Tweak implementation of -Wwrite-strings to better match the behavior of current GCCs:

 * In C, as before, if the "warning flag" is enabled, warnings are produced by
   forcing string literals to have const-qualified types (the produced warnings
   are *not* -Wwrite-strings warnings). However, more recent GCCs (at least 4.4
   onwards) now take -w into account here, so we now do the same.

 * In C++, this flag is entirely sane: it behaves just like any other warning
   flag. Stop triggering -fconst-strings here. This is a bit cleaner, but there's
   no real functionality change except in the case where -Xclang -fno-const-strings
   is also specified.

llvm-svn: 190006

1 files changed, 18 insertions, 7 deletions

diff --git a/clang/lib/Driver/Tools.cpp b/clang/lib/Driver/Tools.cpp
index 524247fe341..dd72fd748ee 100644
--- a/clang/lib/Driver/Tools.cpp
+++ b/clang/lib/Driver/Tools.cpp
@@ -2677,13 +2677,24 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
     Args.AddLastArg(CmdArgs, options::OPT_trigraphs);
   }
 
-  // Map the bizarre '-Wwrite-strings' flag to a more sensible
-  // '-fconst-strings'; this better indicates its actual behavior.
-  if (Args.hasFlag(options::OPT_Wwrite_strings, options::OPT_Wno_write_strings,
-                   false)) {
-    // For perfect compatibility with GCC, we do this even in the presence of
-    // '-w'. This flag names something other than a warning for GCC.
-    CmdArgs.push_back("-fconst-strings");
+  // GCC's behavior for -Wwrite-strings is a bit strange:
+  //  * In C, this "warning flag" changes the types of string literals from
+  //    'char[N]' to 'const char[N]', and thus triggers an unrelated warning
+  //    for the discarded qualifier.
+  //  * In C++, this is just a normal warning flag.
+  //
+  // Implementing this warning correctly in C is hard, so we follow GCC's
+  // behavior for now. FIXME: Directly diagnose uses of a string literal as
+  // a non-const char* in C, rather than using this crude hack.
+  if (!types::isCXX(InputType)) {
+    // FIXME: This should behave just like a warning flag, and thus should also
+    // respect -Weverything, -Wno-everything, -Werror=write-strings, and so on.
+    Arg *WriteStrings =
+        Args.getLastArg(options::OPT_Wwrite_strings,
+                        options::OPT_Wno_write_strings, options::OPT_w);
+    if (WriteStrings &&
+        WriteStrings->getOption().matches(options::OPT_Wwrite_strings))
+      CmdArgs.push_back("-fconst-strings");
   }
 
   // GCC provides a macro definition '__DEPRECATED' when -Wdeprecated is active