[pushed] c++: Aggregate CTAD and string constants.

[pushed] c++: Aggregate CTAD and string constants.

[Jason Merrill]

In CWG discussion, it was suggested that deduction from a string literal
should be to reference-to-const, so that we deduce 'char' rather than 'const
char' for T.

Tested x86_64-pc-linux-gnu, applying to trunk.

gcc/cp/ChangeLog:

	* pt.c (collect_ctor_idx_types): Add 'const' when deducing from
	a string constant.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/class-deduction-aggr7.C: New test.
---
 gcc/cp/pt.c                                        |  9 +++++++--
 gcc/testsuite/g++.dg/cpp2a/class-deduction-aggr7.C | 14 ++++++++++++++
 2 files changed, 21 insertions(+), 2 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/class-deduction-aggr7.C

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index defc2a9abd8..5f43e9c5c69 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -28357,8 +28357,13 @@ collect_ctor_idx_types (tree ctor, tree list, tree elt = NULL_TREE)
       if (TREE_CODE (ftype) == ARRAY_TYPE
 	  && (BRACE_ENCLOSED_INITIALIZER_P (val)
 	      || TREE_CODE (val) == STRING_CST))
-	ftype = (cp_build_reference_type
-		 (ftype, BRACE_ENCLOSED_INITIALIZER_P (val)));
+	{
+	  if (TREE_CODE (val) == STRING_CST)
+	    ftype = cp_build_qualified_type
+	      (ftype, cp_type_quals (ftype) | TYPE_QUAL_CONST);
+	  ftype = (cp_build_reference_type
+		   (ftype, BRACE_ENCLOSED_INITIALIZER_P (val)));
+	}
       list = tree_cons (arg, ftype, list);
     }
 
diff --git a/gcc/testsuite/g++.dg/cpp2a/class-deduction-aggr7.C b/gcc/testsuite/g++.dg/cpp2a/class-deduction-aggr7.C
new file mode 100644
index 00000000000..3505a8c97db
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/class-deduction-aggr7.C
@@ -0,0 +1,14 @@
+// { dg-do compile { target c++20 } }
+
+template <class T, int N>
+struct A
+{
+  T ar[N];
+};
+
+A a = { "foo" };
+
+template<class, class> struct same;
+template<class T> struct same<T,T> {};
+same<decltype (a.ar), char[4]> s;
+

base-commit: 87891d5eafe8d1de90b9d9b056eca81c508d1c77
-- 
2.18.1