c++: Change the default dialect to C++17.

Since GCC 9, C++17 support is no longer experimental.  It was too late
to change the default C++ dialect to C++17 in GCC 10, but I think now
it's time to pull the trigger (C++14 was made the default in GCC 6.1).
We're still missing two C++17 library features, but that shouldn't stop
us.  See
<https://gcc.gnu.org/onlinedocs/libstdc++/manual/status.html#status.iso.2017>
and
<https://gcc.gnu.org/projects/cxx-status.html#cxx17>
for the C++17 status.

I won't list all C++17 features here, but just a few heads-up:

- trigraphs were removed (hardly anyone cares, unless your keyboard is
  missing the # key),
- operator++(bool) was removed (so some tests now run in C++14 and down
  only),
- the keyword register was removed (some legacy code might trip on
  this),
- noexcept specification is now part of the type system and C++17 does
  not allow dynamic exception specifications anymore (the empty throw
  specification is still available, but it is deprecated),
- the evaluation order rules are different in C++17,
- static constexpr data members are now implicitly inline (which makes
  them definitions),
- C++17 requires guaranteed copy elision, meaning that a copy/move
  constructor call might be elided completely.  That means that if
  something relied on a constructor being instantiated via e.g. copying
  a function parameter, it might now fail.

I'll post an update for cxx-status.html and add a new caveat to changes.html
once this is in.

gcc/ChangeLog:
	* doc/invoke.texi (C Dialect Options): Adjust -std default for C++.
	* doc/standards.texi (C Language): Correct the default dialect.
	(C++ Language): Update the default for C++ to gnu++17.

gcc/c-family/ChangeLog:
	* c-opts.c (c_common_init_options): Default to gnu++17.

gcc/testsuite/ChangeLog:
	* c-c++-common/torture/vector-subscript-3.c: In C++17, define away
	the keyword register.
	* g++.dg/cpp1z/attributes-enum-1a.C: Only run pre-C++17.
	* g++.dg/cpp1z/fold7a.C: Likewise.
	* g++.dg/cpp1z/nontype3a.C: Likewise.
	* g++.dg/cpp1z/utf8-2a.C: Likewise.
	* g++.dg/parse/error11.C: Update expected diagnostics for C++17.
	* g++.dg/torture/pr34850.C: Add -Wno-attribute-warning.
	* g++.dg/torture/pr49394.C: In C++17, use noexcept(false).
	* g++.dg/torture/pr82154.C: Use -std=c++14.
	* lib/target-supports.exp: Set to C++17.
	* obj-c++.dg/try-catch-9.mm: Use -Wno-register.

libgomp/ChangeLog:
	* testsuite/libgomp.c++/atomic-3.C: Use -std=gnu++14.

gcc/c-family/c-opts.c

@@ -256,9 +256,9 @@ c_common_init_options (unsigned int decoded_options_count,
 	  }
     }
 
-  /* Set C++ standard to C++14 if not specified on the command line.  */
+  /* Set C++ standard to C++17 if not specified on the command line.  */
   if (c_dialect_cxx ())
-    set_std_cxx14 (/*ISO*/false);
+    set_std_cxx17 (/*ISO*/false);
 
   global_dc->colorize_source_p = true;
 }

gcc/doc/invoke.texi

@@ -2368,7 +2368,6 @@ The name @samp{c++1y} is deprecated.
 @item gnu++14
 @itemx gnu++1y
 GNU dialect of @option{-std=c++14}.
-This is the default for C++ code.
 The name @samp{gnu++1y} is deprecated.
 
 @item c++17
@@ -2379,6 +2378,7 @@ The name @samp{c++1z} is deprecated.
 @item gnu++17
 @itemx gnu++1z
 GNU dialect of @option{-std=c++17}.
+This is the default for C++ code.
 The name @samp{gnu++1z} is deprecated.
 
 @item c++20

gcc/doc/standards.texi

@@ -130,7 +130,7 @@ select an extended version of the C language explicitly with
 extensions).  
 
 The default, if no C language dialect options are given,
-is @option{-std=gnu11}.  
+is @option{-std=gnu17}.
 
 The ISO C standard defines (in clause 4) two classes of conforming
 implementation.  A @dfn{conforming hosted implementation} supports the
@@ -246,7 +246,7 @@ select an extended version of the C++ language explicitly with
 @option{-std=gnu++17} (for C++17 with GNU extensions).  
 
 The default, if
-no C++ language dialect options are given, is @option{-std=gnu++14}.
+no C++ language dialect options are given, is @option{-std=gnu++17}.
 
 @section Objective-C and Objective-C++ Languages
 @cindex Objective-C

gcc/testsuite/c-c++-common/torture/vector-subscript-3.c

@@ -10,6 +10,9 @@ struct vec_s {
     vector short member;
 };
 
+#if defined(__cplusplus) && __cplusplus >= 201703L
+#define register /* nothing */
+#endif
 
 int main () {
   register short vector v0 = {1,2,3,4,5,6,7};

gcc/testsuite/g++.dg/cpp1z/attributes-enum-1a.C

@@ -1,4 +1,5 @@
-// This macro should not be defined without -std=c++17.
+// { dg-do compile { target c++14_down } }
+// This macro should not be defined without c++17.
 
 #ifdef __cpp_enumerator_attributes
 #error __cpp_enumerator_attributes defined

gcc/testsuite/g++.dg/cpp1z/fold7a.C

@@ -1,4 +1,5 @@
-// This macro should not be defined without -std=c++17.
+// { dg-do compile { target c++14_down } }
+// This macro should not be defined without c++17.
 
