go/types: fix bad variable shadowing in offsetsof

Introduced in CL 26995.

Fixes #16902

Change-Id: I8e749f598167e1f8b82cd5e735a7eb5291362e5e
Reviewed-on: https://go-review.googlesource.com/28070
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/types/sizes.go

@@ -167,7 +167,7 @@ func (conf *Config) offsetsof(T *Struct) []int64 {
 	if T.NumFields() > 0 {
 		// compute offsets on demand
 		if s := conf.Sizes; s != nil {
-			offsets := s.Offsetsof(T.fields)
+			offsets = s.Offsetsof(T.fields)
 			// sanity checks
 			if len(offsets) != T.NumFields() {
 				panic("Config.Sizes.Offsetsof returned the wrong number of offsets")

src/go/types/sizes_test.go

@@ -8,6 +8,7 @@ package types_test
 
 import (
 	"go/ast"
+	"go/importer"
 	"go/parser"
 	"go/token"
 	"go/types"
@@ -81,3 +82,31 @@ var s struct {
 		t.Errorf("OffsetsOf(%v) = %v want %v", ts, offsets, []int{0, 4})
 	}
 }
+
+func TestIssue16902(t *testing.T) {
+	const src = `
+package a
+
+import "unsafe"
+
+const _ = unsafe.Offsetof(struct{ x int64 }{}.x)
+`
+	fset := token.NewFileSet()
+	f, err := parser.ParseFile(fset, "x.go", src, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	info := types.Info{Types: make(map[ast.Expr]types.TypeAndValue)}
+	conf := types.Config{
+		Importer: importer.Default(),
+		Sizes:    &types.StdSizes{WordSize: 8, MaxAlign: 8},
+	}
+	_, err = conf.Check("x", fset, []*ast.File{f}, &info)
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, tv := range info.Types {
+		_ = conf.Sizes.Sizeof(tv.Type)
+		_ = conf.Sizes.Alignof(tv.Type)
+	}
+}