Whitespace-sensitive bang patterns (#1087, #17162)

This patch implements a part of GHC Proposal #229 that covers three
operators:

* the bang operator (!)
* the tilde operator (~)
* the at operator (@)

Based on surrounding whitespace, these operators are disambiguated into
bang patterns, lazy patterns, strictness annotations, and type
applications.

This patch does NOT cover ($), ($$), and (-), which are left as future
work.

Metric Increase:
    parsing001
    Naperian
    T4801
Metric Increase (test_env='i386-linux-deb9'):
    haddock.base
    haddock.Cabal
    haddock.compiler
    T14683

28 files changed, 119 insertions, 41 deletions

diff --git a/testsuite/tests/ghc-api/annotations/T10358.stdout b/testsuite/tests/ghc-api/annotations/T10358.stdout
index 604c7dab36..28f516cb5e 100644
--- a/testsuite/tests/ghc-api/annotations/T10358.stdout
+++ b/testsuite/tests/ghc-api/annotations/T10358.stdout
@@ -16,12 +16,12 @@
 ((Test10358.hs:(4,1)-(8,6),AnnSemi), [Test10358.hs:9:1]),
 ((Test10358.hs:(5,3)-(8,6),AnnIn), [Test10358.hs:8:3-4]),
 ((Test10358.hs:(5,3)-(8,6),AnnLet), [Test10358.hs:5:3-5]),
-((Test10358.hs:5:7-16,AnnBang), [Test10358.hs:5:7]),
+((Test10358.hs:5:7-10,AnnBang), [Test10358.hs:5:7]),
 ((Test10358.hs:5:7-16,AnnEqual), [Test10358.hs:5:12]),
 ((Test10358.hs:5:7-16,AnnFunId), [Test10358.hs:5:8-10]),
 ((Test10358.hs:5:7-16,AnnSemi), [Test10358.hs:5:17]),
 ((Test10358.hs:5:14-16,AnnVal), [Test10358.hs:5:15]),
-((Test10358.hs:5:19-32,AnnBang), [Test10358.hs:5:19]),
+((Test10358.hs:5:19-22,AnnBang), [Test10358.hs:5:19]),
 ((Test10358.hs:5:19-32,AnnEqual), [Test10358.hs:5:24]),
 ((Test10358.hs:5:19-32,AnnFunId), [Test10358.hs:5:20-22]),
 ((Test10358.hs:5:19-32,AnnSemi), [Test10358.hs:6:7]),
diff --git a/testsuite/tests/module/mod69.stderr b/testsuite/tests/module/mod69.stderr
index db7487485e..dea161115e 100644
--- a/testsuite/tests/module/mod69.stderr
+++ b/testsuite/tests/module/mod69.stderr
@@ -1,4 +1,4 @@
 
 mod69.hs:3:7: error:
-    Pattern syntax in expression context: x@1
-    Did you mean to enable TypeApplications?
+    @-pattern in expression context: x@1
+    Type application syntax requires a space before '@'
diff --git a/testsuite/tests/module/mod70.stderr b/testsuite/tests/module/mod70.stderr
index 093f166ebd..6e9f854b7a 100644
--- a/testsuite/tests/module/mod70.stderr
+++ b/testsuite/tests/module/mod70.stderr
@@ -1,2 +1,4 @@
 