 #ifdef __cpp_fold_expressions
 #error __cpp_fold_expressions defined

gcc/testsuite/g++.dg/cpp1z/nontype3a.C

@@ -1,4 +1,5 @@
-// This macro should not be defined without -std=c++17.
+// { dg-do compile { target c++14_down } }
+// This macro should not be defined without c++17.
 
 #ifdef __cpp_nontype_template_args
 #error __cpp_nontype_template_args defined

gcc/testsuite/g++.dg/cpp1z/utf8-2a.C

@@ -1,4 +1,5 @@
-// This macro should not be 201411 without -std=c++17.
+// { dg-do compile { target c++14_down } }
+// This macro should not be 201411 without c++17.
 
 #if __cpp_unicode_characters == 201411
 #error Wrong value for __cpp_unicode_characters

gcc/testsuite/g++.dg/parse/error11.C

@@ -1,5 +1,5 @@
 // { dg-do compile }
-// { dg-options "-fshow-column" }"
+// { dg-options "-fshow-column" }
 // Origin: Giovanni Bajo <giovannibajo at gcc dot gnu dot org>
 // Try to find out when the digraph '<:' is used as a mistake, and parse it
 //  correctly to avoid cascaded errors.
@@ -50,12 +50,16 @@ void func(void)
   //  the special error message.
   Foo<: :B> k2;     // { dg-bogus "cannot begin|alternate spelling" "smart error should not be triggered here" }
   Foo[:B> k1;       // { dg-bogus "cannot begin|alternate spelling" "smart error should not be triggered here" } 
-// { dg-error "6:missing template arguments before" "template" { target *-*-* } 51 }
-// { dg-error "9:expected primary-expression before ':' token" "primary" { target *-*-* } 51 }
-// { dg-error "8:expected '\]' before ':' token" "backslash" { target *-*-* } 51 }
-// { dg-error "6:missing template arguments before" "template" { target *-*-* } 52 }
-// { dg-error "7:expected primary-expression before ':' token" "primary" { target *-*-* } 52 }
-// { dg-error "7:expected '\]' before ':' token" "backslash" { target *-*-* } 52 }
+// { dg-error "6:missing template arguments before" "template" { target c++14_down } .-2 }
+// { dg-error "9:expected primary-expression before ':' token" "primary" { target c++14_down } .-3 }
+// { dg-error "8:expected '\]' before ':' token" "backslash" { target c++14_down } .-4 }
+// { dg-error "6:missing template arguments before" "template" { target c++14_down } .-4 }
+// { dg-error "7:expected primary-expression before ':' token" "primary" { target c++14_down } .-5 }
+// { dg-error "7:expected '\]' before ':' token" "backslash" { target c++14_down } .-6 }
+// { dg-error "9:expected identifier" "" { target c++17 } .-8 }
+// { dg-error "8:expected" "" { target c++17 } .-9 }
+// { dg-error "7:expected identifier" "" { target c++17 } .-9 }
+// { dg-error "7:expected" "" { target c++17 } .-10 }
 //
   int Foo[2];
   Foo[::value] = 0;

gcc/testsuite/g++.dg/torture/pr34850.C

@@ -1,6 +1,6 @@
 /* { dg-do compile } */
 /* { dg-skip-if "" { *-*-* } { "-O0" } { "" } } */
-/* { dg-options "-ffat-lto-objects -Wno-return-type" } */
+/* { dg-options "-ffat-lto-objects -Wno-return-type -Wno-attribute-warning" } */
 
 typedef unsigned char uint8_t;
 typedef unsigned int uint32_t;

gcc/testsuite/g++.dg/torture/pr49394.C

@@ -7,6 +7,8 @@ struct Mutex
   ~Mutex ()
 #if __cplusplus <= 201402L
   throw(int)			// { dg-warning "deprecated" "" { target { c++11 && { ! c++17 } } } }
+#else
+  noexcept(false)
 #endif
   {
     if (locked)

gcc/testsuite/g++.dg/torture/pr82154.C

@@ -1,5 +1,6 @@
 // { dg-do compile }
-// { dg-additional-options "-Wno-deprecated" }
+// { dg-additional-options "-std=c++14 -Wno-deprecated" }
+// C++17 does not allow dynamic exception specification.
 
 namespace a {
 int b;

gcc/testsuite/lib/target-supports.exp

@@ -9089,7 +9089,7 @@ proc check_effective_target_c++ { } {
  return 0
 }
 
-set cxx_default "c++14"
+set cxx_default "c++17"
 # Check whether the current active language standard supports the features
 # of C++11/C++14 by checking for the presence of one of the -std flags.
 # This assumes that the default for the compiler is $cxx_default, and that

gcc/testsuite/obj-c++.dg/try-catch-9.mm

@@ -6,7 +6,7 @@
 /* { dg-xfail-run-if "PR23616" { *-*-* } { "-fgnu-runtime" } { "-fnext-runtime" } } */
 /* { dg-xfail-if "Needs OBJC2 ABI" { *-*-darwin* && { lp64 && { ! objc2 } } } { "-fnext-runtime" "-fgnu-runtime" } { "" } } 
 /* { dg-prune-output ".*internal compiler error.*" } */
-/* { dg-options "-fobjc-exceptions -O2" } */
+/* { dg-options "-fobjc-exceptions -O2 -Wno-register" } */
 
 #include "../objc-obj-c++-shared/TestsuiteObject.m"
 #include <stdlib.h>

libgomp/testsuite/libgomp.c++/atomic-3.C

@@ -1,5 +1,6 @@
 // { dg-do run }
-// { dg-options "-Wno-deprecated" }
+// C++17 forbids ++ on bool.
+// { dg-options "-Wno-deprecated -std=gnu++14" }
 
 extern "C" void abort (void);
 bool v, x1, x2, x3, x4, x5, x6;