c++: FIX_TRUNC_EXPR in tsubst [PR102990]

This is a crash where a FIX_TRUNC_EXPR gets into tsubst_copy_and_build
where it hits gcc_unreachable ().

The history of tsubst_copy_and_build/FIX_TRUNC_EXPR is such that it
was introduced in r181478, but it did the wrong thing, whereupon it
was turned into gcc_unreachable () in r258821 (see this thread:
<https://gcc.gnu.org/pipermail/gcc-patches/2018-March/495853.html>).

In a template, we should never create a FIX_TRUNC_EXPR (that's what
conv_unsafe_in_template_p is for).  But in this test we are NOT in
a template when we call digest_nsdmi_init which ends up calling
convert_like, converting 1.0e+0 to int, so convert_to_integer_1
gives us a FIX_TRUNC_EXPR.

But then when we get to parsing f's parameters, we are in a template
when processing decltype(Helpers{}), and since r268321, when the
compound literal isn't instantiation-dependent and the type isn't
type-dependent, finish_compound_literal falls back to the normal
processing, so it calls digest_init, which does fold_non_dependent_init
and since the FIX_TRUNC_EXPR isn't dependent, we instantiate and
therefore crash in tsubst_copy_and_build.

The fateful call to fold_non_dependent_init comes from massage_init_elt,
We shouldn't be calling f_n_d_i on the result of get_nsdmi.  This we can
avoid by eschewing calling f_n_d_i on CONSTRUCTORs; their elements have
already been folded.

	PR c++/102990

gcc/cp/ChangeLog:

	* typeck2.cc (massage_init_elt): Avoid folding CONSTRUCTORs.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/nsdmi-template22.C: New test.
	* g++.dg/cpp0x/nsdmi-template23.C: New test.

3 files changed, 35 insertions, 4 deletions

diff --git a/gcc/cp/typeck2.cc b/gcc/cp/typeck2.cc
index a4c825f..cebe6ac 100644
--- a/gcc/cp/typeck2.cc
+++ b/gcc/cp/typeck2.cc
@@ -1433,10 +1433,15 @@ massage_init_elt (tree type, tree init, int nested, int flags,
     new_flags |= LOOKUP_AGGREGATE_PAREN_INIT;
   init = digest_init_r (type, init, nested ? 2 : 1, new_flags, complain);
   /* When we defer constant folding within a statement, we may want to
-     defer this folding as well.  */
-  tree t = fold_non_dependent_init (init, complain);
-  if (TREE_CONSTANT (t))
-    init = t;
+     defer this folding as well.  Don't call this on CONSTRUCTORs because
+     their elements have already been folded, and we must avoid folding
+     the result of get_nsdmi.  */
+  if (TREE_CODE (init) != CONSTRUCTOR)
+    {
+      tree t = fold_non_dependent_init (init, complain);
+      if (TREE_CONSTANT (t))
+	init = t;
+    }
   return init;
 }
 
diff --git a/gcc/testsuite/g++.dg/cpp0x/nsdmi-template22.C b/gcc/testsuite/g++.dg/cpp0x/nsdmi-template22.C
new file mode 100644
index 0000000..4ed2501
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/nsdmi-template22.C
@@ -0,0 +1,13 @@
+// PR c++/102990
+// { dg-do compile { target c++11 } }
+
+struct knob_t {
+  /* Let's create a FIX_TRUNC_EXPR.  */
+  int value = 1.0;
+};
+
+struct Helpers {
+  knob_t inputs;
+};
+
+template<class> void f(decltype(Helpers{}));
diff --git a/gcc/testsuite/g++.dg/cpp0x/nsdmi-template23.C b/gcc/testsuite/g++.dg/cpp0x/nsdmi-template23.C
new file mode 100644
index 0000000..240cab4
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/nsdmi-template23.C
@@ -0,0 +1,13 @@
+// PR c++/102990
+// { dg-do compile { target c++11 } }
+
+struct knob_t {
+  /* Let's create a FLOAT_EXPR.  */
+  double value = 1UL;
+};
+
+struct Helpers {
+  knob_t inputs;
+};
+
+template<class> void f(decltype(Helpers{}));