c, objc: Add -Wmultiple-parameter-fwd-decl-lists

Warn about this:

	void f(int x; int x; int x);

Add a new diagnostic, -Wmultiple-parameter-fwd-decl-lists, which
diagnoses uses of this obsolescent syntax.

Add this diagnostic in -Wextra.

Forward declarations of parameters are very rarely used.  And functions
that need two forward declaractions of parameters are also quite rare.
This combination results in this code almost not existing in any code
base, which makes adding this to -Wextra okay.  FWIW, I've tried finding
such code using a code search engine, and didn't find any cases (but the
regex for that isn't easy to writei, so I wouldn't trust it).

gcc/c-family/ChangeLog:

	* c.opt: Add -Wmultiple-parameter-fwd-decl-lists

gcc/c/ChangeLog:

	* c-decl.cc (c_scope): Rename {warned > had}_forward_parm_decls.
	(mark_forward_parm_decls): Add
	-Wmultiple-parameter-fwd-decl-lists.

gcc/ChangeLog:

	* doc/extend.texi: Clarify documentation about lists of
	parameter forward declarations, and mention that more than one
	of them are unnecessary.
	* doc/invoke.texi: Document the new
	-Wmultiple-parameter-fwd-decl-lists.

gcc/testsuite/ChangeLog:

	* gcc.dg/Wmultiple-parameter-fwd-decl-lists.c: New test.

Signed-off-by: Alejandro Colomar <alx@kernel.org>

diff --git a/gcc/c-family/c.opt b/gcc/c-family/c.opt
index 3f5e2f0874d94f6610d7a7717ae43b731fbc4469..4fd8770b65c3867573a9b2703a1542ffb881077f 100644 (file)
--- a/gcc/c-family/c.opt
+++ b/gcc/c-family/c.opt
@@ -1086,6 +1086,10 @@ Wmultiple-inheritance
 C++ ObjC++ Var(warn_multiple_inheritance) Warning
 Warn on direct multiple inheritance.
 
+Wmultiple-parameter-fwd-decl-lists
+C ObjC Var(warn_multiple_parameter_fwd_decl_lists) Warning EnabledBy(Wextra)
+Warn for multiple lists of forward declarations of function parameters.
+
 Wmultistatement-macros
 C ObjC C++ ObjC++ Var(warn_multistatement_macros) Warning LangEnabledBy(C ObjC C++ ObjC++,Wall)
 Warn about unsafe macros expanding to multiple statements used as a body of a clause such as if, else, while, switch, or for.

diff --git a/gcc/c/c-decl.cc b/gcc/c/c-decl.cc
index b122e82bfc47d665489a7b7e0355623e565872f9..7e4c7c2ac3f61358780bb8e9bd294c4e9441843e 100644 (file)
--- a/gcc/c/c-decl.cc
+++ b/gcc/c/c-decl.cc
@@ -504,10 +504,8 @@ struct GTY((chain_next ("%h.outer"))) c_scope {
      if these appears in a function definition.  */
   BOOL_BITFIELD had_vla_unspec : 1;
 
-  /* True if we already complained about forward parameter decls
-     in this scope.  This prevents double warnings on
-     foo (int a; int b; ...)  */
-  BOOL_BITFIELD warned_forward_parm_decls : 1;
+  /* True if we parsed a list of forward parameter decls in this scope.  */
+  BOOL_BITFIELD had_forward_parm_decls : 1;
 
   /* True if this is the outermost block scope of a function body.
      This scope contains the parameters, the local variables declared
@@ -6269,12 +6267,14 @@ mark_forward_parm_decls (void)
 {
   struct c_binding *b;
 
-  if (pedantic && !current_scope->warned_forward_parm_decls)
-    {
-      pedwarn (input_location, OPT_Wpedantic,
-              "ISO C forbids forward parameter declarations");
-      current_scope->warned_forward_parm_decls = true;
-    }
+  if (current_scope->had_forward_parm_decls)
+    warning_at (input_location, OPT_Wmultiple_parameter_fwd_decl_lists,
+               "more than one list of forward declarations of parameters");
+  if (pedantic && !current_scope->had_forward_parm_decls)
+    pedwarn (input_location, OPT_Wpedantic,
+            "ISO C forbids forward parameter declarations");
+
+  current_scope->had_forward_parm_decls = true;
 
   for (b = current_scope->bindings; b; b = b->prev)
     if (TREE_CODE (b->decl) == PARM_DECL)

diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
index 7f01f2cb7b884577d8ece81893283f1c7f7fa025..384211f8b6dc9c352d0c6229c9d0123292dfc900 100644 (file)
--- a/gcc/doc/extend.texi
+++ b/gcc/doc/extend.texi
@@ -670,16 +670,21 @@ tester (int len; char data[len][len], int len)
 @end smallexample
 
