Warn about empty Char enumerations (#18402)

Currently the "Enumeration is empty" warning (-Wempty-enumerations)
only fires for numeric literals. This patch adds support for `Char`
literals so that enumerating an empty list of `Char`s will also
trigger the warning.
23 jobs for !3659 with empty-char-enumeration-warning in 403 minutes and 16 seconds (queued for 1 second)
latest detached
Status Job ID Name Coverage
  Lint
passed #388414
lint
ghc-linters

00:01:01

passed #388415
lint
lint-linters

00:01:01

passed #388418
lint
lint-submods

00:00:21

passed #388416
lint
lint-testsuite

00:00:16

passed #388417
lint
typecheck-testsuite

00:00:15

 
  Quick Build
passed #388419
x86_64-linux
hadrian-ghc-in-ghci

00:10:40

 
  Build
passed #388420
x86_64-linux
validate-x86_64-linux-deb9-hadrian

01:15:27

 
  Full Build
failed #388423
aarch64-linux allowed to fail
validate-aarch64-linux-deb9

02:50:32

failed #388424
armv7-linux allowed to fail
validate-armv7-linux-deb9

03:28:14

passed #388425
x86_64-linux
validate-i386-linux-deb9

01:52:31

failed #388432
new-x86_64-windows test allowed to fail
validate-i386-windows

00:02:39

passed #388422
x86_64-darwin
validate-x86_64-darwin

01:12:10

passed #388426
x86_64-linux
validate-x86_64-linux-deb9-debug

02:11:25

passed #388428
x86_64-linux
validate-x86_64-linux-deb9-dwarf

02:02:41

passed #388427
x86_64-linux
validate-x86_64-linux-deb9-integer-simple

01:38:57

passed #388421
x86_64-linux
validate-x86_64-linux-deb9-unreg-hadrian

03:20:00

passed #388429
x86_64-linux
validate-x86_64-linux-fedora27

01:36:09

failed #388431
new-x86_64-windows test allowed to fail
validate-x86_64-windows

02:49:42

failed #388430
new-x86_64-windows test allowed to fail
validate-x86_64-windows-hadrian

03:19:47

 
  Cleanup
passed #388433
x86_64-darwin
cleanup-darwin

00:02:08

 
  Packaging
passed #388434
x86_64-linux
doc-tarball

00:03:54

 
  Testing
manual #388435
x86_64-linux allowed to fail manual
hackage
passed #388436
x86_64-linux
perf-nofib

00:24:36

 
Name Stage Failure
failed
validate-i386-windows Full Build
Git branch:   13-1-stable
GO version: go1.13.8
Built: 2020-06-19T21:12:23+0000
OS/Arch: windows/amd64
Uploading artifacts...
Runtime platform
arch=amd64 os=windows pid=1712 revision=6214287e version=13.1.0
WARNING: junit.xml: no matching files
ERROR: No files to upload
ERROR: Job failed: exit status 1
failed
validate-x86_64-windows-hadrian Full Build
GO version:   go1.13.8
Built: 2020-06-19T21:12:23+0000
OS/Arch: windows/amd64
Uploading artifacts...
Runtime platform
arch=amd64 os=windows pid=7304 revision=6214287e version=13.1.0
junit.xml: found 1 matching files and directories

WARNING: Failed to load system CertPool: crypto/x509: system root pool is not available on Windows
Uploading artifacts as "junit" to coordinator... ok
id=388430 responseStatus=201 Created token=ssr5P99J
ERROR: Job failed: exit status 1
failed
validate-armv7-linux-deb9 Full Build
Uploading artifacts...
ghc-armv7-linux-deb9.tar.xz: found 1 matching files

junit.xml: found 1 matching files

performance-metrics.tsv: found 1 matching files

Uploading artifacts to coordinator... ok
id=388424 responseStatus=201 Created token=kJANpzz2
Uploading artifacts...
junit.xml: found 1 matching files

Uploading artifacts to coordinator... ok
id=388424 responseStatus=201 Created token=kJANpzz2
ERROR: Job failed: exit code 1
failed
validate-aarch64-linux-deb9 Full Build
Uploading artifacts...
ghc-aarch64-linux-deb9.tar.xz: found 1 matching files and directories

junit.xml: found 1 matching files and directories

performance-metrics.tsv: found 1 matching files and directories

Uploading artifacts as "archive" to coordinator... ok
id=388423 responseStatus=201 Created token=Eca8pN6g
Uploading artifacts...
junit.xml: found 1 matching files and directories

Uploading artifacts as "junit" to coordinator... ok
id=388423 responseStatus=201 Created token=Eca8pN6g
ERROR: Job failed: exit code 1
failed
validate-x86_64-windows Full Build
GO version:   go1.13.8
Built: 2020-06-19T21:12:23+0000
OS/Arch: windows/amd64
Uploading artifacts...
Runtime platform
arch=amd64 os=windows pid=2296 revision=6214287e version=13.1.0
junit.xml: found 1 matching files and directories

WARNING: Failed to load system CertPool: crypto/x509: system root pool is not available on Windows
Uploading artifacts as "junit" to coordinator... ok
id=388431 responseStatus=201 Created token=HDdJH1Jx
ERROR: Job failed: exit status 1