[clang] [Clang] skip default argument instantiation for non-defining friend declarations to meet [dcl.fct.default] p4 (PR #115487)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Oleksandr T. (a-tarasyuk)

<details>
<summary>Changes</summary>

Fixes #<!-- -->113324
---

https://github.com/llvm/llvm-project/pull/113777#issuecomment-2463960195
https://github.com/llvm/llvm-project/pull/113777#issuecomment-2464234180



---
Full diff: https://github.com/llvm/llvm-project/pull/115487.diff


3 Files Affected:

- (modified) clang/docs/ReleaseNotes.rst (+2) 
- (modified) clang/lib/Sema/SemaTemplateInstantiateDecl.cpp (+11) 
- (modified) clang/test/CXX/temp/temp.res/p4.cpp (+43) 


``````````diff
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index d4bf05651a63eb..a3bce0e77581fb 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -641,6 +641,8 @@ Bug Fixes to C++ Support
   an implicitly instantiated class template specialization. (#GH51051)
 - Fixed an assertion failure caused by invalid enum forward declarations. (#GH112208)
 - Name independent data members were not correctly initialized from default member initializers. (#GH114069)
+- Fixed an assertion failure caused by invalid default argument substitutions in non-defining
+  friend declarations. (#GH113324).
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
index 5a001843e2ba46..0bbab95001ad8e 100644
--- a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -4694,6 +4694,17 @@ bool Sema::InstantiateDefaultArgument(SourceLocation CallLoc, FunctionDecl *FD,
                                       ParmVarDecl *Param) {
   assert(Param->hasUninstantiatedDefaultArg());
 
+  // FIXME: We don't track member specialization info for non-defining
+  // friend declarations, so we will not be able to later find the function
+  // pattern. As a workaround, don't instantiate the default argument in this
+  // case. This is correct per wording and only an error recovery issue, as per
+  // [dcl.fct.default]p4:
+  //   if a friend declaration D specifies a default argument expression,
+  //   that declaration shall be a definition.
+  if (FD->getFriendObjectKind() != Decl::FOK_None &&
+      !FD->getTemplateInstantiationPattern())
+    return true;
+
   // Instantiate the expression.
   //
   // FIXME: Pass in a correct Pattern argument, otherwise
diff --git a/clang/test/CXX/temp/temp.res/p4.cpp b/clang/test/CXX/temp/temp.res/p4.cpp
index f54d8649f5da88..cf6c45b4c351c5 100644
--- a/clang/test/CXX/temp/temp.res/p4.cpp
+++ b/clang/test/CXX/temp/temp.res/p4.cpp
@@ -185,3 +185,46 @@ template<typename T> struct S {
   friend void X::f(T::type);
 };
 }
+
+namespace GH113324 {
+template <typename = int> struct S1 {
+  friend void f1(S1, int = 0); // expected-error {{friend declaration specifying a default argument must be a definition}}
+  friend void f2(S1 a, S1 = decltype(a){}); // expected-error {{friend declaration specifying a default argument must be a definition}}
+};
+
+template <class T> using alias = int;
+template <typename T> struct S2 {
+  // FIXME: We miss diagnosing the default argument instantiation failure
+  // (forming reference to void)
+  friend void f3(S2, int a = alias<T &>(1)); // expected-error {{friend declaration specifying a default argument must be a definition}}
+};
+
+struct S3 {
+  friend void f4(S3, int = 42) { }
+};
+
+template <bool, class> using __enable_if_t = int;
+template <int v> struct S4 {
+  static const int value = v;
+};
+struct S5 {
+  template <__enable_if_t<S4<true>::value, int> = 0>
+  S5(const char *);
+};
+struct S6 {
+  template <typename a, typename b>
+  friend void f5(int, S6, a, b, S5 = "") { }
+};
+
+void test() {
+  f1(S1<>{});
+  f2(S1<>{});
+  f3(S2<void>());
+
+  S3 s3;
+  f4(s3);
+
+  S6 s6;
+  auto result = f5(0, s6, [] {}, [] {}); // expected-error {{variable has incomplete type 'void}}
+}
+} // namespace GH113324

``````````

</details>


https://github.com/llvm/llvm-project/pull/115487