[clang] Fix crash when creating deduction guide.

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

Differential Revision: https://reviews.llvm.org/D97965

clang/lib/Sema/SemaTemplate.cpp

@@ -2494,6 +2494,12 @@ void Sema::DeclareImplicitDeductionGuides(TemplateDecl *Template,
     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;
   }

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

@@ -416,6 +416,17 @@ 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 {