Implement -Wambiguous-fields

Fixes #18966. Adds a new warning -Wambiguous-fields for uses of field selectors
or record updates that will be rejected in the future, when the DuplicateRecordFields
extension is simplified per https://github.com/ghc-proposals/ghc-proposals/pull/366.

7 files changed, 52 insertions, 0 deletions

diff --git a/testsuite/tests/backpack/should_compile/T13323.stderr b/testsuite/tests/backpack/should_compile/T13323.stderr
index eb49bcbfab..7e637d9dd4 100644
--- a/testsuite/tests/backpack/should_compile/T13323.stderr
+++ b/testsuite/tests/backpack/should_compile/T13323.stderr
@@ -1,6 +1,11 @@
 [1 of 3] Processing p
   [1 of 2] Compiling A[sig]           ( p/A.hsig, nothing )
   [2 of 2] Compiling P                ( p/P.hs, nothing )
+
+T13323.bkp:9:13: warning: [-Wambiguous-fields (in -Wdefault)]
+    The field ‘foo’ belonging to type A is ambiguous.
+    This will not be supported by -XDuplicateRecordFields in future releases of GHC.
+    You can use a qualified import or explicit case analysis to resolve the ambiguity.
 [2 of 3] Processing q
   Instantiating q
   [1 of 1] Compiling A                ( q/A.hs, T13323.out/q/A.o )
@@ -10,5 +15,10 @@
     Instantiating p[A=q:A]
     [1 of 2] Compiling A[sig]           ( p/A.hsig, T13323.out/p/p-HVmFlcYSefiK5n1aDP1v7x/A.o )
     [2 of 2] Compiling P                ( p/P.hs, T13323.out/p/p-HVmFlcYSefiK5n1aDP1v7x/P.o )
+
+T13323.bkp:9:13: warning: [-Wambiguous-fields (in -Wdefault)]
+    The field ‘foo’ belonging to type A is ambiguous.
+    This will not be supported by -XDuplicateRecordFields in future releases of GHC.
+    You can use a qualified import or explicit case analysis to resolve the ambiguity.
   [1 of 2] Compiling R                ( r/R.hs, T13323.out/r/R.o )
   [2 of 2] Instantiating p
diff --git a/testsuite/tests/overloadedrecflds/should_fail/DRFUnused.stderr b/testsuite/tests/overloadedrecflds/should_fail/DRFUnused.stderr
index a9dbd2cdd5..7e75f5c8c7 100644
--- a/testsuite/tests/overloadedrecflds/should_fail/DRFUnused.stderr
+++ b/testsuite/tests/overloadedrecflds/should_fail/DRFUnused.stderr
@@ -1,3 +1,8 @@
 
 DRFUnused.hs:10:16: error: [-Wunused-top-binds (in -Wextra, -Wunused-binds), -Werror=unused-top-binds]
     Defined but not used: ‘foo’
+
+DRFUnused.hs:18:5: warning: [-Wambiguous-fields (in -Wdefault)]
+    The field ‘foo’ belonging to type U is ambiguous.
+    This will not be supported by -XDuplicateRecordFields in future releases of GHC.
+    You can use explicit case analysis to resolve the ambiguity.
diff --git a/testsuite/tests/overloadedrecflds/should_fail/overloadedrecfldsfail06.stderr b/testsuite/tests/overloadedrecflds/should_fail/overloadedrecfldsfail06.stderr
index 3aae5c5061..254931a9bc 100644
--- a/testsuite/tests/overloadedrecflds/should_fail/overloadedrecfldsfail06.stderr
+++ b/testsuite/tests/overloadedrecflds/should_fail/overloadedrecfldsfail06.stderr
@@ -26,3 +26,11 @@ overloadedrecfldsfail06.hs:9:1: error: [-Wunused-imports (in -Wextra), -Werror=u
 overloadedrecfldsfail06.hs:10:1: error: [-Wunused-imports (in -Wextra), -Werror=unused-imports]
     The qualified import of ‘U(x), U’
     from module ‘OverloadedRecFldsFail06_A’ is redundant
+
+overloadedrecfldsfail06.hs:15:22: error: [-Wambiguous-fields (in -Wdefault), -Werror=ambiguous-fields]
+    The record update u {x = True} with type U is ambiguous.
+    This will not be supported by -XDuplicateRecordFields in future releases of GHC.
+
+overloadedrecfldsfail06.hs:18:28: error: [-Wambiguous-fields (in -Wdefault), -Werror=ambiguous-fields]
+    The record update v {P.x = 3} with type V is ambiguous.
+    This will not be supported by -XDuplicateRecordFields in future releases of GHC.
diff --git a/testsuite/tests/overloadedrecflds/should_fail/overloadedrecfldsfail11.stderr b/testsuite/tests/overloadedrecflds/should_fail/overloadedrecfldsfail11.stderr
index 0aa41a2962..9599cddcba 100644
--- a/testsuite/tests/overloadedrecflds/should_fail/overloadedrecfldsfail11.stderr
+++ b/testsuite/tests/overloadedrecflds/should_fail/overloadedrecfldsfail11.stderr
@@ -4,3 +4,8 @@
 overloadedrecfldsfail11.hs:5:15: error: [-Wdeprecations (in -Wdefault), -Werror=deprecations]
     In the use of ‘foo’ (imported from OverloadedRecFldsFail11_A):
     "Warning on a record field"
+
+overloadedrecfldsfail11.hs:5:15: error: [-Wambiguous-fields (in -Wdefault), -Werror=ambiguous-fields]
+    The field ‘foo’ belonging to type S is ambiguous.
+    This will not be supported by -XDuplicateRecordFields in future releases of GHC.
+    You can use a qualified import or explicit case analysis to resolve the ambiguity.
diff --git a/testsuite/tests/overloadedrecflds/should_fail/overloadedrecfldsfail12.hs b/testsuite/tests/overloadedrecflds/should_fail/overloadedrecfldsfail12.hs
index 56092b6ce0..270c43e5d4 100644
--- a/testsuite/tests/overloadedrecflds/should_fail/overloadedrecfldsfail12.hs
+++ b/testsuite/tests/overloadedrecflds/should_fail/overloadedrecfldsfail12.hs
@@ -12,4 +12,7 @@ f e = e { foo = 3, bar = 3 }
 s :: T -> Int
 s = foo
 
