[C++ PATCH] Add test for c++/52269

[Marek Polacek]

Continuing scouring the Bugzilla.  This one compiles fine since r224008
which deferred instantiating constexpr functions until we actually need the
definition.

Tested other compilers too; all of them accept the test.

Bootstrapped/regtested on x86_64-linux, applying to trunk.

2019-06-08  Marek Polacek  <pola...@redhat.com>

        PR c++/52269
        * g++.dg/cpp0x/constexpr-decltype4.C: New test.

diff --git gcc/testsuite/g++.dg/cpp0x/constexpr-decltype4.C 
gcc/testsuite/g++.dg/cpp0x/constexpr-decltype4.C
new file mode 100644
index 00000000000..64af90baee4
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp0x/constexpr-decltype4.C
@@ -0,0 +1,21 @@
+// PR c++/52269
+// { dg-do compile { target c++11 } }
+
+template<typename T>
+int f(T x)
+{
+  return x.get();
+}
+
+// O.K. The body of `f' is not required.
+decltype(f(0)) a;
+
+template<typename T>
+constexpr int g(T x)
+{
+  return x.get();
+}
+
+// Seems to instantiate the body of `g'
+// and results in an error.
+decltype(g(0)) b;