[clang] [Clang] Ensure default arguments in friend declarations are only allowed in defining declarations to prevent multiple reachable declarations (PR #113777)

[Oleksandr T. via cfe-commits]

================
@@ -3430,15 +3430,17 @@ bool Sema::SubstDefaultArgument(
     ContextRAII SavedContext(*this, FD);
     std::unique_ptr<LocalInstantiationScope> LIS;
 
-    if (ForCallExpr) {
+    FunctionDecl *PatternFD =
+        ForCallExpr
+            ? FD->getTemplateInstantiationPattern(/*ForDefinition*/ false)
+            : nullptr;
+    if (PatternFD) {
       // When instantiating a default argument due to use in a call expression,
       // an instantiation scope that includes the parameters of the callee is
       // required to satisfy references from the default argument. For example:
       //   template<typename T> void f(T a, int = decltype(a)());
       //   void g() { f(0); }
       LIS = std::make_unique<LocalInstantiationScope>(*this);
-      FunctionDecl *PatternFD = FD->getTemplateInstantiationPattern(
-          /*ForDefinition*/ false);
----------------
a-tarasyuk wrote:

@zyn0217 @mizvekov I've moved the condition to `InstantiateDefaultArgument` and 
added an additional test. Do the recent updates look acceptable?


https://github.com/llvm/llvm-project/pull/113777
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