cmd/compile: fix missing ddd when building call for function instantiation closure

When building a call expression for function instantiation closure, if
it's a variadic function, the CallExpr.IsDDD must be set for typecheck
to work properly. Otherwise, there will be a mismatch between the
arguments type and the function signature.

Fixes #49516

Change-Id: I0af90ee3fcc3e6c8bba8b20e331e044cbce17985
Reviewed-on: https://go-review.googlesource.com/c/go/+/363314
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/noder/stencil.go

@@ -515,6 +515,7 @@ func (g *genInst) buildClosure(outer *ir.Func, x ir.Node) ir.Node {
 
 	// Build call itself.
 	var innerCall ir.Node = ir.NewCallExpr(pos, ir.OCALL, target.Nname, args)
+	innerCall.(*ir.CallExpr).IsDDD = typ.IsVariadic()
 	if len(formalResults) > 0 {
 		innerCall = ir.NewReturnStmt(pos, []ir.Node{innerCall})
 	}

test/typeparam/issue49516.go

@@ -0,0 +1,26 @@
+// compile -G=3
+
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type Q[T any] struct {
+	s []T
+}
+
+func (q *Q[T]) Push(v ...T) {
+	q.s = append(q.s, v...)
+}
+
+func pushN(push func(*Q[int], ...int), n int) {
+	var q Q[int]
+	for i := 0; i < n; i++ {
+		push(&q, i)
+	}
+}
+
+func f() {
+	pushN((*Q[int]).Push, 100)
+}