Index: lib/Sema/SemaType.cpp
===================================================================
--- lib/Sema/SemaType.cpp
+++ lib/Sema/SemaType.cpp
@@ -1679,8 +1679,16 @@
     // C90 6.5.3 constraints: "The same type qualifier shall not appear more
     // than once in the same specifier-list or qualifier-list, either directly
     // or via one or more typedefs."
-    if (!S.getLangOpts().C99 && !S.getLangOpts().CPlusPlus
-        && TypeQuals & Result.getCVRQualifiers()) {
+    //
+    // C90 and GNU89 do not warn if the TST is from a typedef unless -pedantic
+    // was set. GNU89 additionally does not warn if the TST is from a typeof
+    // expression and -pedantic was not set.
+    if (!S.getLangOpts().C99 && !S.getLangOpts().CPlusPlus &&
+        TypeQuals & Result.getCVRQualifiers() &&
+        (S.Diags.getDiagnosticOptions().Pedantic ||
+         !(DS.getTypeSpecType() == DeclSpec::TST_typename ||
+           (S.getLangOpts().GNUMode &&
+            DS.getTypeSpecType() == DeclSpec::TST_typeofExpr)))) {
       if (TypeQuals & DeclSpec::TQ_const && Result.isConstQualified()) {
         S.Diag(DS.getConstSpecLoc(), diag::ext_duplicate_declspec)
           << "const";
Index: test/Sema/gnu89-const.c
===================================================================
--- /dev/null
+++ test/Sema/gnu89-const.c
@@ -0,0 +1,87 @@
+/*
+RUN: not %clang_cc1 %s -std=c89 -fsyntax-only 2>&1 | FileCheck -check-prefix=CHECK-C89 %s
+RUN: not %clang_cc1 %s -std=c89 -pedantic -fsyntax-only 2>&1 | FileCheck -check-prefix=CHECK-C89-PEDANTIC %s
+RUN: not %clang_cc1 %s -std=c99 -fsyntax-only 2>&1 | FileCheck -check-prefix=CHECK-C99 %s
+RUN: not %clang_cc1 %s -std=c99 -pedantic -fsyntax-only 2>&1 | FileCheck -check-prefix=CHECK-C99-PEDANTIC %s
+RUN: not %clang_cc1 %s -std=c11 -fsyntax-only 2>&1 | FileCheck -check-prefix=CHECK-C11 %s
+RUN: not %clang_cc1 %s -std=c11 -pedantic -fsyntax-only 2>&1 | FileCheck -check-prefix=CHECK-C11-PEDANTIC %s
+RUN: not %clang_cc1 %s -std=c17 -fsyntax-only 2>&1 | FileCheck -check-prefix=CHECK-C17 %s
+RUN: not %clang_cc1 %s -std=c17 -pedantic -fsyntax-only 2>&1 | FileCheck -check-prefix=CHECK-C17-PEDANTIC %s
+
+RUN: %clang_cc1 %s -std=gnu89 -fsyntax-only 2>&1 | FileCheck -check-prefix=CHECK-GNU89 %s
+RUN: %clang_cc1 %s -std=gnu89 -pedantic -fsyntax-only 2>&1 | FileCheck -check-prefix=CHECK-GNU89-PEDANTIC %s
+RUN: %clang_cc1 %s -std=gnu99 -fsyntax-only 2>&1 | FileCheck -check-prefix=CHECK-GNU99 %s
+RUN: %clang_cc1 %s -std=gnu99 -pedantic -fsyntax-only 2>&1 | FileCheck -check-prefix=CHECK-GNU99-PEDANTIC %s
+RUN: %clang_cc1 %s -std=gnu11 -fsyntax-only 2>&1 | FileCheck -check-prefix=CHECK-GNU11 %s
+RUN: %clang_cc1 %s -std=gnu11 -pedantic -fsyntax-only 2>&1 | FileCheck -check-prefix=CHECK-GNU11-PEDANTIC %s
+RUN: %clang_cc1 %s -std=gnu17 -fsyntax-only 2>&1 | FileCheck -check-prefix=CHECK-GNU17 %s
