[PATCH] replace ICE with error for failed template deduction (PR 84355)

[Martin Sebor]

A failed template deduction in template member of a template
triggers an ICE with -std=c++17 due to what seems like
a missing handling of invalid input.  Replacing
the gcc_unreachable() call that causes the ICE with a return
statement indicating the deduction failure eliminates the ICE
and restores sane diagnostics.

Martin

PR c++/84355 - [7/8 Regression] ICE with failing template argument deduction

gcc/cp/ChangeLog:

	PR c++/84355
	* pt.c (unify): Return failure instead of asserting.

gcc/testsuite/ChangeLog:

	PR c++/84355
	* g++.dg/cpp1z/class-deduction48.C: New test.

Index: gcc/cp/pt.c
===================================================================
--- gcc/cp/pt.c	(revision 257713)
+++ gcc/cp/pt.c	(working copy)
@@ -20918,7 +20918,7 @@ unify (tree tparms, tree targs, tree parm, tree ar
 	   && TREE_CODE (tparm) != TYPE_DECL)
 	  || (TREE_CODE (parm) == TEMPLATE_TEMPLATE_PARM
 	      && TREE_CODE (tparm) != TEMPLATE_DECL))
-	gcc_unreachable ();
+	return unify_invalid (explain_p);
 
       if (TREE_CODE (parm) == BOUND_TEMPLATE_TEMPLATE_PARM)
 	{
Index: gcc/testsuite/g++.dg/cpp1z/class-deduction48.C
===================================================================
--- gcc/testsuite/g++.dg/cpp1z/class-deduction48.C	(nonexistent)
+++ gcc/testsuite/g++.dg/cpp1z/class-deduction48.C	(working copy)
@@ -0,0 +1,20 @@
+// PR c++/84355 - ICE with failing template argument deduction
+// { dg-do compile }
+// { dg-options "-std=c++17" }
+
+template <typename T>
+struct A
+{
+  template <int>
+  struct B
+  {
+    B (T);
+  };
+
+  A () {
+    B b (0);   // { dg-error "deduction failed" }
+               // { dg-error "no matching function" "" { target *-*-* } .-1 }
+  }
+};
+
+A<int> a;