Implement -Wredundant-bang-patterns (#17340)

Add new flag '-Wredundant-bang-patterns' that enables checks for "dead" bangs.
Dead bangs are the ones that under no circumstances can force a thunk that
wasn't already forced. Dead bangs are a form of redundant bangs. The new check
is performed in Pattern-Match Coverage Checker along with other checks (namely,
redundant and inaccessible RHSs). Given

    f :: Bool -> Int
    f True = 1
    f !x   = 2

we can detect dead bang patterns by checking whether @x ~ ⊥@ is satisfiable
where the PmBang appears in 'checkGrdTree'. If not, then clearly the bang is
dead. Such a dead bang is then indicated in the annotated pattern-match tree by
a 'RedundantSrcBang' wrapping. In 'redundantAndInaccessibles', we collect
all dead bangs to warn about.

Note that we don't want to warn for a dead bang that appears on a redundant
clause. That is because in that case, we recommend to delete the clause wholly,
including its leading pattern match.

Dead bang patterns are redundant. But there are bang patterns which are
redundant that aren't dead, for example

    f !() = 0

the bang still forces the match variable, before we attempt to match on (). But
it is redundant with the forcing done by the () match. We currently don't
detect redundant bangs that aren't dead.

3 files changed, 104 insertions, 0 deletions

diff --git a/testsuite/tests/pmcheck/should_compile/T17340.hs b/testsuite/tests/pmcheck/should_compile/T17340.hs
new file mode 100644
index 0000000000..fa2ef60812
--- /dev/null
+++ b/testsuite/tests/pmcheck/should_compile/T17340.hs
@@ -0,0 +1,54 @@
+{-# LANGUAGE BangPatterns #-}
+{-# LANGUAGE PatternSynonyms #-}
+module T17340 where
+
+data A = A { a :: () }
+data B = B
+newtype C = C Int
+pattern P = B
+
+f_nowarn :: Bool -> Bool
+f_nowarn !x = x
+
+f :: Bool -> Bool
+f True = False
+f !x   = x
+
+g :: (Int, Int) -> Bool -> ()
+g (a,b) True = ()
+g !x False = ()
+
+data T = MkT !Int
+h :: T -> ()
+h (MkT !x) = ()
+
+k :: Bool -> Int
+k True = 1
+k !_   = 2  -- clause is accessible, so warn for the bang
+
+t :: () -> Bool -> Int
+t _   True  = 1
+t !() True  = 2 -- the clause has inaccessible RHS, warn for the bang
+t _   False = 3
+
+q :: Bool -> Int
+q True  = 1
+q !True = 2 -- clause is redundant, don't warn for the bang
+q False = 3
+
+i :: Bool -> Int
+i True       = 1
+i !x | x     = 2 -- redundant
+     | not x = 3 -- accessible. This one will stay alive, so warn for the bang
+
+newtype T2 a = T2 a
+w :: T2 a -> Bool -> ()
+w _      True = ()
+w (T2 _) True = () -- redundant
+w !_     True = () -- inaccessible
+w _      _    = ()
+
+z :: T2 a -> Bool -> ()
+z _ True                = ()
+z t2 !x | T2 _ <- t2, x = () -- redundant
+        | !_ <- t2, x   = () -- inaccessable
diff --git a/testsuite/tests/pmcheck/should_compile/T17340.stderr b/testsuite/tests/pmcheck/should_compile/T17340.stderr
new file mode 100644
index 0000000000..c31fb2a6f5
--- /dev/null
+++ b/testsuite/tests/pmcheck/should_compile/T17340.stderr
@@ -0,0 +1,48 @@
+
+T17340.hs:15:4: warning: [-Wredundant-bang-patterns]
+    Pattern match has redundant bang
+    In an equation for ‘f’: f x = ...
+
+T17340.hs:19:4: warning: [-Wredundant-bang-patterns]
+    Pattern match has redundant bang
+    In an equation for ‘g’: g x = ...
+
+T17340.hs:27:4: warning: [-Wredundant-bang-patterns]
+    Pattern match has redundant bang
+    In an equation for ‘k’: k _ = ...
+
+T17340.hs:31:1: warning: [-Woverlapping-patterns (in -Wdefault)]
+    Pattern match has inaccessible right hand side
+    In an equation for ‘t’: t !() True = ...
+
+T17340.hs:36:1: warning: [-Woverlapping-patterns (in -Wdefault)]
+    Pattern match is redundant
+    In an equation for ‘q’: q !True = ...
+
+T17340.hs:41:4: warning: [-Wredundant-bang-patterns]
+    Pattern match has redundant bang
+    In an equation for ‘i’: i x = ...
+
+T17340.hs:41:8: warning: [-Woverlapping-patterns (in -Wdefault)]
+    Pattern match is redundant
+    In an equation for ‘i’: i !x | x = ...
+
+T17340.hs:47:1: warning: [-Woverlapping-patterns (in -Wdefault)]
+    Pattern match is redundant
+    In an equation for ‘w’: w (T2 _) True = ...
+
+T17340.hs:48:1: warning: [-Woverlapping-patterns (in -Wdefault)]
+    Pattern match has inaccessible right hand side
+    In an equation for ‘w’: w !_ True = ...
+
+T17340.hs:53:7: warning: [-Wredundant-bang-patterns]
+    Pattern match has redundant bang
+    In an equation for ‘z’: z x = ...
+
+T17340.hs:53:11: warning: [-Woverlapping-patterns (in -Wdefault)]
+    Pattern match is redundant
+    In an equation for ‘z’: z t2 !x | T2 _ <- t2, x = ...
+
+T17340.hs:54:11: warning: [-Woverlapping-patterns (in -Wdefault)]
+    Pattern match has inaccessible right hand side
+    In an equation for ‘z’: z t2 !x | !_ <- t2, x = ...
diff --git a/testsuite/tests/pmcheck/should_compile/all.T b/testsuite/tests/pmcheck/should_compile/all.T
index 054ad82873..51fb76b078 100644
--- a/testsuite/tests/pmcheck/should_compile/all.T
+++ b/testsuite/tests/pmcheck/should_compile/all.T
@@ -124,6 +124,8 @@ test('T18478', collect_compiler_stats('bytes allocated',10), compile,
      ['-fwarn-incomplete-patterns -fwarn-overlapping-patterns'])
 test('T18533', normal, compile,
      ['-fwarn-incomplete-patterns -fwarn-overlapping-patterns'])
+test('T17340', normal, compile,
+     ['-Wredundant-bang-patterns'])
 
 # Other tests
 test('pmc001', [], compile,