[Clang] Look through type sugar when accessing FunctionProtoType (#88428)

This fixes a bug introduced by #84473: if a lambda’s type is type sugar
(e.g. an `AttributedType`), we need to use `getAs()` instead of `cast()`
to retrieve the `FunctionProtoType`.

Author: Sirraide

clang/lib/Sema/SemaExpr.cpp

@@ -20704,7 +20704,7 @@ static void FixDependencyOfIdExpressionsInLambdaWithDependentObjectParameter(
       if (MD->getType().isNull())
         continue;
 
-      const auto *Ty = cast<FunctionProtoType>(MD->getType());
+      const auto *Ty = MD->getType()->getAs<FunctionProtoType>();
       if (!Ty || !MD->isExplicitObjectMemberFunction() ||
           !Ty->getParamType(0)->isDependentType())
         continue;

clang/lib/Sema/SemaExprCXX.cpp

@@ -1488,7 +1488,7 @@ void Sema::MarkThisReferenced(CXXThisExpr *This) {
         if (MD->getType().isNull())
           return false;
 
-        const auto *Ty = cast<FunctionProtoType>(MD->getType());
+        const auto *Ty = MD->getType()->getAs<FunctionProtoType>();
         return Ty && MD->isExplicitObjectMemberFunction() &&
                Ty->getParamType(0)->isDependentType();
       }

clang/test/SemaCXX/cxx2b-deducing-this.cpp

@@ -312,6 +312,18 @@ void TestMutationInLambda() {
       l1();
       l2();
     }
+
+    // Check that we don't crash if the lambda has type sugar.
+    const auto l15 = [=](this auto&&) [[clang::annotate_type("foo")]] [[clang::annotate_type("bar")]] {
+        return x;
+    };
+
+    const auto l16 = [=]() [[clang::annotate_type("foo")]] [[clang::annotate_type("bar")]] {
+        return x;
+    };
+
+    l15();
+    l16();
 }
 
 struct Over_Call_Func_Example {

clang/test/SemaCXX/lambda-expressions.cpp

@@ -762,3 +762,12 @@ template auto t::operator()<int>(int a) const; // expected-note {{in instantiati
 
 }
 #endif
+
+namespace GH84473_bug {
+void f1() {
+  int b;
+  (void) [=] [[gnu::regcall]] () { // expected-warning {{an attribute specifier sequence in this position is a C++23 extension}}
+    (void) b;
+  };
+}
+}