-mod70.hs:3:9: error: Pattern syntax in expression context: ~1
+mod70.hs:3:9: error:
+    Lazy pattern in expression context: ~1
+    Did you mean to add a space after the '~'?
diff --git a/testsuite/tests/parser/should_compile/T1087.hs b/testsuite/tests/parser/should_compile/T1087.hs
new file mode 100644
index 0000000000..9ad85e2b7a
--- /dev/null
+++ b/testsuite/tests/parser/should_compile/T1087.hs
@@ -0,0 +1,14 @@
+{-# LANGUAGE BangPatterns #-}
+
+module T1087 where
+
+prefix_1 = let at a !b = False in at 1 2
+prefix_2 = let (.!.) a !b = False in 1 .!. 2
+
+infix_tilde_1 = let a `at` ~b = False in at 1 2
+infix_tilde_2 = let a .!. ~b = False in 1 .!. 2
+infix_tilde_3 = let ~a .!. b = False in 1 .!. 2
+
+infix_bang_1 = let a .!. !b = False in 1 .!. 2
+infix_bang_2 = let a `at` !b = False in at 1 2
+infix_bang_3 = let !a .!. b = False in 1 .!. 2
diff --git a/testsuite/tests/parser/should_compile/T16619.stderr b/testsuite/tests/parser/should_compile/T16619.stderr
new file mode 100644
index 0000000000..b5dfb89623
--- /dev/null
+++ b/testsuite/tests/parser/should_compile/T16619.stderr
@@ -0,0 +1,3 @@
+
+T16619.hs:2:12: warning:
+    -Wmissing-space-after-bang is deprecated: bang patterns can no longer be written with a space
diff --git a/testsuite/tests/parser/should_compile/all.T b/testsuite/tests/parser/should_compile/all.T
index 3d44e22510..8b919f1b38 100644
--- a/testsuite/tests/parser/should_compile/all.T
+++ b/testsuite/tests/parser/should_compile/all.T
@@ -145,3 +145,7 @@ test('T16339', normal, compile, [''])
 test('T16619', req_th, multimod_compile, ['T16619', '-v0'])
 test('T504', normal, compile, [''])
 test('T515', literate, compile, ['-Wall'])
+test('T1087', normal, compile, [''])
+test('proposal-229a', normal, compile, [''])
+test('proposal-229b', normal, compile, [''])
+test('proposal-229d', normal, compile, [''])
diff --git a/testsuite/tests/parser/should_compile/proposal-229a.hs b/testsuite/tests/parser/should_compile/proposal-229a.hs
new file mode 100644
index 0000000000..c773cee3a2
--- /dev/null
+++ b/testsuite/tests/parser/should_compile/proposal-229a.hs
@@ -0,0 +1,8 @@
+{-# LANGUAGE BangPatterns #-}
+
+module Proposal229a where
+
+data T a b = a :! b
+
+(!) :: x -> T a b -> (x, a, b)
+~u ! !(!m :! !n) = (u, m, n)
diff --git a/testsuite/tests/parser/should_compile/proposal-229b.hs b/testsuite/tests/parser/should_compile/proposal-229b.hs
new file mode 100644
index 0000000000..9182623e54
--- /dev/null
+++ b/testsuite/tests/parser/should_compile/proposal-229b.hs
@@ -0,0 +1,10 @@
+module Proposal229b ((~), (@)) where
+
+(~) :: a -> b -> (a, b)
+x ~ y = (x, y)
+
+(@) :: a -> b -> (a, b)
+x @ y = (x, y)
+
+r :: ((Bool, Bool), Bool)
+r = True ~ False @ True
diff --git a/testsuite/tests/parser/should_compile/proposal-229d.hs b/testsuite/tests/parser/should_compile/proposal-229d.hs
new file mode 100644
index 0000000000..24a57ca872
--- /dev/null
+++ b/testsuite/tests/parser/should_compile/proposal-229d.hs
@@ -0,0 +1,6 @@
+{-# LANGUAGE BangPatterns #-}
+
+module Proposal229d ((!)) where
+
+(!) :: a -> b -> (a, b)
+x ! y = (x,y)   -- parsed as an operator even with BangPatterns enabled
diff --git a/testsuite/tests/parser/should_fail/T14588.stderr b/testsuite/tests/parser/should_fail/T14588.stderr
index cb64103814..2efd9561e8 100644
--- a/testsuite/tests/parser/should_fail/T14588.stderr
+++ b/testsuite/tests/parser/should_fail/T14588.stderr
@@ -1,4 +1,4 @@
 
 T14588.hs:3:19: error:
     Illegal bang-pattern (use BangPatterns):
-    ! x
+    !x
diff --git a/testsuite/tests/parser/should_fail/T16270.stderr b/testsuite/tests/parser/should_fail/T16270.stderr
index f4e90e40fc..a74bdeb8f0 100644
--- a/testsuite/tests/parser/should_fail/T16270.stderr
+++ b/testsuite/tests/parser/should_fail/T16270.stderr
@@ -1,4 +1,7 @@
 
