[dev.typeparams] go/constant: in ToFloat, convert to rational numbers, not floats

Floating-point constants are represented as rational numbers when
possible (i.e., when numerators and denominators are not too large).
If we convert to floats when not necessary, we risk losing precision.

This is the minimal fix for the specific issue, but it's too aggressive:
If the numbers are too large, we still want to convert to floats.
Will address in a separate CL that also does a few related cleanups.

Fixes #43908.

Change-Id: Id575e34fa18361a347c43701cfb4dd7221997f66
Reviewed-on: https://go-review.googlesource.com/c/go/+/286552
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>

src/go/constant/value.go

@@ -871,9 +871,9 @@ func ToInt(x Value) Value {
 func ToFloat(x Value) Value {
 	switch x := x.(type) {
 	case int64Val:
-		return i64tof(x)
+		return i64tor(x)
 	case intVal:
-		return itof(x)
+		return itor(x)
 	case ratVal, floatVal:
 		return x
 	case complexVal:

test/fixedbugs/issue43908.go

@@ -0,0 +1,21 @@
+// run
+
+// Copyright 2021 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.
+
+// Verify exact constant evaluation independent of
+// (mathematically equivalent) expression form.
+
+package main
+
+import "fmt"
+
+const ulp1 = imag(1i + 2i / 3 - 5i / 3)
+const ulp2 = imag(1i + complex(0, 2) / 3 - 5i / 3)
+
+func main() {
+	if ulp1 != ulp2 {
+		panic(fmt.Sprintf("%g != %g\n", ulp1, ulp2))
+	}
+}