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C++ PATCH for c++/65942 (unnecessary instantiation of constexpr fn)

Message ID 556D13D0.7000202@redhat.com
State New
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Commit Message

Jason Merrill June 2, 2015, 2:24 a.m. UTC 
In the discussion of 65942 it was pointed out that there's really no 
reason to instantiate f in this testcase; we (and Clang) had been 
working from the idea that you instantiate a constexpr function when the 
function is mentioned even in unevaluated context, but we might as well 
wait until we actually need the definition.

Tested x86_64-pc-linux-gnu, applying to trunk.  I might apply it to the 
5 branch later.
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Patch

commit 95f0a88be340698e6fed34bb7ac086291a995dd9
Author: Jason Merrill <jason@redhat.com>
Date:   Mon Jun 1 17:24:57 2015 -0400

    	PR c++/65942
    	* decl2.c (mark_used): Don't always instantiate constexpr fns.
    	* constexpr.c (cxx_eval_call_expression): Instantiate them here.

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index 6c0d1fe..5278876 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -1240,6 +1240,15 @@  cxx_eval_call_expression (const constexpr_ctx *ctx, tree t,
 	return build_zero_init (DECL_CONTEXT (fun), NULL_TREE, false);
     }
 
+  /* We can't defer instantiating the function any longer.  */
+  if (!DECL_INITIAL (fun)
+      && DECL_TEMPLOID_INSTANTIATION (fun))
+    {
+      ++function_depth;
+      instantiate_decl (fun, /*defer_ok*/false, /*expl_inst*/false);
+      --function_depth;
+    }
+
   /* If in direct recursive call, optimize definition search.  */
   if (ctx && ctx->call && ctx->call->fundef->decl == fun)
     new_call.fundef = ctx->call->fundef;
diff --git a/gcc/cp/decl2.c b/gcc/cp/decl2.c
index f1b3d0c..8ba19cf 100644
--- a/gcc/cp/decl2.c
+++ b/gcc/cp/decl2.c
@@ -5046,8 +5046,7 @@  mark_used (tree decl, tsubst_flags_t complain)
       && DECL_TEMPLATE_INFO (decl)
       && (decl_maybe_constant_var_p (decl)
 	  || (TREE_CODE (decl) == FUNCTION_DECL
-	      && (DECL_DECLARED_CONSTEXPR_P (decl)
-		  || DECL_OMP_DECLARE_REDUCTION_P (decl)))
+	      && DECL_OMP_DECLARE_REDUCTION_P (decl))
 	  || undeduced_auto_decl (decl))
       && !uses_template_parms (DECL_TI_ARGS (decl)))
     {
diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-decltype2.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-decltype2.C
new file mode 100644
index 0000000..15ca995
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-decltype2.C
@@ -0,0 +1,7 @@ 
+// PR c++/65942
+// { dg-do compile { target c++11 } }
+
+template <typename T> constexpr int f(T t) { return t; }
+template <typename T, typename = decltype(f(T()))> void g(T) { }
+void g(...) { }
+int main() { g(""); }