[clang] Fix single-element array initialization in constexpr

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 llvm#60803

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D145486

(cherry picked from commit af682f0)

Author: Fznamznon

clang/lib/AST/ExprConstant.cpp

@@ -10913,7 +10913,7 @@ bool ArrayExprEvaluator::VisitCXXConstructExpr(const CXXConstructExpr *E,
         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);

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

@@ -0,0 +1,19 @@
+// RUN: %clang_cc1 -std=c++20 -verify %s
+
+// This test makes sure that a single element array doesn't produce
+// spurious errors during constexpr evaluation.
+
+// 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());