compiler: support constant indices with a named type

compiler/asserts.go

@@ -22,7 +22,7 @@ func (c *Compiler) emitLookupBoundsCheck(frame *Frame, arrayLen, index llvm.Valu
 	if index.Type().IntTypeWidth() < arrayLen.Type().IntTypeWidth() {
 		// Sometimes, the index can be e.g. an uint8 or int8, and we have to
 		// correctly extend that type.
-		if indexType.(*types.Basic).Info()&types.IsUnsigned == 0 {
+		if indexType.Underlying().(*types.Basic).Info()&types.IsUnsigned == 0 {
 			index = c.builder.CreateZExt(index, arrayLen.Type(), "")
 		} else {
 			index = c.builder.CreateSExt(index, arrayLen.Type(), "")

testdata/slice.go

@@ -2,6 +2,13 @@ package main
 
 type MySlice [32]byte
 
+type myUint8 uint8
+
+// Indexing into slice with named type (regression test).
+var array = [4]int{
+	myUint8(2): 3,
+}
+
 func main() {
 	l := 5
 	foo := []int{1, 2, 4, 5}