c++: Add -std=c++20.

It's probably past time for this, but definitely now that we're done with
the final committee meeting of C++20.  This patch only adds the option and
adjusts the testsuite to recognize it; more extensive changes can wait for
the published standard.

gcc/ChangeLog
2020-02-15  Jason Merrill  <jason@redhat.com>

	* doc/invoke.texi (C Dialect Options): Add -std=c++20.

gcc/c-family/ChangeLog
2020-02-15  Jason Merrill  <jason@redhat.com>

	* c.opt: Add -std=c++20.

gcc/testsuite/ChangeLog
2020-02-15  Jason Merrill  <jason@redhat.com>

	* lib/target-supports.exp (check_effective_target_c++2a_only): Also
	look for -std=*++20.
	(check_effective_target_concepts): Use check_effective_target_c++2a.

gcc/ChangeLog

+2020-02-15  Jason Merrill  <jason@redhat.com>
+
+	* doc/invoke.texi (C Dialect Options): Add -std=c++20.
+
 2020-02-15  Jakub Jelinek  <jakub@redhat.com>
 
 	PR tree-optimization/93744

gcc/c-family/ChangeLog

+2020-02-15  Jason Merrill  <jason@redhat.com>
+
+	* c.opt: Add -std=c++20.
+
 2020-02-14  Eric Botcazou  <ebotcazou@adacore.com>
 
 	* c-ada-spec.c: Include bitmap.h.

gcc/c-family/c.opt

@@ -2071,7 +2071,11 @@ Conform to the ISO 2017 C++ standard.
 
 std=c++2a
 C++ ObjC++
-Conform to the ISO 2020(?) C++ draft standard (experimental and incomplete support).
+Conform to the ISO 2020 C++ draft standard (experimental and incomplete support).
+
+std=c++20
+C++ ObjC++ Alias(std=c++2a)
+Conform to the ISO 2020 C++ draft standard (experimental and incomplete support).
 
 std=c11
 C ObjC

gcc/doc/invoke.texi

@@ -2078,13 +2078,15 @@ The name @samp{c++1z} is deprecated.
 GNU dialect of @option{-std=c++17}.
 The name @samp{gnu++1z} is deprecated.
 
-@item c++2a
-The next revision of the ISO C++ standard, tentatively planned for
+@item c++20
+@itemx c++2a
+The next revision of the ISO C++ standard, planned for
 2020.  Support is highly experimental, and will almost certainly
 change in incompatible ways in future releases.
 
-@item gnu++2a
-GNU dialect of @option{-std=c++2a}.  Support is highly experimental,
+@item gnu++20
+@itemx gnu++2a
+GNU dialect of @option{-std=c++20}.  Support is highly experimental,
 and will almost certainly change in incompatible ways in future
 releases.
 @end table

gcc/testsuite/ChangeLog

+2020-02-15  Jason Merrill  <jason@redhat.com>
+
+	* lib/target-supports.exp (check_effective_target_c++2a_only): Also
+	look for -std=*++20.
+	(check_effective_target_concepts): Use check_effective_target_c++2a.
+
 2020-02-15  Jakub Jelinek  <jakub@redhat.com>
 
 	PR tree-optimization/93744

gcc/testsuite/lib/target-supports.exp

@@ -8806,7 +8806,7 @@ proc check_effective_target_c++2a_only { } {
     if ![check_effective_target_c++] {
 	return 0
     }
-    if [check-flags { { } { } { -std=c++2a -std=gnu++2a } }] {
+    if [check-flags { { } { } { -std=c++2a -std=gnu++2a -std=c++20 -std=gnu++20 } }] {
 	return 1
     }
     if { $cxx_default == "c++20" && [check-flags { { } { } { } { -std=* } }] } {
@@ -8818,9 +8818,12 @@ proc check_effective_target_c++2a { } {
     return [check_effective_target_c++2a_only]
 }
 
-# Check for C++ Concepts TS support, i.e. -fconcepts flag.
+# Check for C++ Concepts support, i.e. -fconcepts flag.
 proc check_effective_target_concepts { } {
-    return [check-flags { "" { } { -fconcepts -std=*2a } }]
+    if [check_effective_target_c++2a] {
+	return 1
+    }
+    return [check-flags { "" { } { -fconcepts } }]
 }
 
 # Return 1 if expensive testcases should be run.