptr char){.noSideEffect, gcsafe, locks: 0.}
+'''
+"""
+
+type Foo = (proc(_: pointer): void)
+
+
+proc bar(i: ptr[int]) = discard
+proc bar(i: ptr[char]) = discard
+
+
+let fooBar = cast[Foo](bar)