[Sema] add cast from IncompleteArrayType to ConstantArrayType in TryReferenceListInitialization

[HerrCai0907]

Summary

Fixed: https://github.com/llvm/llvm-project/issues/62945
c++20 supports "Permit conversions to arrays of unknown bound".
This need additional cast from IncompleteArrayType to ConstantArrayType
in TryReferenceListInitialization

[shafik]

Thank you for the patch! Please apply clang-format at least one of pre-commit failures is due to that.

[rnk]

clang/lib/CodeGen/CGCall.cpp

4376 ↗

(On Diff #525854)

I think this discards too much information about pointer types. I think it would treat two pointers with interestingly different qualifiers differently. Consider, perhaps, a mismatch in pointer address spaces, which are passed differently (think __ptr32 __ptr64).

I think there is probably some other way to get the type of the parameter after pointer decay. I would ask @efriedma since he's the current IRGen owner or @rjmccall since I think he may have added this prototype assertion originally, I just moved the code around.

[efriedma]

Relevant bit of the AST:

`-ExprWithCleanups 0xd16a780 <line:6:3, col:8> 'void':'void'
  `-CXXOperatorCallExpr 0xd16a678 <col:3, col:8> 'void':'void' '()'
    |-ImplicitCastExpr 0xd16a5a8 <col:4, col:8> 'void (*)(int (&&)[]) const' <FunctionToPointerDecay>
    | `-DeclRefExpr 0xd16a528 <col:4, col:8> 'void (int (&&)[]) const' lvalue CXXMethod 0xd14f7f8 'operator()' 'void (int (&&)[]) const'
    |-ImplicitCastExpr 0xd16a5e8 <col:3> 'const (lambda at <stdin>:5:12)' lvalue <NoOp>
    | `-DeclRefExpr 0xd16a460 <col:3> '(lambda at <stdin>:5:12)':'(lambda at <stdin>:5:12)' lvalue Var 0xd14f628 'f' '(lambda at <stdin>:5:12)':'(lambda at <stdin>:5:12)'
    `-MaterializeTemporaryExpr 0xd16a660 <col:5, col:7> 'int[1]' xvalue
      `-InitListExpr 0xd16a600 <col:5, col:7> 'int[1]'
        `-ImplicitCastExpr 0xd16a640 <col:6> 'int' <LValueToRValue>
          `-DeclRefExpr 0xd16a480 <col:6> 'int' lvalue ParmVar 0xd14f3a8 'a' 'int'

At first glance, I'd consider this a problem with the AST: there should be a no-op cast from int[1] to int[]. We generate such a cast for the following:

void foo(int a) {
  auto f = [](int(&&)[]) {};
  int aa[1] = {a};
  f(static_cast<int(&&)[1]>(aa));
}

[rjmccall]

Yeah, I agree with Eli, there should be a cast here.

[HerrCai0907]

add cast

[erichkeane]

clang/lib/Sema/SemaInit.cpp

4433

Is VK_XValue correct here? The standards quote says it is a PRValue?

[HerrCai0907]

clang/lib/Sema/SemaInit.cpp

4433

The original text of standards in https://eel.is/c++draft/dcl.init.list is

List-initialization of an object or reference of type T is defined as follows:

3.10 Otherwise, if T is a reference type, a prvalue is generated. The prvalue initializes its result object by copy-list-initialization from the initializer list. The prvalue is then used to direct-initialize the reference. The type of the prvalue is the type referenced by T, unless T is “reference to array of unknown bound of U”, in which case the type of the prvalue is the type of x in the declaration U x[] H, where H is the initializer list.

My understand is that a PRValue is generated by XValue initialization list.

By the way, I have try to use VK_PRValue, it cause the assert failed:
can't implicitly cast glvalue to prvalue with this cast kind: NoOp

[HerrCai0907]

ping @erichkeane

[HerrCai0907]

https://github.com/llvm/llvm-project/pull/65918

clang/lib/Sema/SemaInit.cpp

static void TryReferenceListInitialization(Sema &S,
   if (Sequence) {
     if (DestType->isRValueReferenceType() ||
         (T1Quals.hasConst() && !T1Quals.hasVolatile())) {
       Sequence.AddReferenceBindingStep(cv1T1IgnoreAS,
                                        /*BindingTemporary=*/true);
       if (T1Quals.hasAddressSpace())
         Sequence.AddQualificationConversionStep(
             cv1T1, DestType->isRValueReferenceType() ? VK_XValue : VK_LValue);
+      else if (S.getLangOpts().CPlusPlus20 &&
+               isa<IncompleteArrayType>(T1->getUnqualifiedDesugaredType()) &&
+               DestType->isRValueReferenceType()) {
+        // [dcl.init.list] p3.10
+        // unless T is “reference to array of unknown bound of U”, in which case
+        // the type of the prvalue is the type of x in the declaration U x[] H,
+        // where H is the initializer list.
+        Sequence.AddQualificationConversionStep(cv1T1, VK_XValue);
+      }
     } else
       Sequence.SetFailed(
           InitializationSequence::FK_NonConstLValueReferenceBindingToTemporary);
   }
 }
 
 /// Attempt list initialization (C++0x [dcl.init.list])
 static void TryListInitialization(Sema &S,

clang/test/CodeGenCXX/cxx20-p0388-unbound-ary.cpp

 // CHECK-LABEL: @_ZN3One5frob2ERA1_i
 // CHECK-NEXT: entry:
 // CHECK-NEXT: ret ptr %arp
 auto &frob2(int (&arp)[1]) {
   int(&r2)[] = arp;
 
   return r2;
 }
+
+// CHECK-LABEL: @_ZN3One3fooEi
+// CHECK-NEXT: entry:
+// CHECK-NEXT: ret void
+void foo(int a) {
+  auto f = [](int(&&)[]) {};
+  f({a});
+}
+
 } // namespace One