go/types: bad constant integer->string conversion for out-of-bounds integers

[mdempsky]

The package below statically converts the constant 1<<32 to a string (which should be "\uFFFD", because it's outside of the valid Unicode range). It then checks that the length matches the expected length (which is 3).

cmd/compile correctly accepts this package, but go/types and gccgo do not. go/types used to handle this correctly. I believe this is a regression introduced by CL 220844 (6052838).

package p
const _ = -uint(len(string(1<<32)) - len("\uFFFD"))

/cc @griesemer @ianlancetaylor

[griesemer]

The go/types conversion code now incorrectly uses string(rune(codepoint)) where in the past it used the correct string(codepoint).

We probably should revisit commit 6052838 to ensure we didn't introduce other bugs elsewhere.

[gopherbot]

Change https://golang.org/cl/272666 mentions this issue: go/types: fix incorrect string(int) conversion (regression)

[gopherbot]

Change https://golang.org/cl/272667 mentions this issue: [dev.go2go] go/types, cmd/compile/internal/types2: fix incorrect string(int) conversion (regression)

[gopherbot]

Change https://golang.org/cl/272669 mentions this issue: [dev.typeparams] go/types, cmd/compile/internal/types2: fix incorrect string(int) conversion (regression)

[gopherbot]

Change https://golang.org/cl/272611 mentions this issue: compiler: avoid silent truncation for string(1 << 32)