allow multiple parameters using an underscore as name

compiler/ast/astalgo.nim

@@ -207,8 +207,8 @@ proc getNamedParamFromList*(list: PNode, ident: PIdent): PSym =
   ##            if c.instLines: actual.info else: templ.info)
   for i in 1..<list.len:
     let it = list[i].sym
-    if it.name.id == ident.id or
-        sameIgnoreBacktickGensymInfo(it.name.s, ident.s): return it
+    if it.name.s != "_" and (it.name.id == ident.id or
+        sameIgnoreBacktickGensymInfo(it.name.s, ident.s)): return it
 
 proc hashNode(p: RootRef): Hash =
   result = hash(cast[pointer](p))

compiler/backend/ccgtypes.nim

@@ -51,7 +51,11 @@ proc mangleParamName(c: ConfigRef; s: PSym): Rope =
   ## we cannot use 'sigConflicts' here since we don't have access to a BProc.
   ## Fortunately C's scoping rules are sane enough so that that doesn't
   ## cause any trouble.
-  if true:
+  if s.name.s == "_":
+    # multiple parameters can use an underscore as the name. Construct a
+    # locally unique C name by appending the parameter position
+    result = "_p" & $s.position
+  else:
     var res = s.name.s.mangle
     if isKeyword(s.name) or c.cppDefines.contains(res):
       res.add "_0"

compiler/sem/sem.nim

@@ -843,6 +843,7 @@ proc semConstBoolExpr(c: PContext, n: PNode): PNode =
 
 proc semGenericStmt(c: PContext, n: PNode): PNode
 proc semConceptBody(c: PContext, n: PNode): PNode
+proc isDiscardUnderscore(v: PSym): bool
 
 include semtypes, semtempl, semgnrc, semstmts, semexprs
 

compiler/sem/seminst.nim

@@ -104,9 +104,7 @@ proc addParamOrResult(c: PContext, param: PSym)
 
 proc instantiateBody(c: PContext, n, params: PNode, result, orig: PSym) =
   if n[bodyPos].kind != nkEmpty:
-    let procParams = result.typ.n
-    for i in 1..<procParams.len:
-      addDecl(c, procParams[i].sym)
+    addParams(c, result.typ.n)
     maybeAddResult(c, result, result.ast)
 
     inc c.inGenericInst

compiler/sem/semtypes.nim

@@ -1589,6 +1589,9 @@ proc semProcTypeNode(c: PContext, n, genericParams: PNode,
       if argNode.kind == nkError:
         localReport(c.config, argNode)
 
+      # mark underscore parameters as gensyms:
+      discard isDiscardUnderscore(arg)
+
       if a[j].kind == nkPragmaExpr:
         a[j][1] = pragmaDecl(c, arg, a[j][1], paramPragmas)
         # check if we got any errors and if so report them
@@ -1614,7 +1617,7 @@ 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 != "_" and containsOrIncl(check, arg.name.id):
         localReport(c.config, a[j], reportSym(
           rsemParameterRedefinition, arg))
 

tests/lang/s02_core/s02_procedures/t99_underscore_parameter.nim

@@ -0,0 +1,19 @@
+discard """
+  description: '''
+    Multiple parameters can use an underscore as the name, making the
+    parameters inaccessible
+  '''
+"""
+
+proc test1(_: int) = discard
+test1(1)
+
+proc test2(_: int, _: int) = discard
+test2(1, 2)
+
+proc mixed(a: int, _: int) = discard
+mixed(1, 2)
+
+## The same is true for parameters of anonymous procedures.
+let anon = proc(_, _, _: int) = discard
+anon(1, 2, 3)

tests/lang/s02_core/s02_procedures/t99_underscore_parameter_cannot_reference.nim

@@ -0,0 +1,9 @@
+discard """
+  description: '''
+    An underscore parameter cannot be reference in named parameter passing.
+  '''
+  action: reject
+"""
+
+proc test(_: int) = discard
+test(_ = 1)