[gcc r11-9841] c++: var tmpl w/ dependent constrained auto type [PR103341]

[Patrick Palka <ppalka@gcc.gnu.org>]

https://gcc.gnu.org/g:12b11107edfcde6a16ec397a9120687a14254215

commit r11-9841-g12b11107edfcde6a16ec397a9120687a14254215
Author: Patrick Palka <ppalka@redhat.com>
Date:   Fri Jan 28 08:18:28 2022 -0500

    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.c (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.
    
    (cherry picked from commit e272cf95ba048fde60b21aee046c9ca9c9264425)

Diff:
---
 gcc/cp/decl.c                                    | 13 ++++++++++++-
 gcc/testsuite/g++.dg/cpp2a/concepts-class4.C     | 11 +++++++++++
 gcc/testsuite/g++.dg/cpp2a/concepts-var-templ2.C | 13 +++++++++++++
 3 files changed, 36 insertions(+), 1 deletion(-)

diff --git a/gcc/cp/decl.c b/gcc/cp/decl.c
index 8d7d71d0590..bffbd004167 100644
--- a/gcc/cp/decl.c
+++ b/gcc/cp/decl.c
@@ -7860,9 +7860,20 @@ 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 (flag_concepts
+	  && 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" }