[clang][Sema] close IsStandardConversion hole when adding cfi_unchecked_callee (#164592)

Commit b194cf1 changed this function for the case where attribute
`cfi_unchecked_callee` is added in a function conversion. But this
introduces a hole (issue #162798), and it seems the change was
unnecessary: the preceding `TryFunctionConversion` will already allow
adding the `cfi_unchecked_callee` attribute, and will update `FromType`
if it succeeds. So we revert the changes to `IsStandardConversion`. We
also remove the helper function `AddingCFIUncheckedCallee` which is no
longer needed, and simplify the corresponding
`DiscardingCFIUncheckedCallee`.

Fixes: #162798

Author: brunodf-snps

clang/include/clang/Sema/Sema.h

@@ -2681,11 +2681,6 @@ class Sema final : public SemaBase {
   /// function without this attribute.
   bool DiscardingCFIUncheckedCallee(QualType From, QualType To) const;
 
-  /// Returns true if `From` is a function or pointer to a function without the
-  /// `cfi_unchecked_callee` attribute but `To` is a function or pointer to
-  /// function with this attribute.
-  bool AddingCFIUncheckedCallee(QualType From, QualType To) const;
-
   /// This function calls Action when it determines that E designates a
   /// misaligned member due to the packed attribute. This is used to emit
   /// local diagnostics like in reference binding.

clang/lib/Sema/SemaChecking.cpp

@@ -12373,14 +12373,9 @@ static void DiagnoseMixedUnicodeImplicitConversion(Sema &S, const Type *Source,
   }
 }
 
-enum CFIUncheckedCalleeChange {
-  None,
-  Adding,
-  Discarding,
-};
-
-static CFIUncheckedCalleeChange AdjustingCFIUncheckedCallee(QualType From,
-                                                            QualType To) {
+bool Sema::DiscardingCFIUncheckedCallee(QualType From, QualType To) const {
+  From = Context.getCanonicalType(From);
+  To = Context.getCanonicalType(To);
   QualType MaybePointee = From->getPointeeType();
   if (!MaybePointee.isNull() && MaybePointee->getAs<FunctionType>())
     From = MaybePointee;
@@ -12392,25 +12387,10 @@ static CFIUncheckedCalleeChange AdjustingCFIUncheckedCallee(QualType From,
     if (const auto *ToFn = To->getAs<FunctionType>()) {
       if (FromFn->getCFIUncheckedCalleeAttr() &&
           !ToFn->getCFIUncheckedCalleeAttr())
-        return Discarding;
-      if (!FromFn->getCFIUncheckedCalleeAttr() &&
-          ToFn->getCFIUncheckedCalleeAttr())
-        return Adding;
+        return true;
     }
   }
-  return None;
-}
-
-bool Sema::DiscardingCFIUncheckedCallee(QualType From, QualType To) const {
-  From = Context.getCanonicalType(From);
-  To = Context.getCanonicalType(To);
-  return ::AdjustingCFIUncheckedCallee(From, To) == Discarding;
-}
-
-bool Sema::AddingCFIUncheckedCallee(QualType From, QualType To) const {
-  From = Context.getCanonicalType(From);
-  To = Context.getCanonicalType(To);
-  return ::AdjustingCFIUncheckedCallee(From, To) == Adding;
+  return false;
 }
 
 void Sema::CheckImplicitConversion(Expr *E, QualType T, SourceLocation CC,

clang/lib/Sema/SemaOverload.cpp

@@ -2532,15 +2532,12 @@ static bool IsStandardConversion(Sema &S, Expr* From, QualType ToType,
 
   SCS.setToType(2, FromType);
 
-  // If we have not converted the argument type to the parameter type,
-  // this is a bad conversion sequence, unless we're resolving an overload in C.
-  //
-  // Permit conversions from a function without `cfi_unchecked_callee` to a
-  // function with `cfi_unchecked_callee`.
-  if (CanonFrom == CanonTo || S.AddingCFIUncheckedCallee(CanonFrom, CanonTo))
+  if (CanonFrom == CanonTo)
     return true;
 
-  if ((S.getLangOpts().CPlusPlus || !InOverloadResolution))
+  // If we have not converted the argument type to the parameter type,
+  // this is a bad conversion sequence, unless we're resolving an overload in C.
+  if (S.getLangOpts().CPlusPlus || !InOverloadResolution)
     return false;
 
   ExprResult ER = ExprResult{From};

clang/test/Frontend/cfi-unchecked-callee-attribute.cpp

@@ -9,6 +9,7 @@ void (*checked_ptr)(void) = unchecked;  // expected-warning{{implicit conversion
 void (CFI_UNCHECKED_CALLEE *unchecked_ptr)(void) = unchecked;
 void (CFI_UNCHECKED_CALLEE *from_normal)(void) = checked;
 void (CFI_UNCHECKED_CALLEE *c_no_function_decay)(void) = &unchecked;
+void (CFI_UNCHECKED_CALLEE __attribute__((noreturn)) *other_conflict)(void) = &checked; // expected-error{{cannot initialize a variable of type 'void (*)() __attribute__((noreturn)) __attribute__((cfi_unchecked_callee))' with an rvalue of type 'void (*)()'}}
 void (CFI_UNCHECKED_CALLEE *arr[10])(void);
 void (*cfi_elem)(void) = arr[1];  // expected-warning{{implicit conversion from 'void (*)() __attribute__((cfi_unchecked_callee))' to 'void (*)()' discards 'cfi_unchecked_callee' attribute}}
 void (CFI_UNCHECKED_CALLEE *cfi_unchecked_elem)(void) = arr[1];