How can I get rid of deprecated warnings in deprecated functions in GCC?

One way to implement deprecation warnings is to produce warnings on calls to deprecated functions, unless you are calling from a deprecated context. This way legacy code can call legacy code without producing warnings that only amount to noise.

This is a reasonable line of thinking, and it is reflected in the implementations I see in GCC 4.2 (1) and Clang 4.0 (2) on OS X as well as Clang 3.0 (3) on Ubuntu.

(1): i686-apple-darwin11-llvm-g++-4.2 (GCC) 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2336.11.00)
(2): Apple clang version 4.0 (tags/Apple/clang-421.0.57) (based on LLVM 3.1svn)
(3): Ubuntu clang version 3.0-6ubuntu3 (tags/RELEASE_30/final) (based on LLVM 3.0)

However, when I compile with GCC 4.6 (4) on Ubuntu, I get deprecated warnings for all invocations of deprecated functions, independently of context. Is this a regression in functionality? Are there compiler options I can use to get the other behavior?

(4): g++ (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3

Example program:

int __attribute__((deprecated)) a() {
    return 10;
}

int __attribute__((deprecated)) b() {
    return a() * 2; //< I want to get rid of warnings from this line
}

int main() {
    return b(); //< I expect a warning on this line only
}

Output from GCC 4.2 (Yes, I do get the same warning twice. I don't care about that, though):

main.cpp: In function ‘int main()’:
main.cpp:10: warning: ‘b’ is deprecated (declared at main.cpp:5)
main.cpp:10: warning: ‘b’ is deprecated (declared at main.cpp:5)

Output from GCC 4.6:

main.cpp: In function 'int b()':
main.cpp:6:9: warning: 'int a()' is deprecated (declared at main.cpp:1) [-Wdeprecated-declarations]
main.cpp:6:11: warning: 'int a()' is deprecated (declared at main.cpp:1) [-Wdeprecated-declarations]
main.cpp: In function 'int main()':
main.cpp:10:9: warning: 'int b()' is deprecated (declared at main.cpp:5) [-Wdeprecated-declarations]
main.cpp:10:11: warning: 'int b()' is deprecated (declared at main.cpp:5) [-Wdeprecated-declarations]

How can I convince GCC 4.6 that it should give me the same output as GCC 4.2?

Solution

The behaviour you're seeing in GCC 4.2 is caused by an Apple-specific patch to GCC. FSF GCC 4.2.4 warns about the use of a. The specific bit that Apple GCC has that FSF GCC doesn't is:

--- a/gcc/toplev.c
+++ b/gcc/toplev.c
@@ -902,6 +902,9 @@ warn_deprecated_use (tree node)
   if (node == 0 || !warn_deprecated_decl)
     return;

+  if (current_function_decl && TREE_DEPRECATED (current_function_decl))
+    return;
+
   if (DECL_P (node))
     {
       expanded_location xloc = expand_location (DECL_SOURCE_LOCATION (node));

(Available under GPLv2 or later)

You may wish to adapt this patch to a later version of GCC (perhaps no changes are needed, perhaps major changes are needed), and build GCC from source with this patch applied. Or you may report this as a feature request on FSF GCC bugzilla.