go/constant: make constant.Make produce "smallest" const representation

Fixes #42640.

Change-Id: I22b8142b0a47a0f957d1bda28cdfdbb8388cffc4
Reviewed-on: https://go-review.googlesource.com/c/go/+/273086
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/go/constant/value.go

@@ -594,11 +594,11 @@ func Make(x interface{}) Value {
 	case int64:
 		return int64Val(x)
 	case *big.Int:
-		return intVal{x}
+		return makeInt(x)
 	case *big.Rat:
-		return ratVal{x}
+		return makeRat(x)
 	case *big.Float:
-		return floatVal{x}
+		return makeFloat(x)
 	default:
 		return unknownVal{}
 	}

src/go/constant/value_test.go

@@ -620,18 +620,32 @@ func TestUnknown(t *testing.T) {
 	}
 }
 
+type makeTestCase struct {
+	kind      Kind
+	arg, want interface{}
+}
+
+func dup(k Kind, x interface{}) makeTestCase { return makeTestCase{k, x, x} }
+
 func TestMake(t *testing.T) {
-	for _, want := range []interface{}{
+	for _, test := range []makeTestCase{
-		false,
+		{Bool, false, false},
-		"hello",
+		{String, "hello", "hello"},
-		int64(1),
+
-		big.NewInt(10),
+		{Int, int64(1), int64(1)},
-		big.NewFloat(2.0),
+		{Int, big.NewInt(10), int64(10)},
-		big.NewRat(1, 3),
+		{Int, new(big.Int).Lsh(big.NewInt(1), 62), int64(1 << 62)},
+		dup(Int, new(big.Int).Lsh(big.NewInt(1), 63)),
+
+		{Float, big.NewFloat(0), floatVal0.val},
+		dup(Float, big.NewFloat(2.0)),
+		dup(Float, big.NewRat(1, 3)),
 	} {
-		got := Val(Make(want))
+		val := Make(test.arg)
-		if got != want {
+		got := Val(val)
-			t.Errorf("got %v; want %v", got, want)
+		if val.Kind() != test.kind || got != test.want {
+			t.Errorf("got %v (%T, kind = %d); want %v (%T, kind = %d)",
+				got, got, val.Kind(), test.want, test.want, test.kind)
 		}
 	}
 }