[clang] support Wold-style-declaration #78837

SihangZhu · 2024-01-20T09:25:17Z

Storage-class specifiers like static are not the first things in a declaration. According to the C Standard, this usage is obsolescent. This patch add a diagnose to warn it.
This is a counterpart of gcc's Wold-style-declaration.

llvmbot · 2024-01-20T09:25:47Z

@llvm/pr-subscribers-clang

Author: None (SihangZhu)

Changes

Storage-class specifiers like static are not the first things in a declaration. According to the C Standard, this usage is obsolescent. This patch add a diagnose to warn it.
This is a counterpart of gcc's Wold-style-declaration.

Full diff: https://github.com/llvm/llvm-project/pull/78837.diff

4 Files Affected:

(modified) clang/include/clang/Basic/DiagnosticCommonKinds.td (+2)
(modified) clang/include/clang/Basic/DiagnosticGroups.td (+2)
(modified) clang/lib/Parse/ParseDecl.cpp (+11)
(added) clang/test/Parser/old-style-declaration.c (+33)

diff --git a/clang/include/clang/Basic/DiagnosticCommonKinds.td b/clang/include/clang/Basic/DiagnosticCommonKinds.td
index 5544dc88004d9a..0d29316767e950 100644
--- a/clang/include/clang/Basic/DiagnosticCommonKinds.td
+++ b/clang/include/clang/Basic/DiagnosticCommonKinds.td
@@ -84,6 +84,8 @@ def err_param_redefinition : Error<"redefinition of parameter %0">;
 def warn_method_param_redefinition : Warning<"redefinition of method parameter %0">;
 def warn_method_param_declaration : Warning<"redeclaration of method parameter %0">,
   InGroup<DuplicateArgDecl>, DefaultIgnore;
+def warn_old_style_declaration: Warning <"'%0' is not at beginning of declaration">,
+  InGroup<OldStyleDeclaration>, DefaultIgnore;
 def err_invalid_storage_class_in_func_decl : Error<
   "invalid storage class specifier in function declarator">;
 def err_expected_namespace_name : Error<"expected namespace name">;
diff --git a/clang/include/clang/Basic/DiagnosticGroups.td b/clang/include/clang/Basic/DiagnosticGroups.td
index 6765721ae7002c..ebe751a9947971 100644
--- a/clang/include/clang/Basic/DiagnosticGroups.td
+++ b/clang/include/clang/Basic/DiagnosticGroups.td
@@ -578,6 +578,7 @@ def ObjCPointerIntrospect : DiagGroup<"deprecated-objc-pointer-introspection", [
 def ObjCMultipleMethodNames : DiagGroup<"objc-multiple-method-names">;
 def ObjCFlexibleArray : DiagGroup<"objc-flexible-array">;
 def ObjCBoxing : DiagGroup<"objc-boxing">;
+def OldStyleDeclaration : DiagGroup<"old-style-declaration">;
 def CompletionHandler : DiagGroup<"completion-handler">;
 def CalledOnceParameter : DiagGroup<"called-once-parameter", [CompletionHandler]>;
 def OpenCLUnsupportedRGBA: DiagGroup<"opencl-unsupported-rgba">;
@@ -1026,6 +1027,7 @@ def Extra : DiagGroup<"extra", [
     EmptyInitStatement,
     StringConcatation,
     FUseLdPath,
+    OldStyleDeclaration,
   ]>;
 
 def Most : DiagGroup<"most", [
diff --git a/clang/lib/Parse/ParseDecl.cpp b/clang/lib/Parse/ParseDecl.cpp
index 356e7851ec639c..ab1555ebc19088 100644
--- a/clang/lib/Parse/ParseDecl.cpp
+++ b/clang/lib/Parse/ParseDecl.cpp
@@ -3348,6 +3348,7 @@ void Parser::ParseDeclarationSpecifiers(
   while (true) {
     bool isInvalid = false;
     bool isStorageClass = false;
+    bool isFunctionSpecifier = false;
     const char *PrevSpec = nullptr;
     unsigned DiagID = 0;
 
