alternate fix + test for #12094, refs #13804 (#20686)

compiler/semstmts.nim

@@ -370,8 +370,7 @@ proc addToVarSection(c: PContext; result: var PNode; orig, identDefs: PNode) =
     result.add identDefs
 
 proc isDiscardUnderscore(v: PSym): bool =
-  # template generated underscore symbol name starts with _`gensym
-  if v.name.s == "_" or v.name.s.startsWith("_`"):
+  if v.name.s == "_":
     v.flags.incl(sfGenSym)
     result = true
 

compiler/semtempl.nim

@@ -227,7 +227,7 @@ proc addLocalDecl(c: var TemplCtx, n: var PNode, k: TSymKind) =
           closeScope(c)
     let ident = getIdentNode(c, n)
     if not isTemplParam(c, ident):
-      if n.kind != nkSym:
+      if n.kind != nkSym and not (n.kind == nkIdent and n.ident.s == "_"):
         let local = newGenSym(k, ident, c)
         addPrelimDecl(c.c, local)
         styleCheckDef(c.c, n.info, local)

tests/template/tunderscore1.nim

@@ -0,0 +1,11 @@
+discard """
+  errormsg: "undeclared identifier: '_'"
+"""
+
+# issue #12094, #13804
+
+template foo =
+  let _ = 1
+  echo _
+
+foo()