c++: error with constexpr operator() [PR107939]

Similarly to PR107938, this also started with r11-557, whereby cp_finish_decl
can call check_initializer even in a template for a constexpr initializer.

Here we are rejecting

  extern const Q q;

  template<int>
  constexpr auto p = q(0);

even though q has a constexpr operator().  It's deemed non-const by
decl_maybe_constant_var_p because even though 'q' is const it is not
of integral/enum type.

If fun is not a function pointer, we don't know if we're using it as an
lvalue or rvalue, so with this patch we pass 'any' for want_rval.  With
that, p_c_e/VAR_DECL doesn't flat out reject the underlying VAR_DECL.

	PR c++/107939

gcc/cp/ChangeLog:

	* constexpr.cc (potential_constant_expression_1) <case CALL_EXPR>: Pass
	'any' when recursing on a VAR_DECL and not a pointer to function.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/var-templ74.C: Remove dg-error.
	* g++.dg/cpp1y/var-templ77.C: New test.

3 files changed, 39 insertions, 3 deletions

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index 364695b..3079561 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -9179,8 +9179,12 @@ potential_constant_expression_1 (tree t, bool want_rval, bool strict, bool now,
 	  }
 	else if (fun)
           {
-	    if (RECUR (fun, rval))
-	      /* Might end up being a constant function pointer.  */;
+	    if (RECUR (fun, FUNCTION_POINTER_TYPE_P (fun) ? rval : any))
+	      /* Might end up being a constant function pointer.  But it
+		 could also be a function object with constexpr op(), so
+		 we pass 'any' so that the underlying VAR_DECL is deemed
+		 as potentially-constant even though it wasn't declared
+		 constexpr.  */;
 	    else
 	      return false;
           }
diff --git a/gcc/testsuite/g++.dg/cpp1y/var-templ74.C b/gcc/testsuite/g++.dg/cpp1y/var-templ74.C
index 4e2e800..c76a7d9 100644
--- a/gcc/testsuite/g++.dg/cpp1y/var-templ74.C
+++ b/gcc/testsuite/g++.dg/cpp1y/var-templ74.C
@@ -9,7 +9,7 @@ struct Q {
 extern const Q q;
 
 template<int>
-constexpr const Q* p = q(0); // { dg-bogus "not usable" "PR107939" { xfail *-*-* } }
+constexpr const Q* p = q(0);
 
 void
 g ()
diff --git a/gcc/testsuite/g++.dg/cpp1y/var-templ77.C b/gcc/testsuite/g++.dg/cpp1y/var-templ77.C
new file mode 100644
index 0000000..0c56d70
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/var-templ77.C
@@ -0,0 +1,32 @@
+// PR c++/107939
+// { dg-do compile { target c++14 } }
+
+struct Q {
+  struct P {
+    const Q* p;
+  };
+  int n;
+  constexpr P operator()(int) const { return {this}; }
+};
+
+extern const Q q;
+template<int>
+constexpr auto p = q(0);
+static_assert(p<0>.p == &q, "");
+
+constexpr int
+fn (int)
+{
+  return 42;
+}
+
+struct Sur {
+  using FN = int(int);
+  constexpr operator FN*() const { return &fn; }
+};
+
+extern const Sur sur;
+template<int>
+constexpr int aja = sur (0);
+static_assert(aja<0> == 42, "");
+static_assert(sur(1) == 42, "");