cmd/compile: cannot convert (constant 10000 of type float64) to type int64

[NOMORECOFFEE]

What version of Go are you using (go version)?

$ go version
go1.19

Does this issue reproduce with the latest release?

yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env

What did you do?

package main

import (
	"fmt"
)

func main() {
	v := int64((float64(0.03) - float64(0.02)) * 1_000_000)
	fmt.Println(v)
}

https://go.dev/play/p/V0KH5qKf6u4

What did you expect to see?

10000

What did you see instead?

cannot convert (float64(0.03) - float64(0.02)) * 1_000_000 (constant 10000 of type float64) to type int64

[randall77]

The problem is that (float64(0.03) - float64(0.02)) * 1_000_000 is not an integer. 0.03 is not representable exactly as a float64, so the total expression is not exactly an integer. That's what the compiler is complaining about.
The error message could certainly be better.
Note that (0.03 - 0.02) * 1_000_000 does work, because the compiler maintains many more significant digits than a float64 does.

[gopherbot]

Change https://go.dev/cl/442935 mentions this issue: cmd/compile: in compiler errors, print more digits for floats close to an int