Clang is crashing after generating the right diagnostic for a re-declaration of a friend method - Fix for PR47544.

clang/lib/Parse/ParseCXXInlineMethods.cpp

if (Toks) {
       // Skip through until we reach the 'end of default argument' token.
       while (Tok.isNot(tok::eof))
         ConsumeAnyToken();
 
       if (Tok.is(tok::eof) && Tok.getEofData() == Param)
         ConsumeAnyToken();
     } else if (HasUnparsed) {
       assert(Param->hasInheritedDefaultArg());
-      FunctionDecl *Old = cast<FunctionDecl>(LM.Method)->getPreviousDecl();
-      ParmVarDecl *OldParam = Old->getParamDecl(I);
-      assert (!OldParam->hasUnparsedDefaultArg());
+      const FunctionDecl *Old;
+      if (const auto *FunTmpl =
+              dyn_cast<FunctionTemplateDecl>(LM.Method))
+        Old =
+            cast<FunctionDecl>(FunTmpl->getTemplatedDecl())->getPreviousDecl();
+      else
+        Old = cast<FunctionDecl>(LM.Method)->getPreviousDecl();
+      if (Old) {
+        ParmVarDecl *OldParam = const_cast<ParmVarDecl*>(Old->getParamDecl(I));
+        assert(!OldParam->hasUnparsedDefaultArg());
         if (OldParam->hasUninstantiatedDefaultArg())
           Param->setUninstantiatedDefaultArg(
               OldParam->getUninstantiatedDefaultArg());
         else
           Param->setDefaultArg(OldParam->getInit());
       }
     }
+  }
 
   // Parse a delayed exception-specification, if there is one.
   if (CachedTokens *Toks = LM.ExceptionSpecTokens) {
     ParenBraceBracketBalancer BalancerRAIIObj(*this);
 
     // Add the 'stop' token.
     Token LastExceptionSpecToken = Toks->back();
     Token ExceptionSpecEnd;

clang/test/SemaCXX/invalid-decl.cpp

+// RUN: %clang_cc1 -fsyntax-only -verify %s
+
+class test1 {
+  template<typename> friend int bar(bool = true) {} // expected-note {{previous declaration is here}}
+  template<typename> friend int bar(bool); // expected-error {{friend declaration specifying a default argument must be the only declaration}}
+};
+
+class test2 {
+  friend int bar(bool = true) {} // expected-note {{previous declaration is here}}
+  friend int bar(bool); // expected-error{{friend declaration specifying a default argument must be the only declaration}}
+};