[clang] [Clang] prevent crash in early parsing of enable_if for C function declarators with identifier-list parameters (PR #174017)

[via cfe-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Oleksandr T. (a-tarasyuk)

<details>
<summary>Changes</summary>

Fixes #<!-- -->173826

---

This patch resolves an issue where Clang could crash while parsing the `enable_if` attribute on C function declarators with identifier-list parameters.

In C, identifier-list function declarators use parameter identifiers rather than full parameter declarations, as specified by the C spec. Since `enable_if` is parsed early to participate in redeclaration checks

https://github.com/llvm/llvm-project/blob/0f3a9f658a7f459b9aa701ec42a999b645653862/clang/lib/Parse/ParseDecl.cpp#L674-L675

the parser could encounter such identifiers before their parameter declarations exist and incorrectly assume they were already formed, leading to an assertion failure.

This change makes the early `enable_if` parsing path handle parameters that have not yet been declared, preventing the crash while preserving existing behavior for well-formed function prototypes.

---
Full diff: https://github.com/llvm/llvm-project/pull/174017.diff


3 Files Affected:

- (modified) clang/docs/ReleaseNotes.rst (+1) 
- (modified) clang/lib/Parse/ParseDecl.cpp (+3-4) 
- (added) clang/test/Parser/enableif-attr.cpp (+4) 


``````````diff
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 2319ff13f7864..c55ea757d28cb 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -623,6 +623,7 @@ Bug Fixes to C++ Support
 - Fix the result of ``__is_pointer_interconvertible_base_of`` when arguments are qualified and passed via template parameters. (#GH135273)
 - Fixed a crash when evaluating nested requirements in requires-expressions that reference invented parameters. (#GH166325)
 - Fixed a crash when standard comparison categories (e.g. ``std::partial_ordering``) are defined with incorrect static member types. (#GH170015) (#GH56571)
+- Fixed a crash when parsing the ``enable_if`` attribute on C function declarations with identifier-list parameters. (#GH173826)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Parse/ParseDecl.cpp b/clang/lib/Parse/ParseDecl.cpp
index 8688ccf41acb5..0e2caa214f3fd 100644
--- a/clang/lib/Parse/ParseDecl.cpp
+++ b/clang/lib/Parse/ParseDecl.cpp
@@ -680,10 +680,9 @@ void Parser::ParseGNUAttributeArgs(
     PrototypeScope.emplace(this, Scope::FunctionPrototypeScope |
                                      Scope::FunctionDeclarationScope |
                                      Scope::DeclScope);
-    for (unsigned i = 0; i != FTI.NumParams; ++i) {
-      ParmVarDecl *Param = cast<ParmVarDecl>(FTI.Params[i].Param);
-      Actions.ActOnReenterCXXMethodParameter(getCurScope(), Param);
-    }
+    for (unsigned i = 0; i != FTI.NumParams; ++i)
+      Actions.ActOnReenterCXXMethodParameter(
+          getCurScope(), dyn_cast_or_null<ParmVarDecl>(FTI.Params[i].Param));
   }
 
   ParseAttributeArgsCommon(AttrName, AttrNameLoc, Attrs, EndLoc, ScopeName,
diff --git a/clang/test/Parser/enableif-attr.cpp b/clang/test/Parser/enableif-attr.cpp
new file mode 100644
index 0000000000000..6d80a5bd713ce
--- /dev/null
+++ b/clang/test/Parser/enableif-attr.cpp
@@ -0,0 +1,4 @@
+// RUN: %clang_cc1 -x c -fsyntax-only -verify %s
+
+void f1(x) __attribute__((enable_if(1, "")));    // expected-error {{a parameter list without types is only allowed in a function definition}}
+void f2(x, y) __attribute__((enable_if(1, ""))); // expected-error {{a parameter list without types is only allowed in a function definition}}

``````````

</details>


https://github.com/llvm/llvm-project/pull/174017