c++: deducing only from noexcept-spec [PR80951]

The "fail-fast" predicate uses_deducible_template_parms used by
unify_one_argument is neglecting to look inside the noexcept-spec of a
function type.  This causes deduction to spuriously fail whenever the
noexcept-spec is the only part of the type which contains a deducible
template parameter.

	PR c++/80951

gcc/cp/ChangeLog:

	* pt.cc (uses_deducible_template_parms): Consider the
	noexcept-spec of a function type.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1z/noexcept-type25.C: New test.

gcc/cp/pt.cc

@@ -22449,6 +22449,11 @@ uses_deducible_template_parms (tree type)
       for (; parm; parm = TREE_CHAIN (parm))
 	if (uses_deducible_template_parms (TREE_VALUE (parm)))
 	  return true;
+      if (flag_noexcept_type
+	  && TYPE_RAISES_EXCEPTIONS (type)
+	  && TREE_PURPOSE (TYPE_RAISES_EXCEPTIONS (type))
+	  && deducible_expression (TREE_PURPOSE (TYPE_RAISES_EXCEPTIONS (type))))
+	return true;
     }
 
   return false;

gcc/testsuite/g++.dg/cpp1z/noexcept-type25.C

@@ -0,0 +1,13 @@
+// PR c++/80951
+// { dg-do compile { target c++17 } }
+
+void f() noexcept;
+void g();
+
+template<bool E>
+constexpr bool h(void (*)() noexcept(E)) {
+  return E;
+}
+
+static_assert(h(f));
+static_assert(!h(g));