[clang] Rename missing-declarations to useless-declarations

[abrachet]

Summary

Clang's missing-declarations is not the same as gcc's. TLDR: clang has missing-prototypes which works similarly to gcc's missing-declarations for C++ and missing-prototypes for C. A future patch will address this.

The warnings that missing-declarations currently control are either not present on gcc, or are always errors. Changing the name of this flag should not impact many people as it was only being used to suppress the default with no-missing-declarations. The warnings it suppresses are at best useless code or at worst incorrect but clang was more permissive than gcc in those areas.

[aaron.ballman]

Thank you for this, but I'd like to get a better understanding of the long-term goals here (and where we diverge from GCC in terms of some code examples). Also, this should probably be mentioned in the release notes somewhere so that people know what to expect about the changes.

(Adding a few more reviewers for visibility since this is touching an existing diagnostic group. Also, subscribing the mailing lists.)

clang/include/clang/Basic/DiagnosticGroups.td

 def ForLoopAnalysis : DiagGroup<"for-loop-analysis">;
 def LoopAnalysis : DiagGroup<"loop-analysis", [ForLoopAnalysis,
                                                RangeLoopAnalysis]>;
 def MalformedWarningCheck : DiagGroup<"malformed-warning-check">;
 def Main : DiagGroup<"main">;
 def MainReturnType : DiagGroup<"main-return-type">;
 def MaxUnsignedZero : DiagGroup<"max-unsigned-zero">;
 def MissingBraces : DiagGroup<"missing-braces">;
-def MissingDeclarations: DiagGroup<"missing-declarations">;
+def UselessDeclarations: DiagGroup<"useless-declarations">;
 def : DiagGroup<"missing-format-attribute">;
 def : DiagGroup<"missing-include-dirs">;
 def MissingNoreturn : DiagGroup<"missing-noreturn">;
 def MultiChar : DiagGroup<"multichar">;
 def : DiagGroup<"nested-externs">;
 def CXX11LongLong : DiagGroup<"c++11-long-long">;
 def LongLong : DiagGroup<"long-long", [CXX11LongLong]>;
 def ImplicitlyUnsignedLiteral : DiagGroup<"implicitly-unsigned-literal">;

clang/include/clang/Basic/DiagnosticSemaKinds.td

def err_main_global_variable :
     Error<"main cannot be declared as global variable">;
 def warn_main_redefined : Warning<"variable named 'main' with external linkage "
     "has undefined behavior">, InGroup<Main>;
 def ext_main_used : Extension<
   "ISO C++ does not allow 'main' to be used by a program">, InGroup<Main>;
 
 /// parser diagnostics
 def ext_no_declarators : ExtWarn<"declaration does not declare anything">,
-  InGroup<MissingDeclarations>;
+  InGroup<UselessDeclarations>;
 def err_no_declarators : Error<"declaration does not declare anything">;
 def ext_typedef_without_a_name : ExtWarn<"typedef requires a name">,
-  InGroup<MissingDeclarations>;
+  InGroup<UselessDeclarations>;
 def err_typedef_not_identifier : Error<"typedef name must be an identifier">;
 
 def ext_non_c_like_anon_struct_in_typedef : ExtWarn<
   "anonymous non-C-compatible type given name for linkage purposes "
   "by %select{typedef|alias}0 declaration; "
   "add a tag name here">, InGroup<DiagGroup<"non-c-typedef-for-linkage">>;
 def err_non_c_like_anon_struct_in_typedef : Error<
   "anonymous non-C-compatible type given name for linkage purposes "
def warn_deprecated_string_literal_conversion : Warning<
   InGroup<CXX11CompatDeprecatedWritableStr>;
 def ext_deprecated_string_literal_conversion : ExtWarn<
   "ISO C++11 does not allow conversion from string literal to %0">,
   InGroup<WritableStrings>, SFINAEFailure;
 def err_realimag_invalid_type : Error<"invalid type %0 to %1 operator">;
 def err_typecheck_sclass_fscope : Error<
   "illegal storage class on file-scoped variable">;
 def warn_standalone_specifier : Warning<"'%0' ignored on this declaration">,
-  InGroup<MissingDeclarations>;
+  InGroup<UselessDeclarations>;
 def ext_standalone_specifier : ExtWarn<"'%0' is not permitted on a declaration "
-  "of a type">, InGroup<MissingDeclarations>;
+  "of a type">, InGroup<UselessDeclarations>;
 def err_standalone_class_nested_name_specifier : Error<
   "forward declaration of %select{class|struct|interface|union|enum}0 cannot "
   "have a nested name specifier">;
 def err_typecheck_sclass_func : Error<"illegal storage class on function">;
 def err_static_block_func : Error<
   "function declared in block scope cannot have 'static' storage class">;
 def err_typecheck_address_of : Error<"address of %select{bit-field"
   "|vector element|property expression|register variable|matrix element}0 requested">;

clang/test/Parser/MicrosoftExtensions.c

-// RUN: %clang_cc1 -triple i386-mingw32 -fsyntax-only -Wno-missing-declarations -verify -fms-extensions  %s
+// RUN: %clang_cc1 -triple i386-mingw32 -fsyntax-only -Wno-useless-declarations -verify -fms-extensions  %s
 __stdcall int func0(void);
 int __stdcall func(void);
 typedef int (__cdecl *tptr)(void);
 void (*__fastcall fastpfunc)(void);
 extern __declspec(dllimport) void __stdcall VarR4FromDec(void);
 __declspec(deprecated) __declspec(deprecated) char * __cdecl ltoa( long _Val, char * _DstBuf, int _Radix);
 __declspec(safebuffers) __declspec(noalias) __declspec(restrict) void * __cdecl xxx(void *_Memory); /* expected-warning{{__declspec attribute 'safebuffers' is not supported}} */
 typedef __w64 unsigned long ULONG_PTR, *PULONG_PTR;