From 115232b778943be075fc4df991e03d9387563114 Mon Sep 17 00:00:00 2001
From: Patrick Palka <ppalka@redhat.com>
Date: Fri, 15 May 2020 18:51:11 -0400
Subject: [PATCH] c++: decltype of invalid non-dependent expr [PR57943]

We sometimes fail to reject an invalid non-dependent operand to decltype
when inside a template, because finish_decltype_type resolves the
decltype to the TREE_TYPE of the operand before we ever instantiate and
fully process the operand.  Fix this by adding a call to
instantiate_non_dependent_expr_sfinae in finish_decltype_type.

gcc/cp/ChangeLog:

	PR c++/57943
	* semantics.c (finish_decltype_type): Call
	instantiate_non_dependent_expr_sfinae on the expression.

gcc/testsuite/ChangeLog:

	PR c++/57943
	* g++.dg/cpp0x/decltype76.C: New test.
---
 gcc/cp/ChangeLog                        | 6 ++++++
 gcc/cp/semantics.c                      | 8 ++++++++
 gcc/testsuite/ChangeLog                 | 5 +++++
 gcc/testsuite/g++.dg/cpp0x/decltype76.C | 7 +++++++
 4 files changed, 26 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/decltype76.C

diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index acd2d7be844..4b9fc52b6e3 100644
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,3 +1,9 @@
+2020-05-16  Patrick Palka  <ppalka@redhat.com>
+
+	PR c++/57943
+	* semantics.c (finish_decltype_type): Call
+	instantiate_non_dependent_expr_sfinae on the expression.
+
 2020-05-15  Patrick Palka  <ppalka@redhat.com>
 
 	Revert:
diff --git a/gcc/cp/semantics.c b/gcc/cp/semantics.c
index d90816eabc9..64587c791c6 100644
--- a/gcc/cp/semantics.c
+++ b/gcc/cp/semantics.c
@@ -9746,6 +9746,14 @@ finish_decltype_type (tree expr, bool id_expression_or_member_access_p,
 
       return type;
     }
+  else if (processing_template_decl)
+    {
+      ++cp_unevaluated_operand;
+      expr = instantiate_non_dependent_expr_sfinae (expr, complain);
+      --cp_unevaluated_operand;
+      if (expr == error_mark_node)
+	return error_mark_node;
+    }
 
   /* The type denoted by decltype(e) is defined as follows:  */
 
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index 875ac82fb34..a7f777118f2 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,3 +1,8 @@
+2020-05-16  Patrick Palka  <ppalka@redhat.com>
+
+	PR c++/57943
+	* g++.dg/cpp0x/decltype76.C: New test.
+
 2020-05-15  Jason Merrill  <jason@redhat.com>
 
 	* g++.dg/coroutines/coro.h: Always #include <utility>.
diff --git a/gcc/testsuite/g++.dg/cpp0x/decltype76.C b/gcc/testsuite/g++.dg/cpp0x/decltype76.C
new file mode 100644
index 00000000000..239fe6d2a8f
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/decltype76.C
@@ -0,0 +1,7 @@
+// PR c+/57943
+// { dg-do compile { target c++11 } }
+
+struct a { };
+
+template <typename T = decltype (a(0))> // { dg-error "" }
+void f() { }
-- 
2.30.2