Fix crash when using __has_nothrow_copy with inherited constructors

[blastrock]

Summary

Fixes PR30491

[blastrock]

I'm new to this and I'm not sure at all this is the correct way to fix it. Any input is welcome :)

[blastrock]

I got what "shadows" means, this patch can handle a UsingDecl with multiple shadows.

[rsmith]

Do we still need this after D23765 lands?

[blastrock]

No, D23765 seems to fix this issue.

lib/Sema/SemaExprCXX.cpp

         // resolution point.
         if (isa<FunctionTemplateDecl>(ND))
           continue;
-        const CXXConstructorDecl *Constructor = cast<CXXConstructorDecl>(ND);
+        const CXXConstructorDecl *Constructor;
+        // handle inherited constructors
+        if (isa<UsingDecl>(ND))
+        {
+          const auto *UD = cast<UsingDecl>(ND);
+          assert(UD->shadow_size() == 1);
+          Constructor =
+              cast<CXXConstructorDecl>(UD->shadow_begin()->getTargetDecl());
+        }
+        else
+          Constructor = cast<CXXConstructorDecl>(ND);
         if (Constructor->isCopyConstructor(FoundTQs)) {
           FoundConstructor = true;
           const FunctionProtoType *CPT

test/CXX/special/class.copy/p34-cxx11.cpp

+// RUN: %clang_cc1 -fsyntax-only -std=c++11 -verify %s
+// expected-no-diagnostics
+
+struct A {
+  A(A const &) noexcept(false) {}
+};
+
+struct B : public A {
+  using A::A;
+};
+
+static_assert(!__has_nothrow_copy(B), "has_nothrow_copy fails");