Floating-point constants with binary ('p') exponents are not permitted in the source and that has been fixed (issue #9036). E.g.:

package p
const _ = 1p10

is not accepted anymore:
x.go:2: malformed floating point constant

However, it appears that the numbers are still recognized (and not set to 0 or 1) and then make it through the rest of type-checking/constant evaluation:

package p
const _ = 1 << 1p10

x.go:2: malformed floating point constant
x.go:2: stupid shift: 1024

The fact that we hang in Karatsuba is known and is effectively (indirectly) a duplicate of #11327. But once we set 'p' exponent values to 0, the hang should disappear.

Assigning to @dr2chase who fixed issue #9036.

This appears to be fixed. New parser?

$ cat > bug11364.go
package a
var e = 1<<1p10000000<<3
$ go tool compilebug11364.go
go tool compile bug11364.go
bug11364.go:2: syntax error: unexpected p10000000 after top level declaration

and it wastes no time telling me that.

@dr2chase Fixed by you in https://go-review.googlesource.com/c/10450/ I think.