[clang] Fix crash when creating deduction guide.

[adamcz]

Summary

We used to trigger assertion when transforming c-tor with unparsed
default argument. Now we ignore such constructors for this purpose.

[adamcz]

format

[hokein]

Thanks!

clang/test/SemaCXX/cxx1z-class-template-argument-deduction.cpp

425

The diagnostic seems bogus, but I think it is at least better than crashing, I'd add add a FIXME to fix this bogus diagnostic here?.

[adamcz]

expanded a comment

clang/test/SemaCXX/cxx1z-class-template-argument-deduction.cpp

425

I added a line about improving the diagnostic. I'm a bit hesitant to leave a FIXME, since it doesn't seem like something actually worth spending time on. Just a weird corner case on broken code.

clang/lib/Sema/SemaTemplate.cpp

for (NamedDecl *D : LookupConstructors(Transform.Primary)) {
     auto *FTD = dyn_cast<FunctionTemplateDecl>(D);
     auto *CD =
         dyn_cast_or_null<CXXConstructorDecl>(FTD ? FTD->getTemplatedDecl() : D);
     // Class-scope explicit specializations (MS extension) do not result in
     // deduction guides.
     if (!CD || (!FTD && CD->isFunctionTemplateSpecialization()))
       continue;
 
+    // Cannot make a deduction guide when unparsed arguments are present.
+    if (std::any_of(CD->param_begin(), CD->param_end(), [](ParmVarDecl *P) {
+          return !P || P->hasUnparsedDefaultArg();
+        }))
+      continue;
+
     Transform.transformConstructor(FTD, CD);
     AddedAny = true;
   }
 
   // C++17 [over.match.class.deduct]
   //    --  If C is not defined or does not declare any constructors, an
   //    additional function template derived as above from a hypothetical
   //    constructor C().

clang/test/SemaCXX/cxx1z-class-template-argument-deduction.cpp

 template<typename U> struct B {
   B(U u, X<sizeof(u)>);
 };
 B b(0, {});
 }
 
 }
 
+namespace no_crash_on_default_arg {
+class A {
+  template <typename T> class B {
+    B(int c = 1);
+  };
+  // This used to crash due to unparsed default arg above. The diagnostic could
+  // be improved, but the point of this test is to simply check we do not crash.
+  B(); // expected-error {{deduction guide declaration without trailing return type}}
+};
+} // namespace no_crash_on_default_arg
+
 #pragma clang diagnostic push
 #pragma clang diagnostic warning "-Wctad-maybe-unsupported"
 namespace test_implicit_ctad_warning {
 
 template <class T>
 struct Tag {};
 
 template <class T>