c++: unnecessary instantiation of constexpr var [PR99130]

Here the use of 'value' from within an unevaluated context causes us
to overeagerly instantiate it, via maybe_instantiate_decl called from
mark_used, despite the use occurring in a context that doesn't require
a definition.

This seems to only affect constexpr variable specializations, though
we used to have the same issue for constexpr function specializations
until r6-1309-g81371eff9bc7ef made us delay their instantiation until
necessary during constexpr evaluation.

This patch expands upon the r6-1309 fix to make mark_used avoid
unnecessarily instantiating constexpr variable specializations too,
by pulling out from maybe_instantiate_decl the condition

  (decl_maybe_constant_var_p (decl)
   || (TREE_CODE (decl) == FUNCTION_DECL
       && DECL_OMP_DECLARE_REDUCTION_P (decl))
   || undeduced_auto_decl (decl))

into each of its three callers (including mark_used), removing the
problematic first test from mark_used, and simplifying accordingly.
The net result is that only mark_used is changed because the other two
callers, resolve_address_of_overloaded_function and decl_constant_var_p,
already guard the call appropriately.  (This relaxation of mark_used
seems to be safe because during constexpr evaluation we already take
care to instantiate a constexpr variable as necessary via
decl_constant_value etc).

	PR c++/99130

gcc/cp/ChangeLog:

	* decl2.cc (maybe_instantiate_decl): Adjust function comment.
	Check VAR_OR_FUNCTION_DECL_P.  Pull out the disjunction into ...
	(mark_used): ... here, removing the decl_maybe_constant_var_p
	part of it.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/constexpr-decltype5.C: New test.

gcc/cp/decl2.cc

@@ -5381,24 +5381,15 @@ possibly_inlined_p (tree decl)
   return true;
 }
 
-/* Normally, we can wait until instantiation-time to synthesize DECL.
-   However, if DECL is a static data member initialized with a constant
-   or a constexpr function, we need it right now because a reference to
-   such a data member or a call to such function is not value-dependent.
-   For a function that uses auto in the return type, we need to instantiate
-   it to find out its type.  For OpenMP user defined reductions, we need
-   them instantiated for reduction clauses which inline them by hand
-   directly.  */
+/* If DECL is a function or variable template specialization, instantiate
+   its definition now.  */
 
 void
 maybe_instantiate_decl (tree decl)
 {
-  if (DECL_LANG_SPECIFIC (decl)
+  if (VAR_OR_FUNCTION_DECL_P (decl)
+      && DECL_LANG_SPECIFIC (decl)
       && DECL_TEMPLATE_INFO (decl)
-      && (decl_maybe_constant_var_p (decl)
-	  || (TREE_CODE (decl) == FUNCTION_DECL
-	      && DECL_OMP_DECLARE_REDUCTION_P (decl))
-	  || undeduced_auto_decl (decl))
       && !DECL_DECLARED_CONCEPT_P (decl)
       && !uses_template_parms (DECL_TI_ARGS (decl)))
     {
@@ -5700,15 +5691,13 @@ mark_used (tree decl, tsubst_flags_t complain)
       return false;
     }
 
-  /* Normally, we can wait until instantiation-time to synthesize DECL.
-     However, if DECL is a static data member initialized with a constant
-     or a constexpr function, we need it right now because a reference to
-     such a data member or a call to such function is not value-dependent.
-     For a function that uses auto in the return type, we need to instantiate
-     it to find out its type.  For OpenMP user defined reductions, we need
-     them instantiated for reduction clauses which inline them by hand
-     directly.  */
-  maybe_instantiate_decl (decl);
+  /* If DECL has a deduced return type, we need to instantiate it now to
+     find out its type.  For OpenMP user defined reductions, we need them
+     instantiated for reduction clauses which inline them by hand directly.  */
+  if (undeduced_auto_decl (decl)
+      || (TREE_CODE (decl) == FUNCTION_DECL
+	  && DECL_OMP_DECLARE_REDUCTION_P (decl)))
+    maybe_instantiate_decl (decl);
 
   if (processing_template_decl || in_template_function ())
     return true;

gcc/testsuite/g++.dg/cpp0x/constexpr-decltype5.C

@@ -0,0 +1,23 @@
+// PR c++/99130
+// { dg-do compile { target c++11 } }
+
+template<class T>
+struct A {
+  static constexpr int value = T::nonexistent;
+};
+
+using type = const int;
+using type = decltype(A<int>::value);
+
+#if __cpp_variable_templates
+struct B {
+  template<class T>
+  static constexpr int value = T::nonexistent;
+};
+
+template<class T>
+constexpr int value = T::nonexistent;
+
+using type = decltype(B::value<int>);
+using type = decltype(value<int>);
+#endif