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

[Erich Keane via cfe-commits]

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@@ -4692,6 +4692,17 @@ 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
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erichkeane wrote:

We attempt to try to recover whenever possible, as that results in a more 'correct' AST for tooling and for us evaluating it later.  That said, we do have `RecoveryExpr` for cases where we can't really do anything with it, but it needs a bit of testing for every new 'thing', because it is always dependent and that causes consternation with some sections of code.

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