C++ PATCH for c++/88631, CTAD failing for value-initialization

[Marek Polacek]

This PR points out that while we are able to deduce the template arguments
for A{}, we fail for A().

For A{}, the deduction happens in finish_compound_literal:
2789   if (tree anode = type_uses_auto (type))
2790     if (CLASS_PLACEHOLDER_TEMPLATE (anode))
2791       {
2792         type = do_auto_deduction (type, compound_literal, anode, complain,
2793                                   adc_variable_type)

and for A() in build_functional_cast, but there we always give error if there
are no parameters.  That is wrong because even though there are no arguments
to deduce from, we might still be able to deduce from default template
arguments as in the test.  Fixed thus; I'm passing tf_none if there are no
params because we don't want to give redundant diagnostics if the deduction
fails.

Bootstrapped/regtested on x86_64-linux, ok for trunk?

2018-12-30  Marek Polacek  <pola...@redhat.com>

        PR c++/88631 - CTAD failing for value-initialization.
        * typeck2.c (build_functional_cast): Try deducing the template
        arguments even if there are no arguments to deduce from.

        * g++.dg/cpp1z/class-deduction59.C: New test.

diff --git gcc/cp/typeck2.c gcc/cp/typeck2.c
index cc9bf02439b..206a87b94a1 100644
--- gcc/cp/typeck2.c
+++ gcc/cp/typeck2.c
@@ -2147,9 +2147,18 @@ build_functional_cast (tree exp, tree parms, 
tsubst_flags_t complain)
        }
       else if (!parms)
        {
-         if (complain & tf_error)
-           error ("cannot deduce template arguments for %qT from ()", anode);
-         return error_mark_node;
+         /* Even if there are no parameters, we might be able to deduce from
+            default template arguments.  Pass TF_NONE so that we don't
+            generate redundant diagnostics.  */
+         type = do_auto_deduction (type, parms, anode, tf_none,
+                                   adc_variable_type);
+         if (type == error_mark_node)
+           {
+             if (complain & tf_error)
+               error ("cannot deduce template arguments for %qT from ()",
+                      anode);
+             return error_mark_node;
+           }
        }
       else
        type = do_auto_deduction (type, parms, anode, complain,
diff --git gcc/testsuite/g++.dg/cpp1z/class-deduction59.C 
gcc/testsuite/g++.dg/cpp1z/class-deduction59.C
new file mode 100644
index 00000000000..129d723d07c
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp1z/class-deduction59.C
@@ -0,0 +1,12 @@
+// PR c++/88631
+// { dg-do compile { target c++17 } }
+
+template<class T = void>
+class A { };
+
+int main()
+{
+  auto x = A();
+  auto x2 = A{};
+  A y;
+}