[gcc r13-2744] c++: modules and non-dependent auto deduction

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

https://gcc.gnu.org/g:4fac53d652218946b82af4431a801ab1551f0e79

commit r13-2744-g4fac53d652218946b82af4431a801ab1551f0e79
Author: Patrick Palka <ppalka@redhat.com>
Date:   Tue Sep 20 16:08:14 2022 -0400

    c++: modules and non-dependent auto deduction
    
    The modules streaming code seems to rely on the invariant that a
    TEMPLATE_DECL and its DECL_TEMPLATE_RESULT have the same TREE_TYPE.
    But for a non-dependent VAR_DECL with deduced type, the two TREE_TYPEs
    end up diverging: cp_finish_decl deduces the type of the initializer
    ahead of time and updates the TREE_TYPE of the VAR_DECL, but neglects to
    update the corresponding TEMPLATE_DECL as well, which leads to a
    "conflicting global module declaration" error for each of the
    __phase_alignment decls in the below testcase (and for the xtreme-header
    tests if we try including <barrier>).
    
    This patch makes cp_finish_decl update the TREE_TYPE of the corresponding
    TEMPLATE_DECL so that the invariant is maintained.
    
    gcc/cp/ChangeLog:
    
            * decl.cc (cp_finish_decl): After updating the deduced type of a
            VAR_DECL, also update the corresponding TEMPLATE_DECL if there
            is one.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/modules/auto-3.h: New test.
            * g++.dg/modules/auto-3_a.H: New test.
            * g++.dg/modules/auto-3_b.C: New test.

Diff:
---
 gcc/cp/decl.cc                          |  6 ++++++
 gcc/testsuite/g++.dg/modules/auto-3.h   | 10 ++++++++++
 gcc/testsuite/g++.dg/modules/auto-3_a.H |  4 ++++
 gcc/testsuite/g++.dg/modules/auto-3_b.C |  4 ++++
 4 files changed, 24 insertions(+)

diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index 070f673c3a2..80467c19254 100644
--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -8180,6 +8180,12 @@ cp_finish_decl (tree decl, tree init, bool init_const_expr_p,
 	  return;
 	}
       cp_apply_type_quals_to_decl (cp_type_quals (type), decl);
+
+      /* Update the type of the corresponding TEMPLATE_DECL to match.  */
+      if (DECL_LANG_SPECIFIC (decl)
+	  && DECL_TEMPLATE_INFO (decl)
+	  && DECL_TEMPLATE_RESULT (DECL_TI_TEMPLATE (decl)) == decl)
+	TREE_TYPE (DECL_TI_TEMPLATE (decl)) = type;
     }
 
   if (ensure_literal_type_for_constexpr_object (decl) == error_mark_node)
diff --git a/gcc/testsuite/g++.dg/modules/auto-3.h b/gcc/testsuite/g++.dg/modules/auto-3.h
new file mode 100644
index 00000000000..f129433cbcb
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/auto-3.h
@@ -0,0 +1,10 @@
+template<class>
+struct __tree_barrier {
+  static const auto __phase_alignment_1 = 0;
+
+  template<class>
+  static const auto __phase_alignment_2 = 0;
+};
+
+template<class>
+inline auto __phase_alignment_3 = 0;
diff --git a/gcc/testsuite/g++.dg/modules/auto-3_a.H b/gcc/testsuite/g++.dg/modules/auto-3_a.H
new file mode 100644
index 00000000000..25a7a73e73e
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/auto-3_a.H
@@ -0,0 +1,4 @@
+// { dg-additional-options "-fmodule-header" }
+// { dg-module-cmi {} }
+
+#include "auto-3.h"
diff --git a/gcc/testsuite/g++.dg/modules/auto-3_b.C b/gcc/testsuite/g++.dg/modules/auto-3_b.C
new file mode 100644
index 00000000000..03b6d46f476
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/auto-3_b.C
@@ -0,0 +1,4 @@
+// { dg-additional-options "-fmodules-ts -fno-module-lazy" }
+
+#include "auto-3.h"
+import "auto-3_a.H";