[clang] Do not crash on template specialization following a fatal error

[adamcz]

Summary

There was a missing isInvalid() check leading to an attempt to
instantiate template with an empty instantiation stack.

[adamcz]

minor changes to test

clang/lib/Sema/SemaTemplateDeduction.cpp

if (DeduceTemplateArgumentsByTypeMatch(S, P2->getTemplateParameters(),
                                          T2, T1, Info, Deduced, TDF_None,
                                          /*PartialOrdering=*/true))
     return false;
 
   SmallVector<TemplateArgument, 4> DeducedArgs(Deduced.begin(),
                                                Deduced.end());
   Sema::InstantiatingTemplate Inst(S, Info.getLocation(), P2, DeducedArgs,
                                    Info);
+  if (Inst.isInvalid())
+    return false;
+
   auto *TST1 = T1->castAs<TemplateSpecializationType>();
   bool AtLeastAsSpecialized;
   S.runWithSufficientStackSpace(Info.getLocation(), [&] {
     AtLeastAsSpecialized = !FinishTemplateArgumentDeduction(
         S, P2, /*IsPartialOrdering=*/true,
         TemplateArgumentList(TemplateArgumentList::OnStack,
                              TST1->template_arguments()),
         Deduced, Info);

clang/test/SemaCXX/template-specialization-fatal.cpp

+// RUN: %clang_cc1 -verify -fsyntax-only %s
+// Verify clang doesn't assert()-fail on template specialization happening after
+// fatal error.
+
+#include "not_found.h" // expected-error {{'not_found.h' file not found}}
+
+template <class A, class B, class = void>
+struct foo {};
+
+template <class A, class B>
+struct foo<A, B, decltype(static_cast<void (*)(B)>(0)(static_cast<A (*)()>(0)()))> {};