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

* fix #18886 crash on ambiguous proc cast

* follow suggestion

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)