@@ -4092,6 +4093,7 @@ void Parser::ParseDeclarationSpecifiers(
     // function-specifier
     case tok::kw_inline:
       isInvalid = DS.setFunctionSpecInline(Loc, PrevSpec, DiagID);
+      isFunctionSpecifier = true;
       break;
     case tok::kw_virtual:
       // C++ for OpenCL does not allow virtual function qualifier, to avoid
@@ -4104,6 +4106,7 @@ void Parser::ParseDeclarationSpecifiers(
         isInvalid = true;
       } else {
         isInvalid = DS.setFunctionSpecVirtual(Loc, PrevSpec, DiagID);
+        isFunctionSpecifier = true;
       }
       break;
     case tok::kw_explicit: {
@@ -4140,12 +4143,14 @@ void Parser::ParseDeclarationSpecifiers(
       }
       isInvalid = DS.setFunctionSpecExplicit(ExplicitLoc, PrevSpec, DiagID,
                                              ExplicitSpec, CloseParenLoc);
+      isFunctionSpecifier = true;
       break;
     }
     case tok::kw__Noreturn:
       if (!getLangOpts().C11)
         Diag(Tok, diag::ext_c11_feature) << Tok.getName();
       isInvalid = DS.setFunctionSpecNoreturn(Loc, PrevSpec, DiagID);
+      isFunctionSpecifier = true;
       break;
 
     // alignment-specifier
@@ -4552,6 +4557,12 @@ void Parser::ParseDeclarationSpecifiers(
       continue;
     }
 
+    unsigned Specs = DS.getParsedSpecifiers();
+    if (!getLangOpts().CPlusPlus && (isFunctionSpecifier || isStorageClass)) {
+      if (Specs & DeclSpec::PQ_TypeQualifier || DS.hasTypeSpecifier())
+        Diag(Tok, diag::warn_old_style_declaration) << Tok.getName();
+    }
+
     DS.SetRangeEnd(ConsumedEnd.isValid() ? ConsumedEnd : Tok.getLocation());
 
     // If the specifier wasn't legal, issue a diagnostic.
diff --git a/clang/test/Parser/old-style-declaration.c b/clang/test/Parser/old-style-declaration.c
new file mode 100644
index 00000000000000..e7ccea32bfa957
--- /dev/null
+++ b/clang/test/Parser/old-style-declaration.c
@@ -0,0 +1,33 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -Wold-style-declaration %s
+// RUN: %clang_cc1 -fsyntax-only -verify -Wextra %s
+
+static int x0;
+int __attribute__ ((aligned (16))) static x1; // expected-warning {{'static' is not at beginning of declaration}}
+
+extern int x2;
+int extern x3; // expected-warning {{'extern' is not at beginning of declaration}}
+
+typedef int x4;
+int typedef x5; // expected-warning {{'typedef' is not at beginning of declaration}}
+
+void g (int);
+
+void
+f (void)
+{
+  auto int x6 = 0;
+  int auto x7 = 0; // expected-warning {{'auto' is not at beginning of declaration}}
+  register int x8 = 0;
+  int register x9 = 0; // expected-warning {{'register' is not at beginning of declaration}}
+  g (x6 + x7 + x8 + x9);
+}
+
+const static int x10; // expected-warning {{'static' is not at beginning of declaration}}
+
+/* Attributes are OK before storage class specifiers, since some
+   attributes are like such specifiers themselves.  */
+
+__attribute__((format(printf, 1, 2))) static void h (const char *, ...);
+__attribute__((format(printf, 1, 2))) void static i (const char *, ...); // expected-warning {{'static' is not at beginning of declaration}}
+
+static __thread int var = 5; // not-expected-warning {{'__thread' is not at beginning of declaration}}
\ No newline at end of file

SihangZhu · 2024-01-24T07:31:20Z

ping

SihangZhu · 2024-02-19T11:14:12Z

ping

repeat:)

AaronBallman

Thank you for working on this! Please be sure to add release notes to clang/docs/ReleaseNotes.rst so users know about the new diagnostic capabilities.

AaronBallman · 2024-02-20T19:46:49Z

clang/include/clang/Basic/DiagnosticCommonKinds.td

+def warn_old_style_declaration: Warning <"'%0' is not at beginning of declaration">,
+  InGroup<OldStyleDeclaration>, DefaultIgnore;


We don't typically add new diagnostics that are off by default (though this one is in -Wextra so it has a chance of being enabled by users). I actually wonder if we want to enable this by default; the standard makes this an obsolescent feature (see C23 6.11.5p1) and so having an on-by-default warning actually helps alert users to potential future code breakage.

My intuition is that this would not be an overly chatty diagnostic to enable by default; do you have access to a large corpus of C code you could try to compile with your patch?

