Conditionally show plural "s" in warnings

Redundant constraints and defaulting warnings had "constraint(s)" in
their messages; the "s" is now conditional based on the number of things
warned about.

Reviewers: hvr, austin, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1647

3 files changed, 54 insertions, 0 deletions

diff --git a/testsuite/tests/warnings/should_compile/PluralS.hs b/testsuite/tests/warnings/should_compile/PluralS.hs
new file mode 100644
index 0000000000..89b5dbffcd
--- /dev/null
+++ b/testsuite/tests/warnings/should_compile/PluralS.hs
@@ -0,0 +1,27 @@
+-- Test purpose:
+--
+-- Ensure the plural "s" in warnings is only shown if there are more than
+-- one entries
+
+{-# OPTIONS_GHC -Wredundant-constraints #-}
+{-# OPTIONS_GHC -Wtype-defaults #-}
+
+module PluralS () where
+
+
+
+-- Defaulting type classes
+
+defaultingNum = 123 `seq` ()
+
+defaultingNumAndShow = show 123
+
+
+
+-- Redundant constraints
+
+redundantNum :: (Num a, Num a) => a
+redundantNum = 123
+
+redundantMultiple :: (Num a, Show a, Num a, Eq a, Eq a) => a
+redundantMultiple = 123
diff --git a/testsuite/tests/warnings/should_compile/PluralS.stderr b/testsuite/tests/warnings/should_compile/PluralS.stderr
new file mode 100644
index 0000000000..c013e7b73d
--- /dev/null
+++ b/testsuite/tests/warnings/should_compile/PluralS.stderr
@@ -0,0 +1,26 @@
+
+PluralS.hs:15:17: warning:
+    • Defaulting the following constraint to type ‘Integer’
+        Num t0 arising from the literal ‘123’
+    • In the first argument of ‘seq’, namely ‘123’
+      In the expression: 123 `seq` ()
+      In an equation for ‘defaultingNum’: defaultingNum = 123 `seq` ()
+
+PluralS.hs:17:29: warning:
+    • Defaulting the following constraints to type ‘Integer’
+        (Num a0) arising from the literal ‘123’ at PluralS.hs:17:29-31
+        (Show a0) arising from a use of ‘show’ at PluralS.hs:17:24-27
+    • In the first argument of ‘show’, namely ‘123’
+      In the expression: show 123
+      In an equation for ‘defaultingNumAndShow’:
+          defaultingNumAndShow = show 123
+
+PluralS.hs:23:1: warning:
+    • Redundant constraint: Num a
+    • In the type signature for:
+           redundantNum :: (Num a, Num a) => a
+
+PluralS.hs:26:1: warning:
+    • Redundant constraints: (Show a, Num a, Eq a, Eq a)
+    • In the type signature for:
+           redundantMultiple :: (Num a, Show a, Num a, Eq a, Eq a) => a
diff --git a/testsuite/tests/warnings/should_compile/all.T b/testsuite/tests/warnings/should_compile/all.T
index f60468e6ca..a2b1860ba4 100644
--- a/testsuite/tests/warnings/should_compile/all.T
+++ b/testsuite/tests/warnings/should_compile/all.T
@@ -7,6 +7,7 @@ test('T9230', normal, compile_without_flag('-fno-warn-tabs'), [''])
 test('T10908', normal, compile, [''])
 test('T11077', normal, compile, ['-fwarn-missing-exported-sigs'])
 test('T11128', normal, compile, [''])
+test('PluralS', normal, compile, [''])
 
 test('DeprU',
 	extra_clean([