cmd/compile: fix the index variable is shadowed in dictPass

The CL 349613 causes this problem.
In fact, we want to use the outer i to find m.List[i],
but the newly created index variable i in the nearest
for range shadow the outer i.

Fixes golang#48838.

Change-Id: I10f0bd985340f9443eefaadda6fc56e4e7e9a10c
Reviewed-on: https://go-review.googlesource.com/c/go/+/354549
Run-TryBot: Cuong Manh Le <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
Reviewed-by: Dan Scales <[email protected]>
Trust: Dan Scales <[email protected]>

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

@@ -1298,9 +1298,9 @@ func (g *irgen) dictPass(info *instInfo) {
  // Type switch from nonempty interface. We need a *runtime.itab
  // for the dynamic type.
  ix := -1
- for i, ic := range info.dictInfo.itabConvs {
+ for j, ic := range info.dictInfo.itabConvs {
  if ic == m.List[i] {
- ix = info.dictInfo.startItabConv + i
+ ix = info.dictInfo.startItabConv + j
  break
  }
  }

test/typeparam/issue48838.go

@@ -0,0 +1,31 @@
+// run -gcflags=-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 main
+
+func main() {
+ check[string]()
+}
+
+func check[T any]() {
+ var result setter[T]
+ switch result.(type) {
+ case fooA[T]:
+ case fooB[T]:
+ }
+}
+
+type setter[T any] interface {
+ Set(T)
+}
+
+type fooA[T any] struct{}
+
+func (fooA[T]) Set(T) {}
+
+type fooB[T any] struct{}
+
+func (fooB[T]) Set(T) {}