fix #15972 (#15994)

* fix #15972

* add testcase

* more

compiler/semstmts.nim

@@ -739,7 +739,7 @@ proc semForVars(c: PContext, n: PNode; flags: TExprFlags): PNode =
           else:
             v.typ = iter[i]
           n[0][i] = newSymNode(v)
-          if sfGenSym notin v.flags: addDecl(c, v)
+          if sfGenSym notin v.flags and not isDiscardUnderscore(v): addDecl(c, v)
           elif v.owner == nil: v.owner = getCurrOwner(c)
       else:
         var v = symForVar(c, n[0])
@@ -749,7 +749,7 @@ proc semForVars(c: PContext, n: PNode; flags: TExprFlags): PNode =
         # for an example:
         v.typ = iterBase
         n[0] = newSymNode(v)
-        if sfGenSym notin v.flags: addDecl(c, v)
+        if sfGenSym notin v.flags and not isDiscardUnderscore(v): addDecl(c, v)
         elif v.owner == nil: v.owner = getCurrOwner(c)
     else:
       localError(c.config, n.info, errWrongNumberOfVariables)

tests/stmt/tforloop_tuple_multiple_underscore.nim

@@ -0,0 +1,10 @@
+discard """
+  errormsg: "undeclared identifier: '_'"
+"""
+
+iterator iter(): (int, int, int) =
+  yield (1, 1, 2)
+
+
+for (_, i, _) in iter():
+  echo _

tests/stmt/tforloop_tuple_underscore.nim

@@ -0,0 +1,16 @@
+discard """
+  errormsg: "undeclared identifier: '_'"
+"""
+
+iterator iter(): (int, int) =
+  yield (1, 2)
+  yield (3, 4)
+  yield (1, 2)
+  yield (3, 4)
+  yield (1, 2)
+  yield (3, 4)
+
+
+for (_, i) in iter():
+  echo _
+

tests/stmt/tforloop_underscore.nim

@@ -0,0 +1,6 @@
+discard """
+  errormsg: "undeclared identifier: '_'"
+"""
+
+for _ in ["a"]:
+  echo _