`-Wpreferred-type-bitfield-enum-conversion` reported in `include/clang/Basic/LangOptions.def` (FPEvalMethod)

[firewave]

Doing a downstream include-what-you-use build with Clang 21 against LLVM 21 shows the following warning:

In file included from /mnt/s/GitHub/include-what-you-use/iwyu_globals.cc:22:
In file included from /usr/lib/llvm-21/include/clang/AST/PrettyPrinter.h:17:
In file included from /usr/lib/llvm-21/include/clang/Basic/LangOptions.h:663:
/usr/lib/llvm-21/include/clang/Basic/LangOptions.def:350:1: warning: assigning value of preferred signed enum type 'FPEvalMethodKind' to unsigned bit-field 'FPEvalMethod'; negative enumerators of enum 'FPEvalMethodKind' will be converted to positive values [-Wpreferred-type-bitfield-enum-conversion]
  350 | BENIGN_ENUM_LANGOPT(FPEvalMethod, FPEvalMethodKind, 3, FEM_UnsetOnCommandLine, "FP type used for floating point arithmetic")
      | ^
/usr/lib/llvm-21/include/clang/Basic/LangOptions.def:67:6: note: expanded from macro 'BENIGN_ENUM_LANGOPT'
   67 |      COMPATIBLE_ENUM_LANGOPT(Name, Type, Bits, Default, Description)
      |      ^
/usr/lib/llvm-21/include/clang/Basic/LangOptions.def:62:6: note: expanded from macro 'COMPATIBLE_ENUM_LANGOPT'
   62 |      ENUM_LANGOPT(Name, Type, Bits, Default, Description)
      |      ^
/usr/lib/llvm-21/include/clang/Basic/LangOptions.h:661:10: note: expanded from macro 'ENUM_LANGOPT'
  661 |     Name = static_cast<unsigned>(Value);                                       \
      |          ^
/usr/lib/llvm-21/include/clang/Basic/LangOptions.def:350:1: note: consider making the bit-field type signed
  350 | BENIGN_ENUM_LANGOPT(FPEvalMethod, FPEvalMethodKind, 3, FEM_UnsetOnCommandLine, "FP type used for floating point arithmetic")
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/lib/llvm-21/include/clang/Basic/LangOptions.def:67:6: note: expanded from macro 'BENIGN_ENUM_LANGOPT'
   67 |      COMPATIBLE_ENUM_LANGOPT(Name, Type, Bits, Default, Description)
      |      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/lib/llvm-21/include/clang/Basic/LangOptions.def:62:6: note: expanded from macro 'COMPATIBLE_ENUM_LANGOPT'
   62 |      ENUM_LANGOPT(Name, Type, Bits, Default, Description)
      |      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/lib/llvm-21/include/clang/Basic/LangOptions.h:501:3: note: expanded from macro 'ENUM_LANGOPT'
  501 |   unsigned Name : Bits;
      |   ^~~~~~~~
/usr/lib/llvm-21/include/clang/Basic/LangOptions.def:350:1: note: preferred type for bit-field 'FPEvalMethodKind' specified here
/usr/lib/llvm-21/include/clang/Basic/LangOptions.def:67:6: note: expanded from macro 'BENIGN_ENUM_LANGOPT'
   67 |      COMPATIBLE_ENUM_LANGOPT(Name, Type, Bits, Default, Description)
      |      ^
/usr/lib/llvm-21/include/clang/Basic/LangOptions.def:62:6: note: expanded from macro 'COMPATIBLE_ENUM_LANGOPT'
   62 |      ENUM_LANGOPT(Name, Type, Bits, Default, Description)
      |      ^
/usr/lib/llvm-21/include/clang/Basic/LangOptions.h:500:3: note: expanded from macro 'ENUM_LANGOPT'
  500 |   LLVM_PREFERRED_TYPE(Type) \
      |   ^
/usr/lib/llvm-21/include/llvm/Support/Compiler.h:708:47: note: expanded from macro 'LLVM_PREFERRED_TYPE'
  708 | #define LLVM_PREFERRED_TYPE(T) __attribute__((preferred_type(T)))
      |                                               ^

[firewave]

Compiling with Clang 21 against LLVM 20 shows the following:

