go/constant: MakeFloat64(0) must return a value of Float kind

Fixes #42641.

Change-Id: I10fdc7c90054b37ab5b303999015262691c12927
Reviewed-on: https://go-review.googlesource.com/c/go/+/273126
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/go/constant/value.go

@@ -370,16 +370,13 @@ func MakeUint64(x uint64) Value {
 }
 
 // MakeFloat64 returns the Float value for x.
+// If x is -0.0, the result is 0.0.
 // If x is not finite, the result is an Unknown.
 func MakeFloat64(x float64) Value {
 	if math.IsInf(x, 0) || math.IsNaN(x) {
 		return unknownVal{}
 	}
-	// convert -0 to 0
-	if x == 0 {
-		return int64Val(0)
-	}
-	return ratVal{newRat().SetFloat64(x)}
+	return ratVal{newRat().SetFloat64(x + 0)} // convert -0 to 0
 }
 
 // MakeFromLiteral returns the corresponding integer, floating-point,

src/go/constant/value_test.go

@@ -7,6 +7,7 @@ package constant
 import (
 	"fmt"
 	"go/token"
+	"math"
 	"math/big"
 	"strings"
 	"testing"
@@ -620,6 +621,42 @@ func TestUnknown(t *testing.T) {
 	}
 }
 
+func TestMakeFloat64(t *testing.T) {
+	var zero float64
+	for _, arg := range []float64{
+		-math.MaxFloat32,
+		-10,
+		-0.5,
+		-zero,
+		zero,
+		1,
+		10,
+		123456789.87654321e-23,
+		1e10,
+		math.MaxFloat64,
+	} {
+		val := MakeFloat64(arg)
+		if val.Kind() != Float {
+			t.Errorf("%v: got kind = %d; want %d", arg, val.Kind(), Float)
+		}
+
+		// -0.0 is mapped to 0.0
+		got, exact := Float64Val(val)
+		if !exact || math.Float64bits(got) != math.Float64bits(arg+0) {
+			t.Errorf("%v: got %v (exact = %v)", arg, got, exact)
+		}
+	}
+
+	// infinity
+	for sign := range []int{-1, 1} {
+		arg := math.Inf(sign)
+		val := MakeFloat64(arg)
+		if val.Kind() != Unknown {
+			t.Errorf("%v: got kind = %d; want %d", arg, val.Kind(), Unknown)
+		}
+	}
+}
+
 type makeTestCase struct {
 	kind      Kind
 	arg, want interface{}