[Sema] add cast from IncompleteArrayType to ConstantArrayType in TryReferenceListInitialization

[HerrCai0907]

Fixed: #62945
c++20 supports "Permit conversions to arrays of unknown bound". This need additional cast from IncompleteArrayType to ConstantArrayType in TryReferenceListInitialization

[HerrCai0907]

from https://reviews.llvm.org/D151515

[llvmbot]

@llvm/pr-subscribers-clang

Changes

Fixed: #62945
c++20 supports "Permit conversions to arrays of unknown bound". This need additional cast from IncompleteArrayType to ConstantArrayType in TryReferenceListInitialization

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

2 Files Affected:

• (modified) clang/lib/Sema/SemaInit.cpp (+9)
• (modified) clang/test/CodeGenCXX/cxx20-p0388-unbound-ary.cpp (+9)

diff --git a/clang/lib/Sema/SemaInit.cpp b/clang/lib/Sema/SemaInit.cpp
index 93f05e2e47285e4..966d35226eec748 100644
--- a/clang/lib/Sema/SemaInit.cpp
+++ b/clang/lib/Sema/SemaInit.cpp
@@ -4532,6 +4532,15 @@ static void TryReferenceListInitialization(Sema &S,
       if (T1Quals.hasAddressSpace())
         Sequence.AddQualificationConversionStep(
             cv1T1, DestType->isRValueReferenceType() ? VK_XValue : VK_LValue);
+      else if (S.getLangOpts().CPlusPlus20 &&
+               isa(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);
diff --git a/clang/test/CodeGenCXX/cxx20-p0388-unbound-ary.cpp b/clang/test/CodeGenCXX/cxx20-p0388-unbound-ary.cpp
index 78f35a024a54014..a29f4d720c1de4e 100644
--- a/clang/test/CodeGenCXX/cxx20-p0388-unbound-ary.cpp
+++ b/clang/test/CodeGenCXX/cxx20-p0388-unbound-ary.cpp
@@ -23,4 +23,13 @@ auto &frob2(int (&arp)[1]) {
 
   return r2;
 }
+
+// CHECK-LABEL: @_ZN3One3fooEi
+// CHECK-NEXT: entry:
+// CHECK-NEXT: ret void
+void foo(int a) {
+  auto f = [](int(&&)[]) {};
+  f({a});
+}
+
 } // namespace One

[cor3ntin]

@Fznamznon

[Fznamznon]

This needs a release note.

cc @erichkeane

[Fznamznon]

Is qualification conversion right here? I'm not sure we convert qualifiers here.

[HerrCai0907]

I have changed sequence in the latest commit.
The reason to qualification conversion is that in cpp 20, spec. requires support cast from initializer list to reference to array of unknown bound.

for example:

void foo(int a) {
  auto f = [](int(&&)[]) {};
  f({a});
}

{a} should be casted from int[1] to int[].

ast will look like:

      `-CXXOperatorCallExpr 0x12b904d18 <col:3, col:8> 'void':'void' '()'
        |-ImplicitCastExpr 0x12b904c58 <col:4, col:8> 'void (*)(int (&&)[]) const' <FunctionToPointerDecay>
        | `-DeclRefExpr 0x12b904bd8 <col:4, col:8> 'void (int (&&)[]) const' lvalue CXXMethod 0x12b904008 'operator()' 'void (int (&&)[]) const'
        |-ImplicitCastExpr 0x12b904c70 <col:3> 'const (lambda at ID/test.cpp:2:12)' lvalue <NoOp>
        | `-DeclRefExpr 0x12b904b10 <col:3> '(lambda at ID/test.cpp:2:12)':'(lambda at ID/test.cpp:2:12)' lvalue Var 0x12b8ee210 'f' '(lambda at ID/test.cpp:2:12)':'(lambda at ID/test.cpp:2:12)'
        `-MaterializeTemporaryExpr 0x12b904d00 <col:5, col:7> 'int[]':'int[]' xvalue
          `-ImplicitCastExpr 0x12b904ce8 <col:5, col:7> 'int[]':'int[]' <NoOp>
            `-InitListExpr 0x12b904c88 <col:5, col:7> 'int[1]'
              `-ImplicitCastExpr 0x12b904cc8 <col:6> 'int' <LValueToRValue>
                `-DeclRefExpr 0x12b904b30 <col:6> 'int' lvalue ParmVar 0x12b8edfb8 'a' 'int'

[Fznamznon]

@HerrCai0907 I see what you mean and what the patch accomplishes. But my confusion was, why in order to do that AddQualificationConversionStep is called? IMO this is just to convert cv-qualifiers. The goal is to convert one array type to another, right?

[HerrCai0907]

Yes, int[1] to int[] in above example. Because spec. require in this case, type should be int[], but clang will treat initialize list as int[1], which causes crash when IR emit.

[efriedma-quic]

I guess this ends up producing a CK_NoOp CastExpr? That's probably okay, but can we amend the documentation for CK_NoOp to give this as an example?

[HerrCai0907]

I guess this ends up producing a CK_NoOp CastExpr? That's probably okay, but can we amend the documentation for CK_NoOp to give this as an example?

Done

[efriedma-quic]

LGTM