Really an inconsistency.

By the current rule, a call to Sizeof with a argument of a type parameter type is not viewed as constant expression.
So both should compile okay and panic at run time.

[edit]: a complete and a bit modified version:

package main

import "unsafe"

func main() {
  wontBuild(0) // comment on to compile
  panics(0)
}

func wontBuild[T byte](t T) {
  const str = "a"
  _ = str[unsafe.Sizeof(t)]
}

func panics[T byte](t T) {
  const str = "a"
  _ = str[unsafe.Sizeof(t)+0]
}

CC @golang/compiler

Like @go101 said, this is working as intended. unsafe.Sizeof applied to a derived type (for example, a type parameter or a composite type containing one) is a regular value expression. It will still be evaluated at compile-time and optimized like a Go constant. But as far as the type checker is concerned, it's not a Go constant. So it won't emit diagnostics relating to invalid uses of Go constant.

We could change the language spec such that unsafe.Sizeof(t) in your code is recognized as the constant 1, since T must be byte and therefore have size 1. But that would need to go through the proposal process.

So the inconsistency is a compiler dependent behavior?
(More specifically, whether or not the unsafe.Sizeof(t)+0 expression evaluated at run time is compiler dependent?)

So the inconsistency is a compiler dependent behavior?

No.

Thanks to @griesemer for pointing out I misread the issue description. Reopening.

Thanks for the report. @griesemer and I agree both programs should panic.

Running go build -gcflags=-h shows a stack trace in the unified reader. So there's probably just somewhere that we're turning the unsafe.Sizeof expression into an OLITERAL, and then applying bounds checks in typecheck still. types2 handles all constant-related bounds checks in user Go code now, so it's perhaps safe to just remove those checks from typecheck. (These are unrelated to the runtime bounds checks inserted at code generation later in the compiler.)

CC @cuonglm @cagedmantis in case either of you are interested in tackling this. It should be a small fix.

I like that types2 is now considered user Go code 👍🏻

Change https://go.dev/cl/560575 mentions this issue: cmd/compile/internal/typecheck: remove constant bounds check