[clang] Fix single-element array initialization in constexpr

[Fznamznon]

Summary

https://reviews.llvm.org/D130791 added an improvement that in case array
element has a trivial constructor, it is evaluated once and the result is
re-used for remaining elements. Make sure the constructor is evaluated
for single-elements arrays too.

Fixes https://github.com/llvm/llvm-project/issues/60803

[Fznamznon]

Check FinalSize instead of N since it is probably more obvious

[aaron.ballman]

LGTM, thank you for this! Please be sure to file an issue to backport this to the 16.x branch (https://llvm.org/docs/GitHub.html#backporting-fixes-to-the-release-branches).

clang/test/SemaCXX/constexpr-single-element-array.cpp

3–6

Minor rewording and removing the extra / in the comment.

[Fznamznon]

Rebase and update test comment.

[Fznamznon]

Filed https://github.com/llvm/llvm-project/issues/61243 to backport.

clang/lib/AST/ExprConstant.cpp

for (const unsigned N : {1u, FinalSize}) {
         NewValue.getArrayInitializedElt(I).swap(
             Value->getArrayInitializedElt(I));
       Value->swap(NewValue);
 
       if (HadZeroInit)
         for (unsigned I = OldElts; I < N; ++I)
           Value->getArrayInitializedElt(I) = Filler;
 
-      if (HasTrivialConstructor && N == FinalSize) {
+      if (HasTrivialConstructor && N == FinalSize && FinalSize != 1) {
         // If we have a trivial constructor, only evaluate it once and copy
         // the result into all the array elements.
         APValue &FirstResult = Value->getArrayInitializedElt(0);
         for (unsigned I = OldElts; I < FinalSize; ++I)
           Value->getArrayInitializedElt(I) = FirstResult;
       } else {
         for (unsigned I = OldElts; I < N; ++I) {
           if (!VisitCXXConstructExpr(E, ArrayElt,

clang/test/SemaCXX/constexpr-single-element-array.cpp

+// RUN: %clang_cc1 -std=c++20 -verify %s
+
+/// This test makes sure that single element array in constexpr doesn't produce
+/// spurious errors.
+
+/// expected-no-diagnostics
+struct Sub { int x; };
+
+struct S {
+  constexpr S() { Arr[0] = Sub{}; }
+  Sub Arr[1];
+};
+
+constexpr bool test() {
+  S s;
+  return true;
+}
+
+static_assert(test());