add 'viable'

Author: cor3ntin

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -5664,7 +5664,7 @@ def err_explicit_instantiation_not_known : Error<
   "explicit instantiation of %0 does not refer to a function template, "
   "variable template, member function, member class, or static data member">;
 def err_explicit_instantiation_no_candidate : Error<
-    "no candidate for explicit instantiation of %0">;
+    "no viable candidate for explicit instantiation of %0">;
 def note_explicit_instantiation_here : Note<
   "explicit instantiation refers here">;
 def err_explicit_instantiation_data_member_not_instantiated : Error<

clang/test/SemaTemplate/explicit-instantiation-cxx20.cpp

@@ -31,7 +31,7 @@ struct a {
     // expected-note@-1 {{candidate function}}
 };
 template void a<int>::no_candidate();
-// expected-error@-1 {{no candidate for explicit instantiation of 'no_candidate'}}
+// expected-error@-1 {{no viable candidate for explicit instantiation of 'no_candidate'}}
 
 template void a<int>::ambiguous();
 // expected-error@-1 {{partial ordering for explicit instantiation of 'ambiguous' is ambiguous}}