c++: Hard error with tentative parse and CTAD [PR87709]

When parsing e.g. the operand of sizeof, where both types and
expressions are accepted, if during the tentative type parse we
encounter an unexpected template placeholder, we must simulate
an error rather than issue a real error because the expression
parse can still succeed.

gcc/cp/ChangeLog:

	PR c++/87709
	* parser.c (cp_parser_type_id_1): If we see a template
	placeholder, first try simulating an error before issuing
	a real error.

gcc/testsuite/ChangeLog:

	PR c++/87709
	* g++.dg/cpp1z/class-deduction86.C: New test.

diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index fba516efa23d5e74d59bf9f36e825bdce3fe9b54..e1b1617da6826c5d707865d150881045389a3af3 100644 (file)
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -23270,10 +23270,13 @@ cp_parser_type_id_1 (cp_parser *parser, cp_parser_flags flags,
            location_t loc = type_specifier_seq.locations[ds_type_spec];
            if (tree tmpl = CLASS_PLACEHOLDER_TEMPLATE (auto_node))
              {
-               error_at (loc, "missing template arguments after %qT",
-                         auto_node);
-               inform (DECL_SOURCE_LOCATION (tmpl), "%qD declared here",
-                       tmpl);
+               if (!cp_parser_simulate_error (parser))
+                 {
+                   error_at (loc, "missing template arguments after %qT",
+                             auto_node);
+                   inform (DECL_SOURCE_LOCATION (tmpl), "%qD declared here",
+                           tmpl);
+                 }
              }
            else if (parser->in_template_argument_list_p)
              error_at (loc, "%qT not permitted in template argument",

diff --git a/gcc/testsuite/g++.dg/cpp1z/class-deduction86.C b/gcc/testsuite/g++.dg/cpp1z/class-deduction86.C
new file mode 100644 (file)
index 0000000..a198ed2
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/class-deduction86.C
@@ -0,0 +1,16 @@
+// PR c++/87709
+// { dg-do compile { target c++17 } }
+
+template <class T>
+struct lit {
+  lit(T) { }
+};
+
+template <class T>
+int operator+(lit<T>, lit<T>) {
+  return 0;
+}
+
+auto r2 = (lit(0)) + lit(0);
+
+static_assert(sizeof(lit(0)));