[c++17] Fix assertion on synthesizing deduction guides after a fatal error.

[vsapsai]

Summary

After a fatal error Sema::InstantiatingTemplate doesn't allow further
instantiation and doesn't push a CodeSynthesisContext. When we tried to
synthesize implicit deduction guides from constructors we hit the
assertion

Assertion failed: (!CodeSynthesisContexts.empty() && "Cannot perform an instantiation without some context on the " "instantiation stack"), function SubstType, file clang/lib/Sema/SemaTemplateInstantiate.cpp, line 1580.

Fix by avoiding deduction guide synthesis if InstantiatingTemplate is invalid.

rdar://problem/39051732

[vsapsai]

Ping.

[rsmith]

LGTM, thanks!

[vsapsai]

Thanks for the review.

clang/lib/Sema/SemaTemplate.cpp

void Sema::DeclareImplicitDeductionGuides(TemplateDecl *Template,
   // In case we were expanding a pack when we attempted to declare deduction
   // guides, turn off pack expansion for everything we're about to do.
   ArgumentPackSubstitutionIndexRAII SubstIndex(*this, -1);
   // Create a template instantiation record to track the "instantiation" of
   // constructors into deduction guides.
   // FIXME: Add a kind for this to give more meaningful diagnostics. But can
   // this substitution process actually fail?
   InstantiatingTemplate BuildingDeductionGuides(*this, Loc, Template);
+  if (BuildingDeductionGuides.isInvalid())
+    return;
 
   // Convert declared constructors into deduction guide templates.
   // FIXME: Skip constructors for which deduction must necessarily fail (those
   // for which some class template parameter without a default argument never
   // appears in a deduced context).
   bool AddedAny = false;
   for (NamedDecl *D : LookupConstructors(Transform.Primary)) {
     D = D->getUnderlyingDecl();

clang/test/SemaTemplate/instantiate-after-fatal-cxx17.cpp

+// RUN: not %clang_cc1 -std=c++17 -fsyntax-only -ferror-limit 1 %s 2>&1 | FileCheck %s
+
+#error Error 1
+#error Error 2
+// CHECK: fatal error: too many errors emitted, stopping now
+
+namespace rdar39051732 {
+
+  template<class T> struct A {
+    template <class U> A(T&, ...);
+  };
+  // Deduction guide triggers constructor instantiation.
+  template<class T> A(const T&, const T&) -> A<T&>;
+
+}
+