From e272cf95ba048fde60b21aee046c9ca9c9264425 Mon Sep 17 00:00:00 2001
From: Patrick Palka <ppalka@redhat.com>
Date: Fri, 28 Jan 2022 08:18:28 -0500
Subject: [PATCH] c++: var tmpl w/ dependent constrained auto type [PR103341]

When deducing the type of a variable template (or templated static data
member) with a constrained auto type, we might need its template
arguments for satisfaction since the constraint could depend on them.

	PR c++/103341

gcc/cp/ChangeLog:

	* decl.cc (cp_finish_decl): Pass the template arguments of a
	variable template specialization or a templated static data
	member to do_auto_deduction when the auto is constrained.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-class4.C: New test.
	* g++.dg/cpp2a/concepts-var-templ2.C: New test.
---
 gcc/cp/decl.cc                                   | 12 +++++++++++-
 gcc/testsuite/g++.dg/cpp2a/concepts-class4.C     | 11 +++++++++++
 gcc/testsuite/g++.dg/cpp2a/concepts-var-templ2.C | 13 +++++++++++++
 3 files changed, 35 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-class4.C
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-var-templ2.C

diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index 10e6956117e..26ce9bfefec 100644
--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -7958,9 +7958,19 @@ cp_finish_decl (tree decl, tree init, bool init_const_expr_p,
       enum auto_deduction_context adc = adc_variable_type;
       if (VAR_P (decl) && DECL_DECOMPOSITION_P (decl))
 	adc = adc_decomp_type;
+      tree outer_targs = NULL_TREE;
+      if (PLACEHOLDER_TYPE_CONSTRAINTS_INFO (auto_node)
+	  && VAR_P (decl)
+	  && DECL_LANG_SPECIFIC (decl)
+	  && DECL_TEMPLATE_INFO (decl)
+	  && !DECL_FUNCTION_SCOPE_P (decl))
+	/* The outer template arguments might be needed for satisfaction.
+	   (For function scope variables, do_auto_deduction will obtain the
+	   outer template arguments from current_function_decl.)  */
+	outer_targs = DECL_TI_ARGS (decl);
       type = TREE_TYPE (decl) = do_auto_deduction (type, d_init, auto_node,
 						   tf_warning_or_error, adc,
-						   NULL_TREE, flags);
+						   outer_targs, flags);
       if (type == error_mark_node)
 	return;
       if (TREE_CODE (type) == FUNCTION_TYPE)
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-class4.C b/gcc/testsuite/g++.dg/cpp2a/concepts-class4.C
new file mode 100644
index 00000000000..90395f11025
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-class4.C
@@ -0,0 +1,11 @@
+// PR c++/103341
+// { dg-do compile { target c++20 } }
+
+template<class T, class U> concept same_as = __is_same(T, U);
+template<class T>
+struct A {
+  static inline same_as<T> auto value = 0; // { dg-error "constraint" }
+};
+
+template struct A<int>; // { dg-bogus "" }
+template struct A<bool>; // { dg-message "required from here" }
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-var-templ2.C b/gcc/testsuite/g++.dg/cpp2a/concepts-var-templ2.C
new file mode 100644
index 00000000000..e1802aca75f
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-var-templ2.C
@@ -0,0 +1,13 @@
+// PR c++/103341
+// { dg-do compile { target c++20 } }
+
+template<class T, class U> concept same_as = __is_same(T, U);
+template<class T> same_as<T> auto v1a = 1;
+template<class T> same_as<T> auto v1b = T();
+template<class T> same_as<T*> auto v2a = 1; // { dg-error "constraints" }
+template<class T> same_as<T*> auto v2b = T(); // { dg-error "constraints" }
+
+template int v1a<int>;
+template int v1b<int>;
+template int v2a<int>; // { dg-message "required from here" }
+template int v2b<int>; // { dg-message "required from here" }