diff --git a/clang/include/clang/Basic/DiagnosticGroups.td b/clang/include/clang/Basic/DiagnosticGroups.td
--- a/clang/include/clang/Basic/DiagnosticGroups.td
+++ b/clang/include/clang/Basic/DiagnosticGroups.td
@@ -54,6 +54,7 @@
                                     CompoundTokenSplitBySpace]>;
 def CoroutineMissingUnhandledException :
   DiagGroup<"coroutine-missing-unhandled-exception">;
+def DeclarationAfterStatement : DiagGroup<"declaration-after-statement">;
 def DeprecatedExperimentalCoroutine :
   DiagGroup<"deprecated-experimental-coroutine">;
 def DeprecatedCoroutine :
diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -9862,7 +9862,9 @@
 
 def ext_mixed_decls_code : Extension<
   "ISO C90 forbids mixing declarations and code">,
-  InGroup<DiagGroup<"declaration-after-statement">>;
+  InGroup<DeclarationAfterStatement>;
+def warn_mixed_decls_code : Warning<"mixing declarations and code">,
+  InGroup<DeclarationAfterStatement>, DefaultIgnore;
 
 def err_non_local_variable_decl_in_for : Error<
   "declaration of non-local variable in 'for' loop">;
diff --git a/clang/lib/Sema/SemaStmt.cpp b/clang/lib/Sema/SemaStmt.cpp
--- a/clang/lib/Sema/SemaStmt.cpp
+++ b/clang/lib/Sema/SemaStmt.cpp
@@ -410,9 +410,12 @@
                                    ArrayRef<Stmt *> Elts, bool isStmtExpr) {
   const unsigned NumElts = Elts.size();
 
-  // If we're in C89 mode, check that we don't have any decls after stmts.  If
-  // so, emit an extension diagnostic.
-  if (!getLangOpts().C99 && !getLangOpts().CPlusPlus) {
+  // If we're in C89 mode or have -Wdeclaration-after-statement, check that we
+  // don't have any decls after stmts.  If so, emit an extension diagnostic.
+  const unsigned MixedDeclsCodeID = getLangOpts().C99
+                                        ? diag::warn_mixed_decls_code
+                                        : diag::ext_mixed_decls_code;
+  if (!getLangOpts().CPlusPlus && !Diags.isIgnored(MixedDeclsCodeID, L)) {
     // Note that __extension__ can be around a decl.
     unsigned i = 0;
     // Skip over all declarations.
@@ -425,7 +428,7 @@
 
     if (i != NumElts) {
       Decl *D = *cast<DeclStmt>(Elts[i])->decl_begin();
-      Diag(D->getLocation(), diag::ext_mixed_decls_code);
+      Diag(D->getLocation(), MixedDeclsCodeID);
     }
   }
 
diff --git a/clang/test/Sema/warn-mixed-decls-code.c b/clang/test/Sema/warn-mixed-decls-code.c
new file mode 100644
--- /dev/null
+++ b/clang/test/Sema/warn-mixed-decls-code.c
@@ -0,0 +1,24 @@
+// RUN: %clang_cc1 -std=c89 -fsyntax-only -verify -Wall -Wno-comment -DSILENCE %s
+// RUN: %clang_cc1 -std=c99 -fsyntax-only -verify -Wall -pedantic -DSILENCE %s
+// RUN: %clang_cc1 -std=c11 -fsyntax-only -verify -Wall -pedantic -DSILENCE %s
+// RUN: %clang_cc1 -std=c89 -fsyntax-only -verify -Wno-comment -pedantic %s
+// RUN: %clang_cc1 -std=c89 -fsyntax-only -verify -Wno-comment -Wdeclaration-after-statement %s
+// RUN: %clang_cc1 -std=c99 -fsyntax-only -verify -Wdeclaration-after-statement %s
+// RUN: %clang_cc1 -std=c11 -fsyntax-only -verify -Wdeclaration-after-statement %s
+
+#ifdef SILENCE
+  // expected-no-diagnostics
+#endif
+
+void foo(void);
+
+void test1(void) {
+  int a;
+  foo();
+  int b;
+#ifndef SILENCE
+  // expected-warning@-2 {{mixing declarations and code}}
+#endif
+  (void)a;
+  (void)b;
+}