cmd/compile: correct type of CvtBoolToUint8 values

Fixes #40772

Change-Id: I539f07d1f958dacee87d846171a8889d03182d25
Reviewed-on: https://go-review.googlesource.com/c/go/+/248397
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-on: https://go-review.googlesource.com/c/go/+/248401

1 files changed, 19 insertions, 0 deletions

diff --git a/test/fixedbugs/issue40746.go b/test/fixedbugs/issue40746.go
new file mode 100644
index 0000000000..235282fd90
--- /dev/null
+++ b/test/fixedbugs/issue40746.go
@@ -0,0 +1,19 @@
+// compile
+
+// Copyright 2020 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 f(x byte, b bool) byte {
+	var c byte
+	if b {
+		c = 1
+	}
+
+	if int8(c) < 0 {
+		x++
+	}
+	return x
+}