bug(gnovm): prohibit conversion of non-convertible typed constants

[deelawn]

High Level Summary

The Go spec says that constant float values should not be convertible to integer values. Gno guards against conversion of untyped float constants, but not typed constants. For example, both of these examples work in Gno when they should not:

package main

func main() {
    println(int(float64(1.5)))
}

package main

func main() {
    const v float64 = 1.5
    println(int(v))
}

Details

From the Go spec:

A constant value x can be converted to type T if x is representable by a value of T. As a special case, an integer constant x can be explicitly converted to a string type using the same rule as for non-constant x.
-- https://go.dev/ref/spec#Conversions

To interpret the above, it is necessary to know if a typed constant float value x is representable as an integer. Looking at the definition of constant representability:

A constant x is representable by a value of type T, where T is not a type parameter, if one of the following conditions applies:

• x is in the set of values determined by T.
• T is a floating-point type and x can be rounded to T's precision without overflow. Rounding uses IEEE 754 round-to-even rules but with an IEEE negative zero further simplified to an unsigned zero. Note that constant values never result in an IEEE negative zero, NaN, or infinity.
• T is a complex type, and x's components real(x) and imag(x) are representable by values of T's component type (float32 or float64).

-- https://go.dev/ref/spec#Representability

In the case of this particular issue, typed float constant values are not representable as integers because they fail to to satisfy and of the criteria -- most notably that a non-natural number is not contained in the set of possible integer values.

Other Examples

All of these are examples that work in Gno but should not.

Integer Underflow

package main

func main() {
    const a int = -1
    println(uint(a))
}

Integer Overflow

package main

func main() {
    const a int = 270
    println(uint8(a))
}