In file included from /mnt/s/GitHub/include-what-you-use/iwyu_globals.cc:22:
In file included from /usr/lib/llvm-20/include/clang/AST/PrettyPrinter.h:17:
In file included from /usr/lib/llvm-20/include/clang/Basic/LangOptions.h:649:
/usr/lib/llvm-20/include/clang/Basic/LangOptions.def:352:1: warning: assigning value of preferred signed enum type 'FPEvalMethodKind' to unsigned bit-field 'FPEvalMethod'; negative enumerators of enum 'FPEvalMethodKind' will be converted to positive values [-Wpreferred-type-bitfield-enum-conversion]
  352 | BENIGN_ENUM_LANGOPT(FPEvalMethod, FPEvalMethodKind, 2, FEM_UnsetOnCommandLine, "FP type used for floating point arithmetic")
      | ^
/usr/lib/llvm-20/include/clang/Basic/LangOptions.def:67:6: note: expanded from macro 'BENIGN_ENUM_LANGOPT'
   67 |      COMPATIBLE_ENUM_LANGOPT(Name, Type, Bits, Default, Description)
      |      ^
/usr/lib/llvm-20/include/clang/Basic/LangOptions.def:62:6: note: expanded from macro 'COMPATIBLE_ENUM_LANGOPT'
   62 |      ENUM_LANGOPT(Name, Type, Bits, Default, Description)
      |      ^
/usr/lib/llvm-20/include/clang/Basic/LangOptions.h:648:37: note: expanded from macro 'ENUM_LANGOPT'
  648 |   void set##Name(Type Value) { Name = static_cast<unsigned>(Value); }
      |                                     ^
/usr/lib/llvm-20/include/clang/Basic/LangOptions.def:352:1: note: consider making the bit-field type signed
  352 | BENIGN_ENUM_LANGOPT(FPEvalMethod, FPEvalMethodKind, 2, FEM_UnsetOnCommandLine, "FP type used for floating point arithmetic")
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/lib/llvm-20/include/clang/Basic/LangOptions.def:67:6: note: expanded from macro 'BENIGN_ENUM_LANGOPT'
   67 |      COMPATIBLE_ENUM_LANGOPT(Name, Type, Bits, Default, Description)
      |      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/lib/llvm-20/include/clang/Basic/LangOptions.def:62:6: note: expanded from macro 'COMPATIBLE_ENUM_LANGOPT'
   62 |      ENUM_LANGOPT(Name, Type, Bits, Default, Description)
      |      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/lib/llvm-20/include/clang/Basic/LangOptions.h:496:3: note: expanded from macro 'ENUM_LANGOPT'
  496 |   unsigned Name : Bits;
      |   ^~~~~~~~

[AaronBallman]

CC @zahiraam

[zahiraam]

Will take a look.

[nico]

In addition to the location above, it also fires here:

../../clang/include/clang/Basic/LangOptions.h:70:7: warning: assigning value of preferred signed enum type 'FPEvalMethodKind' to unsigned bit-field 'FPEvalMethod'; negative enumerators of enum 'FPEvalMethodKind' will be converted to positive values [-Wpreferred-type-bitfield-enum-conversion]
   70 | class LangOptionsBase {
      |       ^
../../clang/include/clang/Basic/LangOptions.h:507:7: note: in implicit copy constructor for 'clang::LangOptionsBase' first required here
  507 | class LangOptions : public LangOptionsBase {
      |       ^
../../clang/lib/Basic/IdentifierTable.cpp:365:31: note: in implicit copy constructor for 'clang::LangOptions' first required here
  365 |   LangOptions LangOptsNoCPP = LangOpts;
      |                               ^
../../clang/include/clang/Basic/LangOptions.def:356:1: note: consider making the bit-field type signed
  356 | BENIGN_ENUM_LANGOPT(FPEvalMethod, FPEvalMethodKind, 3, FEM_UnsetOnCommandLine, "FP type used for floating point arithmetic")
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../../clang/include/clang/Basic/LangOptions.def:67:6: note: expanded from macro 'BENIGN_ENUM_LANGOPT'
   67 |      COMPATIBLE_ENUM_LANGOPT(Name, Type, Bits, Default, Description)
      |      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../../clang/include/clang/Basic/LangOptions.def:62:6: note: expanded from macro 'COMPATIBLE_ENUM_LANGOPT'
   62 |      ENUM_LANGOPT(Name, Type, Bits, Default, Description)
      |      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../../clang/include/clang/Basic/LangOptions.h:501:3: note: expanded from macro 'ENUM_LANGOPT'
  501 |   unsigned Name : Bits;
      |   ^~~~~~~~
../../clang/include/clang/Basic/LangOptions.def:356:1: note: preferred type for bit-field 'FPEvalMethodKind' specified here
../../clang/include/clang/Basic/LangOptions.def:67:6: note: expanded from macro 'BENIGN_ENUM_LANGOPT'
   67 |      COMPATIBLE_ENUM_LANGOPT(Name, Type, Bits, Default, Description)
      |      ^
../../clang/include/clang/Basic/LangOptions.def:62:6: note: expanded from macro 'COMPATIBLE_ENUM_LANGOPT'
   62 |      ENUM_LANGOPT(Name, Type, Bits, Default, Description)
      |      ^
../../clang/include/clang/Basic/LangOptions.h:500:3: note: expanded from macro 'ENUM_LANGOPT'
  500 |   LLVM_PREFERRED_TYPE(Type) \
      |   ^
../../llvm/include/llvm/Support/Compiler.h:708:47: note: expanded from macro 'LLVM_PREFERRED_TYPE'
  708 | #define LLVM_PREFERRED_TYPE(T) __attribute__((preferred_type(T)))
      |                                               ^
1 warning generated.

