comd/compile/internal/types2: add missing nil check in const conversion

Follow-up on CL 360396.

Fixes #49296.

Change-Id: Ie08f86ae884da4cfd5db557cbf4f721a237dc39f
Reviewed-on: https://go-review.googlesource.com/c/go/+/360796
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

src/cmd/compile/internal/types2/conversions.go

@@ -18,6 +18,8 @@ func (check *Checker) conversion(x *operand, T Type) {
 
 	constConvertibleTo := func(T Type, val *constant.Value) bool {
 		switch t := asBasic(T); {
+		case t == nil:
+			// nothing to do
 		case representableConst(x.val, check, t, val):
 			return true
 		case isInteger(x.typ) && isString(t):

src/cmd/compile/internal/types2/testdata/fixedbugs/issue49296.go2

@@ -0,0 +1,20 @@
+// 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
+
+func _[
+        T0 any,
+        T1 []int,
+        T2 ~float64 | ~complex128 | chan int,
+]() {
+        _ = T0(nil /* ERROR cannot convert untyped nil to T0 */ )
+        _ = T1(1 /* ERROR cannot convert 1 .* to T1 */ )
+        _ = T2(2 /* ERROR cannot convert 2 .* to T2 */ )
+}
+
+// test case from issue
+func f[T interface{[]int}]() {
+	_ = T(1 /* ERROR cannot convert */ )
+}