[PATCH] c++: non-constant non-dependent decltype folding [PR104823]

[Patrick Palka via Gcc-patches]

instantiate_non_dependent_expr_sfinae instantiates only potentially
constant expressions, but when processing a non-dependent decltype
operand we want to instantiate it even if it's non-constant since
such non-dependent decltype is always resolved ahead of time.

Currently finish_decltype_type uses the former function, which causes us
to miss issuing a narrowing diagnostic for S{id(v)} in the below testcase
because we never instantiate this non-constant non-dependent decltype
operand.

So this patch makes finish_decltype_type use i_n_d_e_internal instead of
_sfinae.  And afterward, we need to keep processing_template_decl cleared
for sake of the later call to lvalue_kind, which handles templated and
non-templated COND_EXPR differently.  Otherwise we'd incorrectly reject
the declaration of g in cpp0x/cond2.C with:

  error: ‘g’ declared as function returning a function

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
trunk?

        PR c++/104823

gcc/cp/ChangeLog:

        * semantics.cc (finish_decltype_type): Use i_n_d_e_internal
        instead of _sfinae when instantiating a non-dependent decltype
        operand, and keep processing_template_decl cleared afterward.

gcc/testsuite/ChangeLog:

        * g++.dg/cpp0x/Wnarrowing19.C: New test.
---
 gcc/cp/semantics.cc                       | 11 ++++++++++-
 gcc/testsuite/g++.dg/cpp0x/Wnarrowing19.C |  8 ++++++++
 2 files changed, 18 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/Wnarrowing19.C

diff --git a/gcc/cp/semantics.cc b/gcc/cp/semantics.cc
index 07cae993efe..66d90c2f7be 100644
--- a/gcc/cp/semantics.cc
+++ b/gcc/cp/semantics.cc
@@ -11217,6 +11217,8 @@ finish_decltype_type (tree expr, bool 
id_expression_or_member_access_p,
   /* decltype is an unevaluated context.  */
   cp_unevaluated u;
 
+  processing_template_decl_sentinel ptds (/*reset=*/false);
+
   /* Depending on the resolution of DR 1172, we may later need to distinguish
      instantiation-dependent but not type-dependent expressions so that, say,
      A<decltype(sizeof(T))>::U doesn't require 'typename'.  */
@@ -11232,7 +11234,14 @@ finish_decltype_type (tree expr, bool 
id_expression_or_member_access_p,
     }
   else if (processing_template_decl)
     {
-      expr = instantiate_non_dependent_expr_sfinae (expr, complain);
+      /* Instantiate the non-dependent operand to diagnose any ill-formed
+        expressions.  And keep processing_template_decl cleared for the rest
+        of the function (for sake of the call to lvalue_kind below, which
+        handles templated and non-templated COND_EXPR differently).  */
+      processing_template_decl = 0;
+      /* Since we want to do this even for non-constant expressions, we
+        use i_n_d_e_internal here instead of _sfinae.  */
+      expr = instantiate_non_dependent_expr_internal (expr, complain);
       if (expr == error_mark_node)
        return error_mark_node;
     }
diff --git a/gcc/testsuite/g++.dg/cpp0x/Wnarrowing19.C 
b/gcc/testsuite/g++.dg/cpp0x/Wnarrowing19.C
new file mode 100644
index 00000000000..bd9fd2eb6f9
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/Wnarrowing19.C
@@ -0,0 +1,8 @@
+// PR c++/104823
+// { dg-do compile { target c++11 } }
+
+struct S { S(int); };
+
+double id(double v);
+
+template<class> auto f(double v) -> decltype(S{id(v)}); // { dg-error 
"narrowing" }
-- 
2.35.1.354.g715d08a9e5