cmd/compile: internal compiler error for new version of absdiff.go, failure during noding

[danscales]

Issue for Go 1.18 (generics)

I created a new version of the test test/typeparam/absdiff.go that works fully, now that we've disallowed using a type param on the right-hand-side of a type declaration. (Thanks to @perillo for posting a possible new implementation in #45639 that encouraged me to experiment with getting the test fully working again.)

With the new version, we get an assertion failure in typ0, caused while doing deferred processing of a function body.

Here's the new test case:

package main

import (
	"fmt"
	"math"
)

type Numeric interface {
	~int | ~int8 | ~int16 | ~int32 | ~int64 |
		~uint | ~uint8 | ~uint16 | ~uint32 | ~uint64 | ~uintptr |
		~float32 | ~float64 |
		~complex64 | ~complex128
}

// numericAbs matches a struct containing a numeric type that has an Abs method.
type numericAbs[T Numeric] interface {
	~struct{ Value T }
	Abs() T
}

// AbsDifference computes the absolute value of the difference of
// a and b, where the absolute value is determined by the Abs method.
func absDifference[T Numeric, U numericAbs[T]](a, b U) T {
	d := a.Value - b.Value
	dt := U{Value: d}
	return dt.Abs()
}

// orderedNumeric matches numeric types that support the < operator.
type orderedNumeric interface {
	~int | ~int8 | ~int16 | ~int32 | ~int64 |
		~uint | ~uint8 | ~uint16 | ~uint32 | ~uint64 | ~uintptr |
		~float32 | ~float64
}

// Complex matches the two complex types, which do not have a < operator.
type Complex interface {
	~complex64 | ~complex128
}

// orderedAbs is a helper type that defines an Abs method for
// a struct containing an ordered numeric type.
type orderedAbs[T orderedNumeric] struct {
	Value T
}

func (a orderedAbs[T]) Abs() T {
	if a.Value < 0 {
		return -a.Value
	}
	return a.Value
}

// complexAbs is a helper type that defines an Abs method for
// a struct containing a complex type.
type complexAbs[T Complex] struct {
	Value T
}

func (a complexAbs[T]) Abs() T {
	r := float64(real(a.Value))
	i := float64(imag(a.Value))
	d := math.Sqrt(r*r + i*i)
	return T(complex(d, 0))
}

// OrderedAbsDifference returns the absolute value of the difference
// between a and b, where a and b are of an ordered type.
func OrderedAbsDifference[T orderedNumeric](a, b T) T {
	return absDifference(orderedAbs[T]{a}, orderedAbs[T]{b})
}

// ComplexAbsDifference returns the absolute value of the difference
// between a and b, where a and b are of a complex type.
func ComplexAbsDifference[T Complex](a, b T) T {
	return absDifference(complexAbs[T]{a}, complexAbs[T]{b})
}

func main() {
	if got, want := OrderedAbsDifference(1.0, -2.0), 3.0; got != want {
		panic(fmt.Sprintf("got = %v, want = %v", got, want))
	}
	if got, want := OrderedAbsDifference(-1.0, 2.0), 3.0; got != want {
		panic(fmt.Sprintf("got = %v, want = %v", got, want))
	}
	if got, want := OrderedAbsDifference(-20, 15), 35; got != want {
		panic(fmt.Sprintf("got = %v, want = %v", got, want))
	}

	if got, want := ComplexAbsDifference(5.0+2.0i, 2.0-2.0i), 5+0i; got != want {
		panic(fmt.Sprintf("got = %v, want = %v", got, want))
	}
	if got, want := ComplexAbsDifference(2.0-2.0i, 5.0+2.0i), 5+0i; got != want {
		panic(fmt.Sprintf("got = %v, want = %v", got, want))
	}
}

I have a fix already, which I'll post immediately. We were not properly setting g.curDecl in the deferred function processing, so type params of generic functions were not being made as unique as they should be. This is definitely worth getting in for beta-2 (but not completely required if we miss the cutoff).

@randall77

[gopherbot]

Change https://golang.org/cl/380594 mentions this issue: cmd/compile: new absdiff.go test, fix problem with g.curDecl

[perillo]

@danscales
There is a typo in the absDifference function documentation. AbsDifference should be absDifference.

[danscales]

Thanks @perillo I will incorporate a fix to that typo into the next checkin that I need to make.