c++: Fix ICE with { } as template argument [PR94592]

As an extension (there should be a CWG about this though), we support
braced-init-list as a template argument, but convert_nontype_argument
had trouble digesting them.  We ICEd because of the double coercion we
perform for template arguments: convert_nontype_argument called from
finish_template_type got a { }, and since a class type was involved and
we were in a template, convert_like created an IMPLICIT_CONV_EXPR.  Then
the second conversion of the same argument crashed in constexpr.c
because the IMPLICIT_CONV_EXPR had gotten wrapped in a TARGET_EXPR.
Another issue was that an IMPLICIT_CONV_EXPR leaked to constexpr.c when
building an aggregate init.

We should have instantiated the IMPLICIT_CONV_EXPR in the first call to
convert_nontype_argument, but we didn't, because the call to
is_nondependent_constant_expression returned false because it checks
!BRACE_ENCLOSED_INITIALIZER_P.  Then non_dep was false even though the
expression didn't contain anything dependent and we didn't instantiate
it in convert_nontype_argument.  To fix this, check
BRACE_ENCLOSED_INITIALIZER_P in cxx_eval_outermost_constant_expr rather
than in is_nondependent_*.

	PR c++/94592
	* constexpr.c (cxx_eval_outermost_constant_expr): Return when T is
	a BRACE_ENCLOSED_INITIALIZER_P.
	(is_nondependent_constant_expression): Don't check
	BRACE_ENCLOSED_INITIALIZER_P.
	(is_nondependent_static_init_expression): Likewise.

	* g++.dg/cpp2a/nontype-class34.C: New test.
	* g++.dg/cpp2a/nontype-class35.C: New test.

gcc/cp/ChangeLog

@@ -1,3 +1,12 @@
+2020-04-17  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/94592
+	* constexpr.c (cxx_eval_outermost_constant_expr): Return when T is
+	a BRACE_ENCLOSED_INITIALIZER_P.
+	(is_nondependent_constant_expression): Don't check
+	BRACE_ENCLOSED_INITIALIZER_P.
+	(is_nondependent_static_init_expression): Likewise.
+
 2020-04-20  Patrick Palka  <ppalka@redhat.com>
 
 	PR c++/94628

gcc/cp/constexpr.c

@@ -6534,6 +6534,12 @@ cxx_eval_outermost_constant_expr (tree t, bool allow_non_constant,
   bool non_constant_p = false;
   bool overflow_p = false;
 
+  if (BRACE_ENCLOSED_INITIALIZER_P (t))
+    {
+      gcc_checking_assert (allow_non_constant);
+      return t;
+    }
+
   constexpr_global_ctx global_ctx;
   constexpr_ctx ctx = { &global_ctx, NULL, NULL, NULL, NULL, NULL, NULL,
 			allow_non_constant, strict,
@@ -8295,7 +8301,6 @@ bool
 is_nondependent_constant_expression (tree t)
 {
   return (!type_unknown_p (t)
-	  && !BRACE_ENCLOSED_INITIALIZER_P (t)
 	  && is_constant_expression (t)
 	  && !instantiation_dependent_expression_p (t));
 }
@@ -8307,7 +8312,6 @@ bool
 is_nondependent_static_init_expression (tree t)
 {
   return (!type_unknown_p (t)
-	  && !BRACE_ENCLOSED_INITIALIZER_P (t)
 	  && is_static_init_expression (t)
 	  && !instantiation_dependent_expression_p (t));
 }

gcc/testsuite/ChangeLog

@@ -1,3 +1,9 @@
+2020-04-17  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/94592
+	* g++.dg/cpp2a/nontype-class34.C: New test.
+	* g++.dg/cpp2a/nontype-class35.C: New test.
+
 2020-04-20  Patrick Palka  <ppalka@redhat.com>
 
 	PR c++/94628

gcc/testsuite/g++.dg/cpp2a/nontype-class34.C

@@ -0,0 +1,16 @@
+// PR c++/94592 - ICE with { } as template argument.
+// { dg-do compile { target c++2a } }
+
+struct A {
+  constexpr A() {}
+};
+
+template <A> struct B { };
+
+template<typename> void bar () {
+    B<{}> var;
+}
+
+void fu() {
+    bar<int>();
+}

gcc/testsuite/g++.dg/cpp2a/nontype-class35.C

@@ -0,0 +1,17 @@
+// PR c++/94592 - ICE with { } as template argument.
+// { dg-do compile { target c++2a } }
+
+struct A {
+    int i;
+    constexpr A(int n) : i(n) {}
+};
+
+template <A a> struct B { int i; constexpr B() : i(a.i) { } };
+
+template<typename> void bar () {
+    B<{1}> var;
+}
+
+void fu() {
+    bar<int>();
+}