PR37556: Don't diagnose conflicts between instantiated unqualified

friend declarations and declarations found in inline namespaces within
the target context.

clang/lib/Sema/SemaTemplateInstantiateDecl.cpp

@@ -2053,6 +2053,13 @@ Decl *TemplateDeclInstantiator::VisitFunctionDecl(
     // typedef (C++ [dcl.typedef]p4).
     if (Previous.isSingleTagDecl())
       Previous.clear();
+
+    // Filter out previous declarations that don't match the scope. The only
+    // effect this has is to remove declarations found in inline namespaces
+    // for friend declarations with unqualified names.
+    SemaRef.FilterLookupForScope(Previous, DC, /*Scope*/ nullptr,
+                                 /*ConsiderLinkage*/ true,
+                                 QualifierLoc.hasQualifier());
   }
 
   SemaRef.CheckFunctionDeclaration(/*Scope*/ nullptr, Function, Previous,

clang/test/SemaTemplate/friend.cpp

@@ -122,3 +122,22 @@ namespace qualified_friend_finds_nothing {
   namespace N { void f(int); }
   B<int> bi; // ok?!
 }
+
+namespace PR37556 {
+  inline namespace N { int x1, x2, y1, y2; } // expected-note 2{{previous}}
+  struct X {
+    friend void x1(int);
+    friend void PR37556::x2(int); // expected-error {{different kind}}
+  };
+  template<typename T> struct Y {
+    friend void y1(T);
+    friend void PR37556::y2(T); // expected-error {{different kind}}
+  };
+  template struct Y<int>;
+  template<typename T> struct Z {
+    friend void z1(T);
+    friend void PR37556::z2(T); // expected-error {{does not match any}}
+  };
+  inline namespace N { int z1, z2; }
+  template struct Z<int>;
+}