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.

3 files changed, 19 insertions, 0 deletions

diff --git a/testsuite/tests/warnings/should_compile/T18402.hs b/testsuite/tests/warnings/should_compile/T18402.hs
new file mode 100644
index 0000000000..f18d3b5046
--- /dev/null
+++ b/testsuite/tests/warnings/should_compile/T18402.hs
@@ -0,0 +1,8 @@
+module T18402 where
+
+a = ['b'      .. 'a'] -- empty
+b = ['b', 'a' .. 'c'] -- empty
+c = ['b', 'c' .. 'a'] -- empty
+d = ['a'      .. 'c'] -- not empty
+e = ['a', 'c' .. 'b'] -- not empty
+
diff --git a/testsuite/tests/warnings/should_compile/T18402.stderr b/testsuite/tests/warnings/should_compile/T18402.stderr
new file mode 100644
index 0000000000..1fe12832c9
--- /dev/null
+++ b/testsuite/tests/warnings/should_compile/T18402.stderr
@@ -0,0 +1,9 @@
+
+T18402.hs:3:5: warning: [-Wempty-enumerations (in -Wdefault)]
+    Enumeration is empty
+
+T18402.hs:4:5: warning: [-Wempty-enumerations (in -Wdefault)]
+    Enumeration is empty
+
+T18402.hs:5:5: warning: [-Wempty-enumerations (in -Wdefault)]
+    Enumeration is empty
diff --git a/testsuite/tests/warnings/should_compile/all.T b/testsuite/tests/warnings/should_compile/all.T
index 03c4fcb7b8..de46efcf1f 100644
--- a/testsuite/tests/warnings/should_compile/all.T
+++ b/testsuite/tests/warnings/should_compile/all.T
@@ -30,3 +30,5 @@ test('Overflow', expect_broken_for(16543, ['hpc']), compile, [''])
 
 test('UnusedPackages', normal, multimod_compile,
     ['UnusedPackages.hs', '-package=bytestring -package=base -package=process -package=ghc -Wunused-packages'])
+
+test('T18402', normal, compile, [''])