[gcc r10-9743] c++: alias_ctad_tweaks and constrained dguide [PR95486]

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

https://gcc.gnu.org/g:4806f9157a26cfd66c083bcc01596ff33009c0d6

commit r10-9743-g4806f9157a26cfd66c083bcc01596ff33009c0d6
Author: Patrick Palka <ppalka@redhat.com>
Date:   Wed Jul 29 22:06:41 2020 -0400

    c++: alias_ctad_tweaks and constrained dguide [PR95486]
    
    In the below testcase, we're ICEing from alias_ctad_tweaks ultimately
    because the implied deduction guide for X's user-defined constructor
    already has constraints associated with it.  We then carry over these
    constraints to 'fprime', the overlying deduction guide for the alias
    template Y, via tsubst_decl from alias_ctad_tweaks.  Later in
    alias_ctad_tweaks we call get_constraints followed by set_constraints
    without doing remove_constraints in between, which triggers the !found
    assert in set_constraints.
    
    This patch fixes this issue by adding an intervening call to
    remove_constraints.
    
    gcc/cp/ChangeLog:
    
            PR c++/95486
            * pt.c (alias_ctad_tweaks): Call remove_constraints before
            calling set_constraints.
    
    gcc/testsuite/ChangeLog:
    
            PR c++/95486
            * g++.dg/cpp2a/class-deduction-alias3.C: New test.
    
    (cherry picked from commit 71141b1bd537cc516e485c834c2d36abba3f4544)

Diff:
---
 gcc/cp/pt.c                                         |  5 ++++-
 gcc/testsuite/g++.dg/cpp2a/class-deduction-alias3.C | 11 +++++++++++
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 76c7e428ecd..ae396e056c5 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -28823,7 +28823,10 @@ alias_ctad_tweaks (tree tmpl, tree uguides)
 	    }
 
 	  if (ci)
-	    set_constraints (fprime, ci);
+	    {
+	      remove_constraints (fprime);
+	      set_constraints (fprime, ci);
+	    }
 	}
       else
 	{
diff --git a/gcc/testsuite/g++.dg/cpp2a/class-deduction-alias3.C b/gcc/testsuite/g++.dg/cpp2a/class-deduction-alias3.C
new file mode 100644
index 00000000000..318d4c942ce
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/class-deduction-alias3.C
@@ -0,0 +1,11 @@
+// PR c++/95486
+// { dg-do compile { target c++20 } }
+
+template<class T, class U>
+struct X { X(U) requires __is_same(U, int) {} };
+
+template<class U>
+using Y = X<void, U>;
+
+Y y{1};
+Y z{'a'}; // { dg-error "failed|no match" }