internal/refactor/inline: don't treat blanks as decls in declares()

Plus, a test.

Change-Id: I9d3f4729c1b8da51d771442d9c3f5909f608591e
Reviewed-on: https://go-review.googlesource.com/c/tools/+/534895
Reviewed-by: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Alan Donovan <adonovan@google.com>

internal/refactor/inline/inline.go

@@ -2622,6 +2622,7 @@ func declares(stmts []ast.Stmt) map[string]bool {
 			}
 		}
 	}
+	delete(names, "_")
 	return names
 }
 

internal/refactor/inline/inline_test.go

@@ -388,6 +388,17 @@ func TestBasics(t *testing.T) {
 			`func _(ch chan int) { f(ch) }`,
 			`func _(ch chan int) { <-(<-chan int)(ch) }`,
 		},
+		{
+			// (a regression test for unnecessary braces)
+			"In block elision, blank decls don't count when computing name conflicts.",
+			`func f(x int) { var _ = x; var _ = 3 }`,
+			`func _() { var _ = 1; f(2) }`,
+			`func _() {
+	var _ = 1
+	var _ = 2
+	var _ = 3
+}`,
+		},
 	})
 }