c++: constexpr and lambda capture [PR90212]

This is the same issue as PR86429, just in potential_constant_expression_1
rather than cxx_eval_constant_expression.  As in that case, when we're
trying to evaluate a constant expression within a lambda, we don't have a
constant closure object to refer to, but we can try to refer directly to the
captured variable.

gcc/cp/ChangeLog
2020-05-05  Jason Merrill  <jason@redhat.com>

	PR c++/90212
	* constexpr.c (potential_constant_expression_1): In a lambda
	function, consider a captured variable directly.

gcc/cp/ChangeLog

 2020-05-25  Jason Merrill  <jason@redhat.com>
 
+	PR c++/90212
+	* constexpr.c (potential_constant_expression_1): In a lambda
+	function, consider a captured variable directly.
+
+2020-05-25  Jason Merrill  <jason@redhat.com>
+
 	PR c++/90479
 	* init.c (get_nsdmi): Don't push_to_top_level for a local class.
 

gcc/cp/constexpr.c

@@ -7520,12 +7520,18 @@ potential_constant_expression_1 (tree t, bool want_rval, bool strict, bool now,
 		 variable with automatic storage duration defined outside that
 		 lambda-expression, where the reference would be an
 		 odr-use.  */
+
+	      if (want_rval)
+		/* Since we're doing an lvalue-rvalue conversion, this might
+		   not be an odr-use, so evaluate the variable directly. */
+		return RECUR (DECL_CAPTURED_VARIABLE (t), rval);
+
 	      if (flags & tf_error)
 		{
 		  tree cap = DECL_CAPTURED_VARIABLE (t);
 		  error ("lambda capture of %qE is not a constant expression",
 			 cap);
-		  if (!want_rval && decl_constant_var_p (cap))
+		  if (decl_constant_var_p (cap))
 		    inform (input_location, "because it is used as a glvalue");
 		}
 	      return false;

gcc/testsuite/g++.dg/cpp0x/lambda/lambda-constexpr2.C

+// PR c++/90212
+// { dg-do compile { target c++11 } }
+
+template<typename T> struct tuple {
+    constexpr tuple(T&& t) : t(t) { }
+    int t;
+};
+
+void foo() {
+    constexpr tuple<int> v1{1};
+    constexpr auto v2 = v1;
+    [&]{ constexpr auto v2 = v1; };
+}