Comment 1:

"A constant may be given a type explicitly by a constant declaration or conversion,
or implicitly when used in a variable declaration or an assignment or as an operand
in an expression. ... For instance, 3.0 can be given any integer or any
floating-point type, while 2147483648.0 (equal to 1<<31) can be given the types
float32, float64, or uint32 but not int32 or string."
Constants, The Go Programming Language Specification.
http://golang.org/doc/go_spec.html#Constants
Aren't you are asking for the size of and alignment of an untyped constant, which are
undefined? The constant 3 can be given any integer or any floating-point type. Give
it a type e.g. float(3), int64(3), etc. More meaningful error reporting would be nice.
"Sizeof returns the size in bytes occupied by the value v. The size is that of the
"top level" of the value only. For instance, if v is a slice, it returns the size of
the slice descriptor, not the size of the memory referenced by the slice."
func Sizeof. http://golang.org/pkg/unsafe/#Sizeof 
String literals are typed, they are strings. Aren't the size of and alignment of a
string returned by unsafe actually the size of and alignment of the string descriptor?

Comment 2:

The size of an untyped constant is undefined, I agree. If no one does anything about
this part, I can't complain too much.
The internal compiler error definitely seems like a bug to me, though.
Also note that string literals are not typed. I can say:
type S string
var s S = "A string"
If string literals were typed, that would fail to compile. I think it's inconsistent
that I get a somewhat reasonable value for the size of a string literal, but not for
a numeric literal.

Comment 3:

Owner changed to r...@golang.org.

Status changed to Accepted.

Comment 4:

changeset:   5525:330d5e8777eb
user:        Russ Cox <rsc@golang.org>
date:        Mon May 24 14:18:15 2010 -0700
summary:     gc: fix unsafe.Sizeof on ideal constants

Status changed to Fixed.