+t :: S -> Bool
+t = foo
+
 main = return ()
diff --git a/testsuite/tests/overloadedrecflds/should_fail/overloadedrecfldsfail12.stderr b/testsuite/tests/overloadedrecflds/should_fail/overloadedrecfldsfail12.stderr
index e17c9f8573..fe8ac81ef9 100644
--- a/testsuite/tests/overloadedrecflds/should_fail/overloadedrecfldsfail12.stderr
+++ b/testsuite/tests/overloadedrecflds/should_fail/overloadedrecfldsfail12.stderr
@@ -12,3 +12,13 @@ overloadedrecfldsfail12.hs:10:20: error: [-Wdeprecations (in -Wdefault), -Werror
 overloadedrecfldsfail12.hs:13:5: error: [-Wdeprecations (in -Wdefault), -Werror=deprecations]
     In the use of ‘foo’ (imported from OverloadedRecFldsFail12_A):
     "Deprecated foo"
+
+overloadedrecfldsfail12.hs:13:5: error: [-Wambiguous-fields (in -Wdefault), -Werror=ambiguous-fields]
+    The field ‘foo’ belonging to type T is ambiguous.
+    This will not be supported by -XDuplicateRecordFields in future releases of GHC.
+    You can use a qualified import or explicit case analysis to resolve the ambiguity.
+
+overloadedrecfldsfail12.hs:16:5: error: [-Wambiguous-fields (in -Wdefault), -Werror=ambiguous-fields]
+    The field ‘foo’ belonging to type S is ambiguous.
+    This will not be supported by -XDuplicateRecordFields in future releases of GHC.
+    You can use explicit case analysis to resolve the ambiguity.
diff --git a/testsuite/tests/rename/should_compile/T11167_ambig.stderr b/testsuite/tests/rename/should_compile/T11167_ambig.stderr
new file mode 100644
index 0000000000..5320b42149
--- /dev/null
+++ b/testsuite/tests/rename/should_compile/T11167_ambig.stderr
@@ -0,0 +1,11 @@
+
+T11167_ambig.hs:10:13: warning: [-Wambiguous-fields (in -Wdefault)]
+    The field ‘runContT’ belonging to type ContT r m a is ambiguous.
+    This will not be supported by -XDuplicateRecordFields in future releases of GHC.
+    You can use explicit case analysis to resolve the ambiguity.
+
+T11167_ambig.hs:17:9: warning: [-Wambiguous-fields (in -Wdefault)]
+    The field ‘runContT’ belonging to type forall a.
+                                           ContT () IO a is ambiguous.
+    This will not be supported by -XDuplicateRecordFields in future releases of GHC.
+    You can use explicit case analysis to resolve the ambiguity.