c++: ICE with template code in constexpr [PR104284]

Since r9-6073 cxx_eval_store_expression preevaluates the value to
be stored, and that revealed a crash where a template code (here,
code=IMPLICIT_CONV_EXPR) leaks into cxx_eval*.

It happens because we're performing build_vec_init while processing
a template, which calls get_temp_regvar which creates an INIT_EXPR.
This INIT_EXPR's RHS contains an rvalue conversion so we create an
IMPLICIT_CONV_EXPR.  Its operand is not type-dependent and the whole
INIT_EXPR is not type-dependent.  So we call build_non_dependent_expr
which, with -fchecking=2, calls fold_non_dependent_expr.  At this
point the expression still has an IMPLICIT_CONV_EXPR, which ought to
be handled in instantiate_non_dependent_expr_internal.  However,
tsubst_copy_and_build doesn't handle INIT_EXPR; it will just call
tsubst_copy which does nothing when args is null.  So we fail to
replace the IMPLICIT_CONV_EXPR and ICE.

The problem is that we call build_vec_init in a template in the
first place.  We can avoid doing so by checking p_t_d before
calling build_aggr_init in check_initializer.

	PR c++/104284

gcc/cp/ChangeLog:

	* decl.cc (check_initializer): Don't call build_aggr_init in
	a template.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/constexpr-104284-1.C: New test.
	* g++.dg/cpp1y/constexpr-104284-2.C: New test.
	* g++.dg/cpp1y/constexpr-104284-3.C: New test.
	* g++.dg/cpp1y/constexpr-104284-4.C: New test.

5 files changed, 139 insertions, 0 deletions

diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index 68741bbf..69f60a6 100644
--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -7332,6 +7332,10 @@ check_initializer (tree decl, tree init, int flags, vec<tree, va_gc> **cleanups)
 	   && !(init && BRACE_ENCLOSED_INITIALIZER_P (init)
 		&& CP_AGGREGATE_TYPE_P (type)
 		&& (CLASS_TYPE_P (type)
+		    /* The call to build_aggr_init below could end up
+		       calling build_vec_init, which may break when we
+		       are processing a template.  */
+		    || processing_template_decl
 		    || !TYPE_NEEDS_CONSTRUCTING (type)
 		    || type_has_extended_temps (type))))
 	  || (DECL_DECOMPOSITION_P (decl) && TREE_CODE (type) == ARRAY_TYPE))
diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-104284-1.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-104284-1.C
new file mode 100644
index 0000000..809c26a
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-104284-1.C
@@ -0,0 +1,34 @@
+// PR c++/104284
+// { dg-do compile { target c++14 } }
+// { dg-additional-options "-fchecking=2" }
+
+struct S {
+  char c{};
+};
+
+auto x1 = [](auto) { constexpr S s[]{{}}; };
+auto x2 = [](auto) { constexpr S s[]{{'a'}}; };
+#if __cpp_designated_initializers >= 201707L
+auto x3 = [](auto) { constexpr S s[]{{.c = 'a'}}; };
+#endif
+auto x4 = [](auto) { constexpr S s[]{'a'}; };
+auto x5 = [](auto) { constexpr S s[]{{{}}}; };
+
+template<class>
+constexpr void g ()
+{
+  constexpr S s1[]{{}};
+  static_assert(s1[0].c == '\0', "");
+  constexpr S s2[]{{'a'}};
+  static_assert(s2[0].c == 'a', "");
+#if __cpp_designated_initializers >= 201707L
+  constexpr S s3[]{{.c = 'a'}};
+  static_assert(s3[0].c == 'a', "");
+#endif
+  constexpr S s4[]{'a'};
+  static_assert(s4[0].c == 'a', "");
+  constexpr S s5[]{{{}}};
+  static_assert(s5[0].c == '\0', "");
+}
+
+static_assert ((g<int>(), true), "");
diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-104284-2.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-104284-2.C
new file mode 100644
index 0000000..704d37d
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-104284-2.C
@@ -0,0 +1,33 @@
+// PR c++/104284
+// { dg-do compile { target c++14 } }
+// { dg-additional-options "-fchecking=2" }
+
+struct S {
+  char a;
+  constexpr S() : a{'a'} { }
+  constexpr S(char a_) : a{a_} { }
+};
+
+auto x1 = [](auto) { constexpr S s[]{{}}; };
+auto x2 = [](auto) { constexpr S s[]{{'a'}}; };
+auto x3 = [](auto) { constexpr S s[]{'a'}; };
+auto x4 = [](auto) { constexpr S s[]{{{}}}; };
+
+template<typename>
+constexpr void g()
+{
+  constexpr S s1[]{{}};
+  static_assert(s1[0].a == 'a', "");
+  constexpr S s2[]{{'a'}};
+  static_assert(s2[0].a == 'a', "");
+  constexpr S s3[]{'a'};
+  static_assert(s3[0].a == 'a', "");
+  constexpr S s4[]{{{}}};
+  static_assert(s4[0].a == '\0', "");
+}
+
+void
+f ()
+{
+  g<int>();
+}
diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-104284-3.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-104284-3.C
new file mode 100644
index 0000000..6f23b25
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-104284-3.C
@@ -0,0 +1,33 @@
+// PR c++/104284
+// { dg-do compile { target c++14 } }
+// { dg-additional-options "-fchecking=2" }
+// Like constexpr-104284.C, but the function template is not
+// constexpr.  In that case, we were still calling build_vec_init
+// in a template, just not crashing.
+
+struct S {
+  char c{};
+};
+
+template<class>
+void g ()
+{
+  constexpr S s1[]{{}};
+  static_assert(s1[0].c == '\0', "");
+  constexpr S s2[]{{'a'}};
+  static_assert(s2[0].c == 'a', "");
+#if __cpp_designated_initializers >= 201707L
+  constexpr S s3[]{{.c = 'a'}};
+  static_assert(s3[0].c == 'a', "");
+#endif
+  constexpr S s4[]{'a'};
+  static_assert(s4[0].c == 'a', "");
+  constexpr S s5[]{{{}}};
+  static_assert(s5[0].c == '\0', "");
+}
+
+void
+f ()
+{
+  g<int>();
+}
diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-104284-4.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-104284-4.C
new file mode 100644
index 0000000..a99d325
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-104284-4.C
@@ -0,0 +1,35 @@
+// PR c++/104284
+// { dg-do run { target c++14 } }
+// { dg-additional-options "-fchecking=2" }
+
+struct S {
+  char c{};
+};
+
+template<class>
+constexpr void g ()
+{
+  S s1[]{{}};
+  if (s1[0].c != '\0')
+    __builtin_abort ();
+  S s2[]{{'a'}};
+  if (s2[0].c != 'a')
+    __builtin_abort ();
+#if __cpp_designated_initializers >= 201707L
+  S s3[]{{.c = 'a'}};
+  if (s3[0].c != 'a')
+    __builtin_abort ();
+#endif
+  S s4[]{'a'};
+  if (s4[0].c != 'a')
+    __builtin_abort ();
+  S s5[]{{{}}};
+  if (s5[0].c != '\0')
+    __builtin_abort ();
+}
+
+int
+main ()
+{
+  g<int>();
+}