[PATCH] c++: var tmpl w/ dependent constrained auto type [PR103341]

[Patrick Palka via Gcc-patches]

When deducing the type of a variable template specialization with a
constrained auto type, we might need the template arguments for
satisfaction in case the constraint is dependent.

Bootstrapped and reegtested on x86_64-pc-linux-gnu, does this look OK
for trunk and perhaps 11?

        PR c++/103341

gcc/cp/ChangeLog:

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

gcc/testsuite/ChangeLog:

        * g++.dg/cpp2a/concepts-var-templ2.C: New test.
---
 gcc/cp/decl.cc                                   |  7 ++++++-
 gcc/testsuite/g++.dg/cpp2a/concepts-var-templ2.C | 13 +++++++++++++
 2 files changed, 19 insertions(+), 1 deletion(-)
 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..b698fa9b95a 100644
--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -7958,9 +7958,14 @@ 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 (is_constrained_auto (auto_node)
+         && variable_template_specialization_p (decl))
+       /* The outer template arguments might be needed for satisfaction.  */
+       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-var-templ2.C 
b/gcc/testsuite/g++.dg/cpp2a/concepts-var-templ2.C
new file mode 100644
index 00000000000..56cead5e8c3
--- /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{1};
+template<class T> same_as<T*> auto v2a{T()}; // { 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" }
-- 
2.35.0