Look through typedefs in getFunctionTypeWithExceptionSpec

[sberg]

Summary

Fixes https://bugs.llvm.org/show_bug.cgi?id=40658

[sberg]

The change itself should probably be uncontroversial (the bad cast had been there ever since getFunctionTypeWithExceptionSpec had been introduced with r221918), but I'm not sure about the test: It tests the relevant code somewhat indirectly; is it fine in clang/test/AST/?; or is such a test even overkill?

[sberg]

committed for now to get the crash fixed; if there are issues with the test they can be addressed later

lib/AST/ASTContext.cpp

QualType ASTContext::getFunctionTypeWithExceptionSpec(
   if (const auto *AT = dyn_cast<AttributedType>(Orig))
     return getAttributedType(
         AT->getAttrKind(),
         getFunctionTypeWithExceptionSpec(AT->getModifiedType(), ESI),
         getFunctionTypeWithExceptionSpec(AT->getEquivalentType(), ESI));
 
   // Anything else must be a function type. Rebuild it with the new exception
   // specification.
-  const auto *Proto = cast<FunctionProtoType>(Orig);
+  const auto *Proto = Orig->getAs<FunctionProtoType>();
   return getFunctionType(
       Proto->getReturnType(), Proto->getParamTypes(),
       Proto->getExtProtoInfo().withExceptionSpec(ESI));
 }
 
 bool ASTContext::hasSameFunctionTypeIgnoringExceptionSpec(QualType T,
                                                           QualType U) {
   return hasSameType(T, U) ||

test/AST/function-alias.cpp

+// RUN: %clang_cc1 -std=c++17 -fsyntax-only %s
+
+// Verify that ASTContext::getFunctionTypeWithExceptionSpec (called through
+// ASTContext::hasSameFunctionTypeIgnoringExceptionSpec from
+// ExprEvaluatorBase::handleCallExpr in lib/AST/ExprConstant.cpp) does not crash
+// for a type alias.
+
+constexpr int f() noexcept { return 0; }
+
+using F = int();
+
+constexpr int g(F * p) { return p(); }
+
+constexpr int n = g(f);