+T16270.hs:2:12: warning:
+    -Werror=missing-space-after-bang is deprecated: bang patterns can no longer be written with a space
+
 T16270.hs:7:1: warning: [-Wtabs (in -Wdefault)]
     Tab character found here, and in five further locations.
     Please use spaces instead.
@@ -46,10 +49,9 @@ T16270.hs:23:10: error:
     Perhaps you intended to use GADTs or a similar language
     extension to enable syntax: data T where
 
-T16270.hs:25:12: error: [-Wmissing-space-after-bang (in -Wdefault), -Werror=missing-space-after-bang]
-    Did you forget to enable BangPatterns?
-    If you mean to bind (!) then perhaps you want
-    to add a space after the bang for clarity.
+T16270.hs:25:12: error:
+    Illegal bang-pattern (use BangPatterns):
+    !i
 
 T16270.hs:27:9: error:
     Multi-way if-expressions need MultiWayIf turned on
@@ -57,13 +59,13 @@ T16270.hs:27:9: error:
 T16270.hs:29:9: error:
     Multi-way if-expressions need MultiWayIf turned on
 
-T16270.hs:32:6: Illegal lambda-case (use LambdaCase)
+T16270.hs:32:6: error: Illegal lambda-case (use LambdaCase)
 
-T16270.hs:35:5:
+T16270.hs:35:5: error:
     Use NumericUnderscores to allow underscores in integer literals
 
-T16270.hs:37:5:
-    primitive string literal must contain only characters <= '/xFF'
+T16270.hs:37:5: error:
+    primitive string literal must contain only characters <= '\xFF'
 
 T16270.hs:43:1: error:
     parse error (possibly incorrect indentation or mismatched brackets)
diff --git a/testsuite/tests/parser/should_fail/T17162.hs b/testsuite/tests/parser/should_fail/T17162.hs
new file mode 100644
index 0000000000..6419da7544
--- /dev/null
+++ b/testsuite/tests/parser/should_fail/T17162.hs
@@ -0,0 +1,13 @@
+-- {-# LANGUAGE NoBangPatterns #-}
+
+module T17162 where
+
+charIsRepresentable :: TextEncoding -> Char -> IO Bool
+charIsRepresentable !enc c =
+  withCString enc [c]
+              (\cstr -> do str <- peekCString enc cstr
+                           case str of
+                             [ch] | ch == c -> pure True
+                             _ -> pure False)
+    `catch`
+       \(_ :: IOException) -> pure False
diff --git a/testsuite/tests/parser/should_fail/T17162.stderr b/testsuite/tests/parser/should_fail/T17162.stderr
new file mode 100644
index 0000000000..d621e08ccc
--- /dev/null
+++ b/testsuite/tests/parser/should_fail/T17162.stderr
@@ -0,0 +1,4 @@
+
+T17162.hs:6:21: error:
+    Illegal bang-pattern (use BangPatterns):
+    !enc
diff --git a/testsuite/tests/parser/should_fail/T3811b.stderr b/testsuite/tests/parser/should_fail/T3811b.stderr
index f4e44c603c..65de1d5a75 100644
--- a/testsuite/tests/parser/should_fail/T3811b.stderr
+++ b/testsuite/tests/parser/should_fail/T3811b.stderr
@@ -1,4 +1,4 @@
 
 T3811b.hs:4:14: error:
     Cannot parse data constructor in a data/newtype declaration:
-      ! B
+      !B
diff --git a/testsuite/tests/parser/should_fail/T3811c.stderr b/testsuite/tests/parser/should_fail/T3811c.stderr
index 431318e268..52f081bbe6 100644
--- a/testsuite/tests/parser/should_fail/T3811c.stderr
+++ b/testsuite/tests/parser/should_fail/T3811c.stderr
@@ -1,5 +1,6 @@
 
