re PR c++/78131 (Inconsistent evaluation for `constexpr` lambdas in templates…

re PR c++/78131 (Inconsistent evaluation for `constexpr` lambdas in templates between `static_assert`, `if constexpr(…)` and `constexpr` variables)

2017-10-04  Paolo Carlini  <paolo.carlini@oracle.com>

	PR c++/78131
	* g++.dg/cpp1z/constexpr-lambda17.C: New.

From-SVN: r253431

gcc/testsuite/ChangeLog

 2017-10-04  Paolo Carlini  <paolo.carlini@oracle.com>
 
+	PR c++/78131
+	* g++.dg/cpp1z/constexpr-lambda17.C: New.
+
+2017-10-04  Paolo Carlini  <paolo.carlini@oracle.com>
+
 	PR c++/78018
 	* g++.dg/cpp1y/lambda-generic-78018.C: New.
 

gcc/testsuite/g++.dg/cpp1z/constexpr-lambda17.C

+// PR c++/78131
+// { dg-options -std=c++17 }
+
+template <typename TF>
+constexpr auto f(TF)
+{
+    return [](auto...) constexpr { return true; };
+}   
+
+// Compiles and works as intended.
+template <typename T0>
+void ok_0(T0)
+{
+    static_assert(f([](auto x) -> decltype(x){})(T0{}));
+}
+
+// Compiles and works as intended.
+template <typename T0>
+void ok_1(T0)
+{
+    constexpr auto a = f([](auto x) -> decltype(x){})(T0{});
+    if constexpr(a) { }
+}
+
+// Compile-time error!
+template <typename T0>
+void fail_0(T0)
+{
+    if constexpr(f([](auto x) -> decltype(x){})(T0{})) { } 
+}