c++: Fix value-init crash in template [PR93676]

Since <https://gcc.gnu.org/ml/gcc-patches/2015-02/msg00556.html> we
attempt to value-initialize in build_vec_init even when there's no
initializer but the type has a constexpr default constructor.  But
build_value_init doesn't work in templates, and build_vec_init
creates a lot of garbage that would not be used anyway, so don't
call it in a template.

	PR c++/93676 - value-init crash in template.
	* init.c (build_new_1): Don't call build_vec_init in a template.

	* g++.dg/cpp0x/nsdmi-template19.C: New test.

gcc/cp/ChangeLog

 2020-02-26  Marek Polacek  <polacek@redhat.com>
 
+	PR c++/93676 - value-init crash in template.
+	* init.c (build_new_1): Don't call build_vec_init in a template.
+
+2020-02-26  Marek Polacek  <polacek@redhat.com>
+
 	PR c++/93862 - ICE with static_cast when converting from int[].
 	* call.c (reference_compatible_p): No longer static.
 	* cp-tree.h (reference_compatible_p): Declare.

gcc/cp/init.c

@@ -3511,13 +3511,17 @@ build_new_1 (vec<tree, va_gc> **placement, tree type, tree nelts,
 	  explicit_value_init_p = true;
 	}
 
-      if (processing_template_decl && explicit_value_init_p)
+      if (processing_template_decl)
 	{
+	  /* Avoid an ICE when converting to a base in build_simple_base_path.
+	     We'll throw this all away anyway, and build_new will create
+	     a NEW_EXPR.  */
+	  tree t = fold_convert (build_pointer_type (elt_type), data_addr);
 	  /* build_value_init doesn't work in templates, and we don't need
 	     the initializer anyway since we're going to throw it away and
 	     rebuild it at instantiation time, so just build up a single
 	     constructor call to get any appropriate diagnostics.  */
-	  init_expr = cp_build_fold_indirect_ref (data_addr);
+	  init_expr = cp_build_fold_indirect_ref (t);
 	  if (type_build_ctor_call (elt_type))
 	    init_expr = build_special_member_call (init_expr,
 						   complete_ctor_identifier,

gcc/testsuite/ChangeLog

 2020-02-26  Marek Polacek  <polacek@redhat.com>
 
+	PR c++/93676 - value-init crash in template.
+	* g++.dg/cpp0x/nsdmi-template19.C: New test.
+
+2020-02-26  Marek Polacek  <polacek@redhat.com>
+
 	PR c++/93862 - ICE with static_cast when converting from int[].
 	* g++.dg/cpp0x/rv-cast7.C: New test.
 

gcc/testsuite/g++.dg/cpp0x/nsdmi-template19.C

+// PR c++/93676 - value-init crash in template.
+// { dg-do compile { target c++11 } }
+
+struct P {
+  int x = 0;
+};
+
+template<class T>
+struct S {
+  S() { new P[2][2]; }
+};
+
+S<int> s;