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.

2 files changed, 15 insertions, 1 deletions

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 15be7b6..a7ab2bd 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -28620,7 +28620,10 @@ 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 0000000..318d4c9
--- /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" }