fixes #9076

compiler/ccgstmts.nim

@@ -1159,4 +1159,4 @@ proc genStmts(p: BProc, t: PNode) =
   if isPush: pushInfoContext(p.config, t.info)
   expr(p, t, a)
   if isPush: popInfoContext(p.config)
-  internalAssert p.config, a.k in {locNone, locTemp, locLocalVar}
+  internalAssert p.config, a.k in {locNone, locTemp, locLocalVar, locExpr}

compiler/semstmts.nim

@@ -114,7 +114,7 @@ const
   skipForDiscardable = {nkIfStmt, nkIfExpr, nkCaseStmt, nkOfBranch,
     nkElse, nkStmtListExpr, nkTryStmt, nkFinally, nkExceptBranch,
     nkElifBranch, nkElifExpr, nkElseExpr, nkBlockStmt, nkBlockExpr,
-    nkHiddenStdConv}
+    nkHiddenStdConv, nkHiddenDeref}
 
 proc implicitlyDiscardable(n: PNode): bool =
   var n = n
@@ -601,6 +601,8 @@ proc semForVars(c: PContext, n: PNode; flags: TExprFlags): PNode =
   inc(c.p.nestedLoopCounter)
   openScope(c)
   n.sons[length-1] = semExprBranch(c, n.sons[length-1], flags)
+  if efInTypeof notin flags:
+    discardCheck(c, n.sons[length-1], flags)
   closeScope(c)
   dec(c.p.nestedLoopCounter)
 

tests/discard/tneedsdiscard_in_for.nim

@@ -0,0 +1,22 @@
+discard """
+  line: 22
+  errormsg: '''expression 'premultiply(app.gradient[i])' is of type 'Rgba8' and has to be discarded'''
+"""
+
+# bug #9076
+type
+  Rgba8 = object
+
+proc premultiply*(c: var Rgba8): var Rgba8 =
+  discard
+
+type
+  App = ref object
+    gradient: seq[Rgba8]
+
+method onDraw(app: App) {.base.} =
+  var
+    width  = 100'f64
+
+  for i in 0..<width.int:
+    app.gradient[i].premultiply()