-T3811c.hs:6:11: error:
-    Strictness annotation applied to a compound type.
-    Did you mean to add parentheses?
-      !(Show D)
+T3811c.hs:6:10: error:
+    Illegal class instance: ‘!Show D’
+      Class instances must be of the form
+        context => C ty_1 ... ty_n
+      where ‘C’ is a class
diff --git a/testsuite/tests/parser/should_fail/T3811f.stderr b/testsuite/tests/parser/should_fail/T3811f.stderr
index 2d31fa86cf..783a89e284 100644
--- a/testsuite/tests/parser/should_fail/T3811f.stderr
+++ b/testsuite/tests/parser/should_fail/T3811f.stderr
@@ -1,5 +1,3 @@
 
-T3811f.hs:4:8: error:
-    Strictness annotation applied to a compound type.
-    Did you mean to add parentheses?
-      !(Foo a)
+T3811f.hs:4:7: error:
+    Malformed head of type or class declaration: !Foo a
diff --git a/testsuite/tests/parser/should_fail/all.T b/testsuite/tests/parser/should_fail/all.T
index 2fc7f3d326..c4a7a4f67b 100644
--- a/testsuite/tests/parser/should_fail/all.T
+++ b/testsuite/tests/parser/should_fail/all.T
@@ -161,3 +161,5 @@ test('patFail006', normal, compile_fail, [''])
 test('patFail007', normal, compile_fail, [''])
 test('patFail008', normal, compile_fail, [''])
 test('patFail009', normal, compile_fail, [''])
+test('T17162', normal, compile_fail, [''])
+test('proposal-229c', normal, compile_fail, [''])
diff --git a/testsuite/tests/parser/should_fail/proposal-229c.hs b/testsuite/tests/parser/should_fail/proposal-229c.hs
new file mode 100644
index 0000000000..344311b2a1
--- /dev/null
+++ b/testsuite/tests/parser/should_fail/proposal-229c.hs
@@ -0,0 +1,6 @@
+{-# LANGUAGE NoBangPatterns #-}
+
+module Proposal229c (f) where
+
+-- should recommend to enable BangPatterns instead of parsing as an infix operator
+f !x = x
diff --git a/testsuite/tests/parser/should_fail/proposal-229c.stderr b/testsuite/tests/parser/should_fail/proposal-229c.stderr
new file mode 100644
index 0000000000..965801a3c3
--- /dev/null
+++ b/testsuite/tests/parser/should_fail/proposal-229c.stderr
@@ -0,0 +1,4 @@
+
+proposal-229c.hs:6:3: error:
+    Illegal bang-pattern (use BangPatterns):
+    !x
diff --git a/testsuite/tests/parser/should_fail/strictnessDataCon_A.stderr b/testsuite/tests/parser/should_fail/strictnessDataCon_A.stderr
index c02d2ee974..27e6c709a5 100644
--- a/testsuite/tests/parser/should_fail/strictnessDataCon_A.stderr
+++ b/testsuite/tests/parser/should_fail/strictnessDataCon_A.stderr
@@ -1,3 +1,3 @@
 
 strictnessDataCon_A.hs:1:27: error:
-    Strictness annotation cannot appear in this position.
+    Operator applied to too few arguments: !
diff --git a/testsuite/tests/rename/should_fail/T12879.stderr b/testsuite/tests/rename/should_fail/T12879.stderr
index 1b3559c255..0c6b7f36f5 100644
--- a/testsuite/tests/rename/should_fail/T12879.stderr
+++ b/testsuite/tests/rename/should_fail/T12879.stderr
@@ -1,4 +1,4 @@
 
