[clang] [clang] Preserve UDL nodes in RemoveNestedImmediateInvocation (PR #66641)

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https://github.com/zyn0217 created https://github.com/llvm/llvm-project/pull/66641

D63960 performs a tree transformation on AST to prevent evaluating constant expressions eagerly by removing recorded immediate consteval invocations from subexpressions. (See https://reviews.llvm.org/D63960#inline-600736 for its motivation.)

However, the UDL node has been replaced with a CallExpr in the default TreeTransform since ca844ab0 (inadvertently?). This confuses clangd as it relies on the exact AST node type to decide whether or not to present inlay hints for an expression.

With regard to the fix, I think it's enough to return the UDL expression as-is during the transformation: We've bound it to temporary in its construction, and there's no ConstantExpr to visit under a UDL.

Fixes https://github.com/llvm/llvm-project/issues/63898.

>From ba9aa0ffe006ecdd8685c7e62be8c4c776391ec2 Mon Sep 17 00:00:00 2001
From: Younan Zhang <zyn7109 at gmail.com>
Date: Mon, 18 Sep 2023 19:55:45 +0800
Subject: [PATCH] [clang] Preserve UDL nodes in RemoveNestedImmediateInvocation

D63960 performs a tree transformation on AST to prevent evaluating
constant expressions eagerly by removing recorded immediate consteval
invocations from subexpressions. (See https://reviews.llvm.org/D63960#inline-600736
for its motivation.)

However, the UDL node has been replaced with a CallExpr in the default
TreeTransform since ca844ab0 (inadvertently?). This confuses clangd
as it relies on the exact AST node type to decide whether or not to
present inlay hints for an expression.

With regard to the fix, I think it's enough to return the UDL expression
as-is during the transformation: We've bound it to temporary
in its construction, and there's no ConstantExpr to visit under a UDL.

Fixes https://github.com/llvm/llvm-project/issues/63898.
---
 clang/lib/Sema/SemaExpr.cpp               |  5 ++++-
 clang/test/AST/ast-dump-udl-consteval.cpp | 17 +++++++++++++++++
 2 files changed, 21 insertions(+), 1 deletion(-)
 create mode 100644 clang/test/AST/ast-dump-udl-consteval.cpp

diff --git a/clang/lib/Sema/SemaExpr.cpp b/clang/lib/Sema/SemaExpr.cpp
index 92496b03ecabe54..104da822aae2930 100644
--- a/clang/lib/Sema/SemaExpr.cpp
+++ b/clang/lib/Sema/SemaExpr.cpp
@@ -18438,7 +18438,10 @@ static void RemoveNestedImmediateInvocation(
       DRSet.erase(cast<DeclRefExpr>(E->getCallee()->IgnoreImplicit()));
       return Base::TransformCXXOperatorCallExpr(E);
     }
-    /// Base::TransformInitializer skip ConstantExpr so we need to visit them
+    /// Base::TransformUserDefinedLiteral doesn't preserve the
+    /// UserDefinedLiteral node.
+    ExprResult TransformUserDefinedLiteral(UserDefinedLiteral *E) { return E; }
+    /// Base::TransformInitializer skips ConstantExpr so we need to visit them
     /// here.
     ExprResult TransformInitializer(Expr *Init, bool NotCopyInit) {
       if (!Init)
diff --git a/clang/test/AST/ast-dump-udl-consteval.cpp b/clang/test/AST/ast-dump-udl-consteval.cpp
new file mode 100644
index 000000000000000..9da53f725172aba
--- /dev/null
+++ b/clang/test/AST/ast-dump-udl-consteval.cpp
@@ -0,0 +1,17 @@
+// RUN: %clang_cc1 -xc++ -std=c++23 -ast-dump %s | FileCheck %s
+
+int inline consteval operator""_u32(unsigned long long val) {
+  return val;
+}
+
+void udl() {
+  (void)(0_u32 + 1_u32);
+}
+
+// CHECK: `-BinaryOperator {{.+}} <col:10, col:18> 'int' '+'
+// CHECK-NEXT: |-ConstantExpr {{.+}} <col:10> 'int'
+// CHECK-NEXT: | |-value: Int 0
+// CHECK-NEXT: | `-UserDefinedLiteral {{.+}} <col:10> 'int'
+// CHECK: `-ConstantExpr {{.+}} <col:18> 'int'
+// CHECK-NEXT:   |-value: Int 1
+// CHECK-NEXT:   `-UserDefinedLiteral {{.+}} <col:18> 'int'