Bugfix #34804 A universal catch should be able to deal with a passthrough throw.

[sgolemon]

Summary

Given a function such as:

void foo() noexcept try {
  throw;
} catch(...) {
}

The clang-5.0 and later behavior is to assume the throw can't be
caught because its child element is checked first. This is incorrect.

We already have logic checking for a catch-all block which can
cope with this, so just move that higher in the priority.

[sgolemon]

Fixed in parallel by https://reviews.llvm.org/D39013

clang/lib/Sema/AnalysisBasedWarnings.cpp

 enum ThrowState {
   FoundNoPathForThrow,
   FoundPathForThrow,
   FoundPathWithNoThrowOutFunction,
 };
 
 static bool isThrowCaught(const CXXThrowExpr *Throw,
                           const CXXCatchStmt *Catch) {
+  const Type *CaughtType = Catch->getCaughtType().getTypePtrOrNull();
+  if (!CaughtType)
+    return true;
   const Type *ThrowType = nullptr;
   if (Throw->getSubExpr())
     ThrowType = Throw->getSubExpr()->getType().getTypePtrOrNull();
   if (!ThrowType)
     return false;
-  const Type *CaughtType = Catch->getCaughtType().getTypePtrOrNull();
-  if (!CaughtType)
-    return true;
   if (ThrowType->isReferenceType())
     ThrowType = ThrowType->castAs<ReferenceType>()
                     ->getPointeeType()
                     ->getUnqualifiedDesugaredType();
   if (CaughtType->isReferenceType())
     CaughtType = CaughtType->castAs<ReferenceType>()
                      ->getPointeeType()
                      ->getUnqualifiedDesugaredType();

clang/test/SemaCXX/warn-throw-out-noexcept-func.cpp

 }
 void n_ShouldNotDiag() noexcept {
   try {
     S s = S{};
     throw s;
   } catch (const S &s) {
   }
 }
-void o_ShouldDiag() noexcept { //expected-note {{function declared non-throwing here}}
+void o_ShouldNotDiag() noexcept {
   try {
-    throw; //expected-warning {{has a non-throwing exception specification but}}
+    throw;
   } catch (...) {
   }
 }
 
 #define NOEXCEPT noexcept
 void with_macro() NOEXCEPT { //expected-note {{function declared non-throwing here}}
   throw 1; // expected-warning {{has a non-throwing exception specification but}}
 }