Index: test/Sema/inline.c
===================================================================
--- test/Sema/inline.c	(revision 96477)
+++ test/Sema/inline.c	(working copy)
@@ -1,6 +1,22 @@
-// RUN: %clang_cc1 -fsyntax-only -verify %s
+// RUN: %clang_cc1 -std=gnu89 -fsyntax-only -verify %s
 
 // Check that we don't allow illegal uses of inline
 inline int a; // expected-error{{'inline' can only appear on functions}}
 typedef inline int b; // expected-error{{'inline' can only appear on functions}}
 int d(inline int a); // expected-error{{'inline' can only appear on functions}}
+
+// PR5253
+// GNU Extension: check that we can redefine an extern inline function
+extern inline int f(int a) {return a;}
+int f(int b) {return b;} // expected-note{{previous definition is here}}
+// And now check that we can't redefine a normal function
+int f(int c) {return c;} // expected-error{{redefinition of 'f'}}
+
+// Check that we can redefine an extern inline function as a static function
+extern inline int g(int a) {return a;}
+static int g(int b) {return b;}
+
+// Check that we ensure the types of the two definitions are the same
+extern inline int h(int a) {return a;} // expected-note{{previous definition is here}}
+int h(short b) {return b;}  // expected-error{{conflicting types for 'h'}}
+
Index: lib/Sema/SemaDecl.cpp
===================================================================
--- lib/Sema/SemaDecl.cpp	(revision 96477)
+++ lib/Sema/SemaDecl.cpp	(working copy)
@@ -956,9 +956,14 @@
   QualType OldQType = Context.getCanonicalType(Old->getType());
   QualType NewQType = Context.getCanonicalType(New->getType());
 
+  // Don't complain about this if we're in GNU89 mode and the old function
+  // is an extern inline function.
   if (!isa<CXXMethodDecl>(New) && !isa<CXXMethodDecl>(Old) &&
       New->getStorageClass() == FunctionDecl::Static &&
-      Old->getStorageClass() != FunctionDecl::Static) {
+      Old->getStorageClass() != FunctionDecl::Static &&
+      !(LangOpts.GNUMode && !LangOpts.C99 && !LangOpts.CPlusPlus &&
+        Old->isInlineSpecified() &&
+        Old->getStorageClass() == FunctionDecl::Extern)) {
     Diag(New->getLocation(), diag::err_static_non_static)
       << New;
     Diag(Old->getLocation(), PrevDiag);
@@ -4062,8 +4067,13 @@
   CurFunctionNeedsScopeChecking = false;
 
   // See if this is a redefinition.
+  // But don't complain if we're in GNU89 mode and the previous definition
+  // was an extern inline function.
   const FunctionDecl *Definition;
-  if (FD->getBody(Definition)) {
+  if (FD->getBody(Definition) &&
+      !(LangOpts.GNUMode && !LangOpts.C99 && !LangOpts.CPlusPlus &&
+        Definition->isInlineSpecified() &&
+        Definition->getStorageClass() == FunctionDecl::Extern)) {
     Diag(FD->getLocation(), diag::err_redefinition) << FD->getDeclName();
     Diag(Definition->getLocation(), diag::note_previous_definition);
   }
