underscores for routine parameters (#21192)

* underscores for routine parameters

fixes #13443, fixes #13804, refs #21121

* add changelog + more tests

* support generics and ensure inferred lambdas work

compiler/lookups.nim

@@ -323,6 +323,7 @@ proc wrongRedefinition*(c: PContext; info: TLineInfo, s: string;
 # xxx pending bootstrap >= 1.4, replace all those overloads with a single one:
 # proc addDecl*(c: PContext, sym: PSym, info = sym.info, scope = c.currentScope) {.inline.} =
 proc addDeclAt*(c: PContext; scope: PScope, sym: PSym, info: TLineInfo) =
+  if sym.name.s == "_": return
   let conflict = scope.addUniqueSym(sym)
   if conflict != nil:
     if sym.kind == skModule and conflict.kind == skModule and sym.owner == conflict.owner:

compiler/semtempl.nim

@@ -642,8 +642,9 @@ proc semTemplateDef(c: PContext, n: PNode): PNode =
     # body by the absence of the sfGenSym flag:
     for i in 1..<s.typ.n.len:
       let param = s.typ.n[i].sym
-      param.flags.incl sfTemplateParam
-      param.flags.excl sfGenSym
+      if param.name.s != "_":
+        param.flags.incl sfTemplateParam
+        param.flags.excl sfGenSym
       if param.typ.kind != tyUntyped: allUntyped = false
   else:
     s.typ = newTypeS(tyProc, c)

compiler/semtypes.nim

@@ -1372,7 +1372,9 @@ proc semProcTypeNode(c: PContext, n, genericParams: PNode,
       inc(counter)
       if def != nil and def.kind != nkEmpty:
         arg.ast = copyTree(def)
-      if containsOrIncl(check, arg.name.id):
+      if arg.name.s == "_":
+        arg.flags.incl(sfGenSym)
+      elif containsOrIncl(check, arg.name.id):
         localError(c.config, a[j].info, "attempt to redefine: '" & arg.name.s & "'")
       result.n.add newSymNode(arg)
       rawAddSon(result, finalType)