[gcc(refs/vendors/ARM/heads/arm-struct-reorg-wip)] c++: constrained lambda inside template [PR92633]

[Tamar Christina]

https://gcc.gnu.org/g:40912c25b57b4e21aae20a4896624387aa13f7ed

commit 40912c25b57b4e21aae20a4896624387aa13f7ed
Author: Patrick Palka <ppalka@redhat.com>
Date:   Mon Jun 1 21:37:04 2020 -0400

    c++: constrained lambda inside template [PR92633]
    
    When regenerating a constrained lambda during instantiation of an
    enclosing template, we are forgetting to substitute into the lambda's
    constraints.  Fix this by substituting through the constraints during
    tsubst_lambda_expr.
    
    gcc/cp/ChangeLog:
    
            PR c++/92633
            PR c++/92838
            * pt.c (tsubst_function_decl): Don't do set_constraints when
            regenerating a lambda.
            (tsubst_lambda_expr): Substitute into the lambda's constraints
            and do set_constraints here.
    
    gcc/testsuite/ChangeLog:
    
            PR c++/92633
            PR c++/92838
            * g++.dg/cpp2a/concepts-lambda11.C: New test.
            * g++.dg/cpp2a/concepts-lambda12.C: New test.
    
    (cherry picked from commit c2449995ca36ea955e3c6d4ee7f0d401b512c897)

Diff:
---
 gcc/cp/pt.c                                    | 16 +++++++++++++++-
 gcc/testsuite/g++.dg/cpp2a/concepts-lambda11.C | 17 +++++++++++++++++
 gcc/testsuite/g++.dg/cpp2a/concepts-lambda12.C | 15 +++++++++++++++
 3 files changed, 47 insertions(+), 1 deletion(-)

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 7bff120c2fa..b5bd13e12ad 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -13861,7 +13861,10 @@ tsubst_function_decl (tree t, tree args, tsubst_flags_t complain,
      don't substitute through the constraints; that's only done when
      they are checked.  */
   if (tree ci = get_constraints (t))
-    set_constraints (r, ci);
+    /* Unless we're regenerating a lambda, in which case we'll set the
+       lambda's constraints in tsubst_lambda_expr.  */
+    if (!lambda_fntype)
+      set_constraints (r, ci);
 
   if (DECL_FRIEND_P (t) && DECL_FRIEND_CONTEXT (t))
     SET_DECL_FRIEND_CONTEXT (r,
@@ -19039,6 +19042,17 @@ tsubst_lambda_expr (tree t, tree args, tsubst_flags_t complain, tree in_decl)
 	  finish_member_declaration (fn);
 	}
 
+      if (tree ci = get_constraints (oldfn))
+	{
+	  /* Substitute into the lambda's constraints.  */
+	  if (oldtmpl)
+	    ++processing_template_decl;
+	  ci = tsubst_constraint_info (ci, args, complain, in_decl);
+	  if (oldtmpl)
+	    --processing_template_decl;
+	  set_constraints (fn, ci);
+	}
+
       /* Let finish_function set this.  */
       DECL_DECLARED_CONSTEXPR_P (fn) = false;
 
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-lambda11.C b/gcc/testsuite/g++.dg/cpp2a/concepts-lambda11.C
new file mode 100644
index 00000000000..dd9cd4e2344
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-lambda11.C
@@ -0,0 +1,17 @@
+// PR c++/92838
+// { dg-do compile { target c++20 } }
+
+template<int N>
+auto foo()
+{
+  [] () requires (N != 0) { }(); // { dg-error "no match" }
+  [] () requires (N == 0) { }();
+
+  [] <int M=1> () requires (N == M) { }(); // { dg-error "no match" }
+  [] <int M=1> () requires (N != M) { }();
+}
+
+void bar()
+{
+  foo<0>();
+}
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-lambda12.C b/gcc/testsuite/g++.dg/cpp2a/concepts-lambda12.C
new file mode 100644
index 00000000000..2bc9fd0bb25
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-lambda12.C
@@ -0,0 +1,15 @@
+// PR c++/92633
+// { dg-do compile { target c++20 } }
+
+template<class A, class B>
+concept different_than = !__is_same_as(A, B);
+
+template<class B>
+auto diff(B) {
+    return [](different_than<B> auto a) {};
+}
+
+int main() {
+    diff(42)("");
+    diff(42)(42); // { dg-error "no match" }
+}