c-family: Enable -fpermissive for C and ObjC

Future changes will treat some C front end warnings similar to
-Wnarrowing.

gcc/

	* doc/invoke.texi (Warning Options): Mention C diagnostics
	for -fpermissive.

gcc/c-family/

	* c.opt (fpermissive): Enable for C and ObjC.
	* c-opts.cc (c_common_post_options): Enable -fpermissive.

3 files changed, 21 insertions, 3 deletions

diff --git a/gcc/c-family/c-opts.cc b/gcc/c-family/c-opts.cc
index d30d413..fbabd18 100644
--- a/gcc/c-family/c-opts.cc
+++ b/gcc/c-family/c-opts.cc
@@ -854,6 +854,20 @@ c_common_post_options (const char **pfilename)
       && flag_unsafe_math_optimizations == 0)
     flag_fp_contract_mode = FP_CONTRACT_OFF;
 
+  /* C language modes before C99 enable -fpermissive by default, but
+     only if -pedantic-errors is not specified.  Also treat
+     -fno-permissive as a subset of -pedantic-errors that does not
+     reject certain GNU extensions also present the defaults for later
+     language modes.  */
+  if (!c_dialect_cxx ()
+      && !flag_isoc99
+      && !global_dc->m_pedantic_errors
+      && !OPTION_SET_P (flag_permissive))
+    {
+      flag_permissive = 1;
+      global_dc->m_permissive = 1;
+    }
+
   /* If we are compiling C, and we are outside of a standards mode,
      we can permit the new values from ISO/IEC TS 18661-3 for
      FLT_EVAL_METHOD.  Otherwise, we must restrict the possible values to
diff --git a/gcc/c-family/c.opt b/gcc/c-family/c.opt
index 26f629fb..d81177f 100644
--- a/gcc/c-family/c.opt
+++ b/gcc/c-family/c.opt
@@ -2116,7 +2116,7 @@ C ObjC C++ ObjC++
 Look for and use PCH files even when preprocessing.
 
 fpermissive
-C++ ObjC++ Var(flag_permissive)
+C ObjC C++ ObjC++ Var(flag_permissive)
 Downgrade conformance errors to warnings.
 
 fplan9-extensions
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 2caa7ce..d0b55fb 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -6174,13 +6174,17 @@ errors by @option{-pedantic-errors}.  For instance:
 Downgrade some required diagnostics about nonconformant code from
 errors to warnings.  Thus, using @option{-fpermissive} allows some
 nonconforming code to compile.  Some C++ diagnostics are controlled
-only by this flag, but it also downgrades some diagnostics that have
-their own flag:
+only by this flag, but it also downgrades some C and C++ diagnostics
+that have their own flag:
 
 @gccoptlist{
 -Wnarrowing @r{(C++)}
 }
 
+The @option{-fpermissive} option is the default for historic C language
+modes (@option{-std=c89}, @option{-std=gnu89}, @option{-std=c90},
+@option{-std=gnu90}).
+
 @opindex Wall
 @opindex Wno-all
 @item -Wall