[gcc r14-10096] c++/modules: deduced return type merging [PR114795]

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

https://gcc.gnu.org/g:4f9401d1a802325e5dfa2db841945e1a9c59a980

commit r14-10096-g4f9401d1a802325e5dfa2db841945e1a9c59a980
Author: Patrick Palka <ppalka@redhat.com>
Date:   Tue Apr 23 14:01:22 2024 -0400

    c++/modules: deduced return type merging [PR114795]
    
    When merging an imported function template specialization with an
    existing one, if the existing one has an undeduced return type and the
    imported one's is already deduced, we need to propagate the deduced type
    since once we install the imported definition we won't get a chance to
    deduce it by normal means.
    
    So this patch makes is_matching_decl propagate the deduced return
    type alongside our propagation of the exception specification.
    Another option would be to propagate it later when installing the
    imported definition from read_function_def, but it seems preferable
    to do it sooner rather than later.
    
            PR c++/114795
    
    gcc/cp/ChangeLog:
    
            * module.cc (trees_in::is_matching_decl): Propagate deduced
            function return type.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/modules/auto-4_a.H: New test.
            * g++.dg/modules/auto-4_b.C: New test.
    
    Reviewed-by: Jason Merrill <jason@redhat.com>

Diff:
---
 gcc/cp/module.cc                        |  9 +++++++++
 gcc/testsuite/g++.dg/modules/auto-4_a.H | 14 ++++++++++++++
 gcc/testsuite/g++.dg/modules/auto-4_b.C | 15 +++++++++++++++
 3 files changed, 38 insertions(+)

diff --git a/gcc/cp/module.cc b/gcc/cp/module.cc
index d94d8ff4df9..c35e70b8cb8 100644
--- a/gcc/cp/module.cc
+++ b/gcc/cp/module.cc
@@ -11537,6 +11537,15 @@ trees_in::is_matching_decl (tree existing, tree decl, bool is_typedef)
       else if (!DEFERRED_NOEXCEPT_SPEC_P (d_spec)
 	       && !comp_except_specs (d_spec, e_spec, ce_type))
 	goto mismatch;
+
+      /* Similarly if EXISTING has an undeduced return type, but DECL's
+	 is already deduced.  */
+      if (undeduced_auto_decl (existing) && !undeduced_auto_decl (decl))
+	{
+	  dump (dumper::MERGE)
+	    && dump ("Propagating deduced return type to %N", existing);
+	  TREE_TYPE (existing) = change_return_type (TREE_TYPE (d_type), e_type);
+	}
     }
   else if (is_typedef)
     {
diff --git a/gcc/testsuite/g++.dg/modules/auto-4_a.H b/gcc/testsuite/g++.dg/modules/auto-4_a.H
new file mode 100644
index 00000000000..0f7cd262dfa
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/auto-4_a.H
@@ -0,0 +1,14 @@
+// PR c++/114795
+// { dg-additional-options "-fmodule-header" }
+// { dg-module-cmi {} }
+
+template<class T>
+struct A {
+  auto f() { return T(); }
+};
+
+template<class T>
+void g() {
+  A<int> a;
+  a.f();
+}
diff --git a/gcc/testsuite/g++.dg/modules/auto-4_b.C b/gcc/testsuite/g++.dg/modules/auto-4_b.C
new file mode 100644
index 00000000000..378684ef6d0
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/auto-4_b.C
@@ -0,0 +1,15 @@
+// PR c++/114795
+// { dg-additional-options "-fmodules-ts -fno-module-lazy" }
+
+template<class T>
+struct A {
+  auto f() { return T(); }
+};
+
+A<int> a;
+
+import "auto-4_a.H";
+
+int main() {
+  g<int>(); // { dg-bogus "before deduction of 'auto'" "" { target *-*-* } 0 }
+}