AaronBallman · 2024-02-20T19:49:56Z

clang/lib/Parse/ParseDecl.cpp

@@ -4104,6 +4106,7 @@ void Parser::ParseDeclarationSpecifiers(
        isInvalid = true;
      } else {
        isInvalid = DS.setFunctionSpecVirtual(Loc, PrevSpec, DiagID);
+        isFunctionSpecifier = true;


C doesn't have virtual functions, so no need for this change.

AaronBallman · 2024-02-20T19:50:14Z

clang/lib/Parse/ParseDecl.cpp

@@ -4140,12 +4143,14 @@ void Parser::ParseDeclarationSpecifiers(
      }
      isInvalid = DS.setFunctionSpecExplicit(ExplicitLoc, PrevSpec, DiagID,
                                             ExplicitSpec, CloseParenLoc);
+      isFunctionSpecifier = true;


C also doesn't have explicit.

AaronBallman · 2024-02-20T19:55:47Z

clang/lib/Parse/ParseDecl.cpp

@@ -3348,6 +3348,7 @@ void Parser::ParseDeclarationSpecifiers(
  while (true) {
    bool isInvalid = false;
    bool isStorageClass = false;
+    bool isFunctionSpecifier = false;


Hmmm, what's obsoleted are storage class specifiers that are not at the start of the declaration. e.g.,

int static i = 12;

and function specifiers are not storage class specifiers. So technically, this is not obsolete:

void _Noreturn func(void);

GCC does warn about this, and I think that's reasonable to also mimic. I'll check with WG14 whether function specifiers should also be obsoleted similar to storage class specifiers.

AaronBallman · 2024-02-20T19:57:16Z

clang/test/Parser/old-style-declaration.c

+  auto int x6 = 0;
+  int auto x7 = 0; // expected-warning {{'auto' is not at beginning of declaration}}


This is going to cause problems when we update to -std=c23, so it might be good to guard this on __STDC_VERSION__ to save ourselves some headaches later.

AaronBallman · 2024-02-20T20:00:27Z

clang/test/Parser/old-style-declaration.c

+__attribute__((format(printf, 1, 2))) static void h (const char *, ...);
+__attribute__((format(printf, 1, 2))) void static i (const char *, ...); // expected-warning {{'static' is not at beginning of declaration}}
+
+static __thread int var = 5; // not-expected-warning {{'__thread' is not at beginning of declaration}}


You should add tests for: _Noreturn, inline, _Thread_local, constexpr (C23), auto (C23 usage, not C89 usage).

I think it also makes sense to add a test for alignas but I think we should allow that in any position under the same logic for allowing attributes.

Also, please add a newline to the end of the file.

AaronBallman · 2024-02-20T20:02:49Z

clang/test/Parser/old-style-declaration.c

+// RUN: %clang_cc1 -fsyntax-only -verify -Wextra %s
+


It would be good to add a RUN line along these lines:

Suggested change

// RUN: %clang_cc1 -fsyntax-only -verify -Wextra %s

// RUN: %clang_cc1 -fsyntax-only -verify -Wextra %s

// RUN: %clang_cc1 -fsyntax-only -verify=cpp -Wextra -x c++ %s

// cpp-no-diagnostics

and group the code that's invalid in C++ under #ifndef __cplusplus, to show that the diagnostics are not generated in C++, only in C.

[clang] support Wold-style-declaration as gcc

773b14c

llvmbot added clang Clang issues not falling into any other category clang:frontend Language frontend issues, e.g. anything involving "Sema" labels Jan 20, 2024

hstk30-hw requested review from sam-mccall, aaronpuchert, cor3ntin and AaronBallman and removed request for sam-mccall January 20, 2024 09:27

SihangZhu changed the title ~~[clang] support Wold-style-declaration as gcc~~ [clang] support Wold-style-declaration Jan 20, 2024

Merge branch 'main' into old-style-declaration

c1f6e0c

thesamesam added the clang:diagnostics New/improved warning or error message in Clang, but not in clang-tidy or static analyzer label Jan 24, 2024

AaronBallman reviewed Feb 20, 2024

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AaronBallman added the c label Feb 20, 2024

AaronBallman reviewed Feb 20, 2024

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		def warn_old_style_declaration: Warning <"'%0' is not at beginning of declaration">,
		InGroup<OldStyleDeclaration>, DefaultIgnore;

		auto int x6 = 0;
		int auto x7 = 0; // expected-warning {{'auto' is not at beginning of declaration}}

[clang] support Wold-style-declaration #78837

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