 @cindex parameter forward declaration
-The @samp{int len} before the semicolon is a @dfn{parameter forward
-declaration}, and it serves the purpose of making the name @code{len}
-known when the declaration of @code{data} is parsed.
-
-You can write any number of such parameter forward declarations in the
-parameter list.  They can be separated by commas or semicolons, but the
-last one must end with a semicolon, which is followed by the ``real''
-parameter declarations.  Each forward declaration must match a ``real''
-declaration in parameter name and data type.  ISO C99 does not support
-parameter forward declarations.
+The @samp{int len} before the semicolon
+is a @dfn{parameter forward declaration},
+and it serves the purpose of making the name @code{len} known
+when the declaration of @code{data} is parsed.
+
+Lists of parameter forward declarations are terminated by semicolons,
+and parameter forward declarations are separated within such lists by commas,
+just like in the regular list of parameter declarations.
+
+You can write any number of lists of parameter forward declaration,
+but using more than one is unnecessary.
+The last semicolon is followed by the list of parameter declarations.
+Each parameter forward declaration must match
+a parameter declaration in parameter name and data type.
+ISO C99 does not support parameter forward declarations.
 
 @node Zero Length
 @subsection Arrays of Length Zero

diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index cea83f62c5f037da361ec8d841ecb7c9337165f9..99607a09b89c8b09792c1003ec4e851b3ef88c5f 100644 (file)
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -540,7 +540,9 @@ Objective-C and Objective-C++ Dialects}.
 @gccoptlist{-Wbad-function-cast -Wdeprecated-non-prototype -Wfree-labels
 -Wmissing-declarations -Wmissing-parameter-name -Wmissing-parameter-type
 -Wdeclaration-missing-parameter-type -Wmissing-prototypes
--Wmissing-variable-declarations -Wnested-externs -Wold-style-declaration
+-Wmissing-variable-declarations
+-Wmultiple-parameter-fwd-decl-lists
+-Wnested-externs -Wold-style-declaration
 -Wold-style-definition -Wstrict-prototypes -Wtraditional
 -Wtraditional-conversion -Wdeclaration-after-statement -Wpointer-sign}
 
@@ -6683,6 +6685,7 @@ name is still supported, but the newer name is more descriptive.)
 -Wmissing-parameter-name @r{(C/ObjC only)}
 -Wmissing-parameter-type @r{(C/ObjC only)}
 -Wold-style-declaration @r{(C/ObjC only)}
+-Wmultiple-parameter-fwd-decl-lists @r{(C/ObjC only)}
 -Woverride-init @r{(C/ObjC only)}
 -Wredundant-move @r{(C++ and Objective-C++ only)}
 -Wshift-negative-value @r{(in C++11 to C++17 and in C99 and newer)}
@@ -10536,6 +10539,13 @@ is not considered an old-style definition in C23 mode, because it is
 equivalent to @samp{(void)} in that case, but is considered an
 old-style definition for older standards.
 
+@opindex Wmultiple-parameter-fwd-decl-lists
+@opindex Wno-multiple-parameter-fwd-decl-lists
+@item -Wmultiple-parameter-fwd-decl-lists @r{(C and Objective-C only)}
+Warn if more than one list of forward declarations of parameters
+appears in a function prototype.
+This warning is also enabled by @option{-Wextra}.
+
 @opindex Wdeprecated-non-prototype
 @opindex Wno-deprecated-non-prototype
 @item -Wdeprecated-non-prototype @r{(C and Objective-C only)}

diff --git a/gcc/testsuite/gcc.dg/Wmultiple-parameter-fwd-decl-lists.c b/gcc/testsuite/gcc.dg/Wmultiple-parameter-fwd-decl-lists.c
new file mode 100644 (file)
index 0000000..c3edbf6
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/Wmultiple-parameter-fwd-decl-lists.c
@@ -0,0 +1,6 @@
+/* { dg-do compile } */
+/* { dg-options "-Wmultiple-parameter-fwd-decl-lists" } */
+
+void f(int n, int m; int n, int m);
+void g(int n; int m; int n, int m); /* { dg-warning "more than one list of forward declarations" } */
+void h(int n; int n; int n); /* { dg-warning "more than one list of forward declarations" } */