[clang] [clang][bytecode] Prefer ParmVarDecls as function parameters (PR #153952)

[Timm Baeder via cfe-commits]

https://github.com/tbaederr created https://github.com/llvm/llvm-project/pull/153952

We might create a local temporary variable for a ParmVarDecl, in which case a DeclRefExpr for that ParmVarDecl should _still_ result in us choosing the parameter, not that local.

>From bbf6cd50e1f41838605cfca1817dff43c08e3827 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Timm=20B=C3=A4der?= <tbaeder at redhat.com>
Date: Sat, 16 Aug 2025 15:07:58 +0200
Subject: [PATCH] [clang][bytecode] Prefer ParmVarDecls as function parameters

We might create a local temporary variable for a ParmVarDecl,
in which case a DeclRefExpr for that ParmVarDecl should _still_ result
in us choosing the parameter, not that local.
---
 clang/lib/AST/ByteCode/Compiler.cpp   | 30 ++++++++++++++-------------
 clang/test/AST/ByteCode/functions.cpp | 19 +++++++++++++++++
 2 files changed, 35 insertions(+), 14 deletions(-)

diff --git a/clang/lib/AST/ByteCode/Compiler.cpp b/clang/lib/AST/ByteCode/Compiler.cpp
index 8e651cf060620..6c6c8d41d3b93 100644
--- a/clang/lib/AST/ByteCode/Compiler.cpp
+++ b/clang/lib/AST/ByteCode/Compiler.cpp
@@ -6745,6 +6745,22 @@ bool Compiler<Emitter>::visitDeclRef(const ValueDecl *D, const Expr *E) {
   // value.
   bool IsReference = D->getType()->isReferenceType();
 
+  // Function parameters.
+  // Note that it's important to check them first since we might have a local
+  // variable created for a ParmVarDecl as well.
+  if (const auto *PVD = dyn_cast<ParmVarDecl>(D)) {
+    if (Ctx.getLangOpts().CPlusPlus && !Ctx.getLangOpts().CPlusPlus11 &&
+        !D->getType()->isIntegralOrEnumerationType()) {
+      return this->emitInvalidDeclRef(cast<DeclRefExpr>(E),
+                                      /*InitializerFailed=*/false, E);
+    }
+    if (auto It = this->Params.find(PVD); It != this->Params.end()) {
+      if (IsReference || !It->second.IsPtr)
+        return this->emitGetParam(classifyPrim(E), It->second.Offset, E);
+
+      return this->emitGetPtrParam(It->second.Offset, E);
+    }
+  }
   // Local variables.
   if (auto It = Locals.find(D); It != Locals.end()) {
     const unsigned Offset = It->second.Offset;
@@ -6762,20 +6778,6 @@ bool Compiler<Emitter>::visitDeclRef(const ValueDecl *D, const Expr *E) {
 
     return this->emitGetPtrGlobal(*GlobalIndex, E);
   }
-  // Function parameters.
-  if (const auto *PVD = dyn_cast<ParmVarDecl>(D)) {
-    if (Ctx.getLangOpts().CPlusPlus && !Ctx.getLangOpts().CPlusPlus11 &&
-        !D->getType()->isIntegralOrEnumerationType()) {
-      return this->emitInvalidDeclRef(cast<DeclRefExpr>(E),
-                                      /*InitializerFailed=*/false, E);
-    }
-    if (auto It = this->Params.find(PVD); It != this->Params.end()) {
-      if (IsReference || !It->second.IsPtr)
-        return this->emitGetParam(classifyPrim(E), It->second.Offset, E);
-
-      return this->emitGetPtrParam(It->second.Offset, E);
-    }
-  }
 
   // In case we need to re-visit a declaration.
   auto revisit = [&](const VarDecl *VD) -> bool {
diff --git a/clang/test/AST/ByteCode/functions.cpp b/clang/test/AST/ByteCode/functions.cpp
index 3c00de0102e5a..5272bedd037d1 100644
--- a/clang/test/AST/ByteCode/functions.cpp
+++ b/clang/test/AST/ByteCode/functions.cpp
@@ -713,3 +713,22 @@ namespace EnableIfWithTemporary {
   struct A { ~A(); };
   int &h() __attribute__((enable_if((A(), true), ""))); // both-warning {{clang extension}}
 }
+
+namespace LocalVarForParmVarDecl {
+  struct Iter {
+    void *p;
+  };
+  constexpr bool bar2(Iter A) {
+    return true;
+  }
+  constexpr bool bar(Iter A, bool b) {
+    if (b)
+      return true;
+
+    return bar(A, true);
+  }
+  consteval int foo() {
+    return bar(Iter(), false);
+  }
+  static_assert(foo());
+}