cmd/fix, x/tools/internal/refactor/inline: inlining 1 << f() may produce a constant (1 << 123) that is too large

[zigo101]

Go version

go version go1.26.1 linux/amd64

Output of go env in your module/workspace:

.

What did you do?

package main

//go:fix inline
func foo() int {
	return 123
}

func main() {
	_ = 1 << foo()
}

What did you see happen?

fails to compile now

What did you expect to see?

still compiles

[zigo101]

Similar one:

package main

type T = [8]byte
var a T

//go:fix inline
func foo() T {
    return T{}
}

func main() {
	if foo() == a {
	}
}

main will be rewritten as

func main() {
	if T{} == a {
	}
}

which fails to compile now.

[adonovan]

The missing information in the bug report is that the result, 1 << 123, fails to compile because it exceeds the limit for constants. This is a related problem to the "fallible" constants one described in last week's blog post, but the subtlety is that the constant expression spans both the original code and the newly substituted subtree. This will need some thought.

[gabyhelp]

Related Issues

• cmd/fix: go fix doesn't consider constant-ness of len/cap/unsafe.Sizeof etc calls in generic functions #78169
• cmd/fix: incorrect fix that breaks compilation #78149
• cmd/fix: `go fix` does not inline nested inlines #78164
• cmd/fix: 'go fix -inline .' says 'applied 2 of 3 fixes' but actually applies none #77750 (closed)
• go fix: go fix replaces pointer to a function argument with new(argument) #77665 (closed)
• cmd/fix,x/tools/go/analysis/passes/inline: adds unnecessary import for argument type #77610 (closed)

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[zigo101]

@adonovan
This is not a constant specific problem. Please see #78170 (comment)

[zigo101]

another one:

package main

//go:fix inline
func foo(v any) {
	println(v)
}

func main() {
	foo(1)
}

main will be re-written as

func main() {
	println(1)
}

which changes the old behavior,

[zigo101]

similar:

package main

//go:fix inline
func foo(v *int) {
	println(v)
}

func main() {
	foo(nil)
}

main will be rewritten as

func main() {
	println(nil)
}

which doesn't compile now.

[adonovan]

Please include "did X, got Y, want Z" information in your bug reports. Stating merely Y="fails to compile" or omitting Y and Z entirely does not make for easy reading. Thanks.

[zigo101]

Generally want Z is behavior not changed.

I added got Y for all cases now.