 T12879.hs:4:7: error:
-    Pattern syntax in expression context: x@x
+    @-pattern in expression context: x@x
     Type application syntax requires a space before '@'
diff --git a/testsuite/tests/rename/should_fail/rnfail016.stderr b/testsuite/tests/rename/should_fail/rnfail016.stderr
index 47436132f2..6ed450ce0b 100644
--- a/testsuite/tests/rename/should_fail/rnfail016.stderr
+++ b/testsuite/tests/rename/should_fail/rnfail016.stderr
@@ -1,4 +1,4 @@
 
 rnfail016.hs:6:7: error:
-    Pattern syntax in expression context: x@x
-    Did you mean to enable TypeApplications?
+    @-pattern in expression context: x@x
+    Type application syntax requires a space before '@'
diff --git a/testsuite/tests/rename/should_fail/rnfail016a.stderr b/testsuite/tests/rename/should_fail/rnfail016a.stderr
index 3a59ee7478..544cf58cac 100644
--- a/testsuite/tests/rename/should_fail/rnfail016a.stderr
+++ b/testsuite/tests/rename/should_fail/rnfail016a.stderr
@@ -1,2 +1,2 @@
 
-rnfail016a.hs:6:7: error: Pattern syntax in expression context: ~x
+rnfail016a.hs:6:7: error: parse error on input ‘~’
diff --git a/testsuite/tests/rename/should_fail/rnfail051.stderr b/testsuite/tests/rename/should_fail/rnfail051.stderr
index 9c45a6168b..c1f4f43a2f 100644
--- a/testsuite/tests/rename/should_fail/rnfail051.stderr
+++ b/testsuite/tests/rename/should_fail/rnfail051.stderr
@@ -1,3 +1,3 @@
 
 rnfail051.hs:7:17: error:
-    Pattern syntax in expression context: _ -> putStrLn "_"
+    View pattern in expression context: _ -> putStrLn "_"
diff --git a/testsuite/tests/th/T12411.stderr b/testsuite/tests/th/T12411.stderr
index 1f344323bd..22f7de0190 100644
--- a/testsuite/tests/th/T12411.stderr
+++ b/testsuite/tests/th/T12411.stderr
@@ -1,4 +1,6 @@
 
 T12411.hs:4:1: error:
-    Pattern syntax in expression context: pure@Q
-    Did you mean to enable TypeApplications?
+    Illegal visible type application ‘@Q’
+      Perhaps you intended to use TypeApplications
+
+T12411.hs:4:7: error: Not in scope: type constructor or class ‘Q’
diff --git a/testsuite/tests/typecheck/should_fail/T14761b.stderr b/testsuite/tests/typecheck/should_fail/T14761b.stderr
index 08a319cde3..af557c4725 100644
--- a/testsuite/tests/typecheck/should_fail/T14761b.stderr
+++ b/testsuite/tests/typecheck/should_fail/T14761b.stderr
@@ -1,5 +1,2 @@
 
-T14761b.hs:5:21: error:
-    Strictness annotation applied to a compound type.
-    Did you mean to add parentheses?
-      !(Maybe Int)
+T14761b.hs:5:19: error: Operator applied to too few arguments: !
diff --git a/testsuite/tests/typecheck/should_fail/T15527.stderr b/testsuite/tests/typecheck/should_fail/T15527.stderr
index dd03a0a0ca..8908b17218 100644
--- a/testsuite/tests/typecheck/should_fail/T15527.stderr
+++ b/testsuite/tests/typecheck/should_fail/T15527.stderr
@@ -1,4 +1,4 @@
 
 T15527.hs:4:6: error:
-    Pattern syntax in expression context: (.)@Int
-    Did you mean to enable TypeApplications?
+    Illegal visible type application ‘@Int’
+      Perhaps you intended to use TypeApplications
diff --git a/testsuite/tests/typecheck/should_fail/T7210.stderr b/testsuite/tests/typecheck/should_fail/T7210.stderr
index 4d7cb38a4d..d0ca04a84e 100644
--- a/testsuite/tests/typecheck/should_fail/T7210.stderr
+++ b/testsuite/tests/typecheck/should_fail/T7210.stderr
@@ -1,5 +1,7 @@
 
-T7210.hs:5:20: error:
-    Strictness annotation applied to a compound type.
-    Did you mean to add parentheses?
-      !(IntMap Int)
+T7210.hs:5:19: error:
+    • Unexpected strictness annotation: !IntMap
+      strictness annotation cannot appear nested inside a type
+    • In the type ‘!IntMap Int’
+      In the definition of data constructor ‘C’
+      In the data declaration for ‘T’