c++/modules: relax diagnostic about GMF contents

Issuing a hard error when the GMF doesn't consist only of preprocessing
directives happens to be inconvenient for automated testcase reduction
via cvise.  This patch relaxes this diagnostic into a pedwarn that can
be disabled with -Wno-global-module.

gcc/c-family/ChangeLog:

	* c.opt (Wglobal-module): New warning.

gcc/cp/ChangeLog:

	* parser.cc (cp_parser_translation_unit): Relax GMF contents
	error into a pedwarn.

gcc/ChangeLog:

	* doc/invoke.texi (-Wno-global-module): Document.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/friend-6_a.C: Pass -Wno-global-module instead
	of -Wno-pedantic.  Remove now unnecessary preprocessing
	directives from GMF.

Reviewed-by: Jason Merrill <jason@redhat.com>

4 files changed, 15 insertions, 7 deletions

diff --git a/gcc/c-family/c.opt b/gcc/c-family/c.opt
index b7a4a1a..56cccf2 100644
--- a/gcc/c-family/c.opt
+++ b/gcc/c-family/c.opt
@@ -802,6 +802,10 @@ Wframe-address
 C ObjC C++ ObjC++ Var(warn_frame_address) Warning LangEnabledBy(C ObjC C++ ObjC++,Wall)
 Warn when __builtin_frame_address or __builtin_return_address is used unsafely.
 
+Wglobal-module
+C++ ObjC++ Var(warn_global_module) Warning Init(1)
+Warn about the global module fragment not containing only preprocessing directives.
+
 Wif-not-aligned
 C ObjC C++ ObjC++ Var(warn_if_not_aligned) Init(1) Warning
 Warn when the field in a struct is not aligned.
diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index a310b9e..e32acfc 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -5253,9 +5253,9 @@ cp_parser_translation_unit (cp_parser* parser)
 	      if (!warned)
 		{
 		  warned = true;
-		  error_at (token->location,
-			    "global module fragment contents must be"
-			    " from preprocessor inclusion");
+		  pedwarn (token->location, OPT_Wglobal_module,
+			   "global module fragment contents must be"
+			   " from preprocessor inclusion");
 		}
 	    }
 	}
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index bdf05be..2390d47 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -256,7 +256,7 @@ in the following sections.
 -Wdeprecated-copy -Wdeprecated-copy-dtor
 -Wno-deprecated-enum-enum-conversion -Wno-deprecated-enum-float-conversion
 -Weffc++ -Wno-elaborated-enum-base
--Wno-exceptions -Wextra-semi  -Wno-inaccessible-base
+-Wno-exceptions -Wextra-semi -Wno-global-module -Wno-inaccessible-base
 -Wno-inherited-variadic-ctor  -Wno-init-list-lifetime
 -Winvalid-constexpr -Winvalid-imported-macros
 -Wno-invalid-offsetof  -Wno-literal-suffix
@@ -4774,6 +4774,12 @@ undefined behavior at runtime.  This warning is enabled by default.
 @item -Wextra-semi @r{(C++, Objective-C++ only)}
 Warn about redundant semicolons after in-class function definitions.
 
+@opindex Wno-global-module
+@opindex Wglobal-module
+@item -Wno-global-module @r{(C++ and Objective-C++ only)}
+Disable the diagnostic for when the global module fragment of a module
+unit does not consist only of preprocessor directives.
+
 @opindex Winaccessible-base
 @opindex Wno-inaccessible-base
 @item -Wno-inaccessible-base @r{(C++, Objective-C++ only)}
diff --git a/gcc/testsuite/g++.dg/modules/friend-6_a.C b/gcc/testsuite/g++.dg/modules/friend-6_a.C
index 7493e8f..13f7710 100644
--- a/gcc/testsuite/g++.dg/modules/friend-6_a.C
+++ b/gcc/testsuite/g++.dg/modules/friend-6_a.C
@@ -1,11 +1,9 @@
-// { dg-additional-options "-fmodules-ts -Wno-pedantic" }
+// { dg-additional-options "-fmodules-ts -Wno-global-module" }
 // { dg-module-cmi friend_6 }
 
 module;
-# 1 "" 1
 template <typename> struct Trans_NS___cxx11_basic_string {
   template <typename> friend class basic_stringbuf;
 };
 template struct Trans_NS___cxx11_basic_string<char>;
-# 6 "" 2
 export module friend_6;