[zahiraam]

There is a WIP fix/discussion here #137661

[nico]

Background here, as far as I understand it:

1. Long ago (1592d88, late 2021), FPEvalMethodKind with FEM_Indeterminable landed
2. [Clang] Consider preferred_type in bitfield warnings (#116760) #116785 made clang warn on this
3. Ensure bit-fields storing FPEvalMethodKind are wide enough to do so #136515 and [clang] Remove FEM_Indeterminable #137247 look like fix attempts. The latter got reverted and is now being reviewed at [clang] Remove FEM_Indeterminable #137661

Since this is a very noisy warning and actively being worked on, I tried silencing the warning until the fix is here like so:

% git diff
diff --git a/clang/include/clang/Basic/LangOptions.def b/clang/include/clang/Basic/LangOptions.def
index 85ca523c4415..1258a349ebf0 100644
--- a/clang/include/clang/Basic/LangOptions.def
+++ b/clang/include/clang/Basic/LangOptions.def
@@ -347,7 +347,17 @@ BENIGN_ENUM_LANGOPT(DefaultFPContractMode, FPModeKind, 2, FPM_Off, "FP contracti
 COMPATIBLE_LANGOPT(ExpStrictFP, 1, false, "Enable experimental strict floating point")
 BENIGN_LANGOPT(RoundingMath, 1, false, "Do not assume default floating-point rounding behavior")
 BENIGN_ENUM_LANGOPT(FPExceptionMode, FPExceptionModeKind, 2, FPE_Default, "FP Exception Behavior Mode type")
+
+#if defined(__clang__)
+// FIXME: Remove this once the warning is fixed, https://llvm.org/PR137600
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wpreferred-type-bitfield-enum-conversion"
+#endif
 BENIGN_ENUM_LANGOPT(FPEvalMethod, FPEvalMethodKind, 3, FEM_UnsetOnCommandLine, "FP type used for floating point arithmetic")
+#if defined(__clang__)
+#pragma clang diagnostic pop
+#endif
+
 ENUM_LANGOPT(Float16ExcessPrecision, ExcessPrecisionKind, 2, FPP_Standard, "Intermediate truncation behavior for Float16 arithmetic")
 ENUM_LANGOPT(BFloat16ExcessPrecision, ExcessPrecisionKind, 2, FPP_Standard, "Intermediate truncation behavior for BFloat16 arithmetic")
 LANGOPT(NoBitFieldTypeAlign , 1, 0, "bit-field type alignment")

That mostly works, but we still get the warning in #137600 (comment) since that one is emitted for class LangOptionsBase and is harder to pragma. Still, that seems better than what we currently have, so I'll land that for now.

[nico]

Once this is fixed, please also revert 01e0296.

[zahiraam]

Once this is fixed, please also revert 01e0296.

Thanks. Will do that.

[firewave]

There's also #137603 in the 20.x branch.