c++: Enable coroutines with -std=c++20.

Now that GCC 10 is out it seems time.  People can still choose to disable
coroutines with -fno-coroutines.

This also switches the coroutines testsuite to run in C++20 mode.  The
change to coro.h is only necessary for co-await-11-forwarding.C; we could
alternatively #include <utility> just in that file.

gcc/c-family/ChangeLog
2020-05-15  Jason Merrill  <jason@redhat.com>

	* c-opts.c (set_std_cxx20): Set flag_coroutines.

gcc/testsuite/ChangeLog
2020-05-15  Jason Merrill  <jason@redhat.com>

	* g++.dg/coroutines/coro.h: Always #include <utility>.
	* g++.dg/coroutines/coroutines.exp (DEFAULT_COROFLAGS): Use
	-std=c++20.

5 files changed, 17 insertions, 12 deletions

diff --git a/gcc/c-family/ChangeLog b/gcc/c-family/ChangeLog
index 7bd78c0..da4be41 100644
--- a/gcc/c-family/ChangeLog
+++ b/gcc/c-family/ChangeLog
@@ -1,3 +1,7 @@
+2020-05-15  Jason Merrill  <jason@redhat.com>
+
+	* c-opts.c (set_std_cxx20): Set flag_coroutines.
+
 2020-05-13  Jason Merrill  <jason@redhat.com>
 
 	* c.opt (std=c++20): Make c++2a the alias.
diff --git a/gcc/c-family/c-opts.c b/gcc/c-family/c-opts.c
index bd617d3..7695e88 100644
--- a/gcc/c-family/c-opts.c
+++ b/gcc/c-family/c-opts.c
@@ -1722,7 +1722,7 @@ set_std_cxx17 (int iso)
   lang_hooks.name = "GNU C++17";
 }
 
-/* Set the C++ 202a draft standard (without GNU extensions if ISO).  */
+/* Set the C++ 2020 standard (without GNU extensions if ISO).  */
 static void
 set_std_cxx20 (int iso)
 {
@@ -1734,9 +1734,10 @@ set_std_cxx20 (int iso)
   flag_isoc94 = 1;
   flag_isoc99 = 1;
   flag_isoc11 = 1;
-  /* C++20 includes concepts. */
+  /* C++20 includes coroutines. */
+  flag_coroutines = true;
   cxx_dialect = cxx20;
-  lang_hooks.name = "GNU C++17"; /* Pretend C++17 until standardization.  */
+  lang_hooks.name = "GNU C++20";
 }
 
 /* Args to -d specify what to dump.  Silently ignore
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index fb93dbb..875ac82 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,3 +1,9 @@
+2020-05-15  Jason Merrill  <jason@redhat.com>
+
+	* g++.dg/coroutines/coro.h: Always #include <utility>.
+	* g++.dg/coroutines/coroutines.exp (DEFAULT_COROFLAGS): Use
+	-std=c++20.
+
 2020-05-15  Segher Boessenkool  <segher@kernel.crashing.org>
 
 	* gcc.target/powerpc/vec-gnb-0.c: Use int128 effective target.
diff --git a/gcc/testsuite/g++.dg/coroutines/coro.h b/gcc/testsuite/g++.dg/coroutines/coro.h
index fccfe41..02d2660 100644
--- a/gcc/testsuite/g++.dg/coroutines/coro.h
+++ b/gcc/testsuite/g++.dg/coroutines/coro.h
@@ -2,20 +2,12 @@
 
 #include <coroutine>
 
-#  if __clang__
-#    include <utility>
-#  endif
-
 namespace coro = std;
 
 #elif __has_include(<experimental/coroutine>)
 
 #include <experimental/coroutine>
 
-#  if __clang__
-#    include <utility>
-#  endif
-
 namespace coro = std::experimental;
 
 #else
@@ -143,6 +135,8 @@ extern "C" int printf (const char *, ...);
 
 #include <cstdlib> /* for abort () */
 
+#include <utility> /* for std::forward */
+
 #ifndef OUTPUT
 #  define PRINT(X)
 #  define PRINTF (void)
diff --git a/gcc/testsuite/g++.dg/coroutines/coroutines.exp b/gcc/testsuite/g++.dg/coroutines/coroutines.exp
index e7fd4da..1bef98a 100644
--- a/gcc/testsuite/g++.dg/coroutines/coroutines.exp
+++ b/gcc/testsuite/g++.dg/coroutines/coroutines.exp
@@ -29,7 +29,7 @@ if ![info exists DEFAULT_CXXFLAGS] then {
 }
 
 set DEFAULT_COROFLAGS $DEFAULT_CXXFLAGS
-lappend DEFAULT_COROFLAGS "-std=c++17" "-fcoroutines"
+lappend DEFAULT_COROFLAGS "-std=c++20"
 
 dg-init