+RUN: %clang_cc1 %s -std=gnu17 -pedantic -fsyntax-only 2>&1 | FileCheck -check-prefix=CHECK-GNU17-PEDANTIC %s
+*/
+
+const const int c_i;
+/*
+CHECK-C89: 21:7: warning: duplicate 'const' declaration specifier
+CHECK-C89-PEDANTIC: 21:7: warning: duplicate 'const' declaration specifier
+CHECK-C99: 21:7: warning: duplicate 'const' declaration specifier
+CHECK-C99-PEDANTIC: 21:7: warning: duplicate 'const' declaration specifier
+CHECK-C11: 21:7: warning: duplicate 'const' declaration specifier
+CHECK-C11-PEDANTIC: 21:7: warning: duplicate 'const' declaration specifier
+CHECK-C17: 21:7: warning: duplicate 'const' declaration specifier
+CHECK-C17-PEDANTIC: 21:7: warning: duplicate 'const' declaration specifier
+
+CHECK-GNU89: 21:7: warning: duplicate 'const' declaration specifier
+CHECK-GNU89-PEDANTIC: 21:7: warning: duplicate 'const' declaration specifier
+CHECK-GNU99: 21:7: warning: duplicate 'const' declaration specifier
+CHECK-GNU99-PEDANTIC: 21:7: warning: duplicate 'const' declaration specifier
+CHECK-GNU11: 21:7: warning: duplicate 'const' declaration specifier
+CHECK-GNU11-PEDANTIC: 21:7: warning: duplicate 'const' declaration specifier
+CHECK-GNU17: 21:7: warning: duplicate 'const' declaration specifier
+CHECK-GNU17-PEDANTIC: 21:7: warning: duplicate 'const' declaration specifier
+*/
+
+typedef const int t;
+const t c_i2;
+/*
+CHECK-C89-NOT: 43:1: warning: duplicate 'const' declaration specifier
+CHECK-C89-PEDANTIC: 43:1: warning: duplicate 'const' declaration specifier
+          ^ NOTE: special case
+CHECK-C99-NOT: 43:1: warning: duplicate 'const' declaration specifier
+CHECK-C99-PEDANTIC-NOT: 43:1: warning: duplicate 'const' declaration specifier
+CHECK-C11-NOT: 43:1: warning: duplicate 'const' declaration specifier
+CHECK-C11-PEDANTIC-NOT: 43:1: warning: duplicate 'const' declaration specifier
+CHECK-C17-NOT: 43:1: warning: duplicate 'const' declaration specifier
+CHECK-C17-PEDANTIC-NOT: 43:1: warning: duplicate 'const' declaration specifier
+
+CHECK-GNU89-NOT: 43:1: warning: duplicate 'const' declaration specifier
+CHECK-CNU89-PEDANTIC: 43:1: warning: duplicate 'const' declaration specifier
+CHECK-CNU99-NOT: 43:1: warning: duplicate 'const' declaration specifier
+CHECK-CNU99-PEDANTIC-NOT: 43:1: warning: duplicate 'const' declaration specifier
+CHECK-CNU11-NOT: 43:1: warning: duplicate 'const' declaration specifier
+CHECK-CNU11-PEDANTIC-NOT: 43:1: warning: duplicate 'const' declaration specifier
+CHECK-CNU17-NOT: 43:1: warning: duplicate 'const' declaration specifier
+CHECK-CNU17-PEDANTIC-NOT: 43:1: warning: duplicate 'const' declaration specifier
+*/
+
+const int c_i3;
+const typeof(c_i) c_i4;
+/*
+CHECK-C89: 66:19: error: expected function body after function declarator
+CHECK-C89-PEDANTIC: 66:19: error: expected function body after function declarator
+CHECK-C99: 66:19: error: expected function body after function declarator
+CHECK-C99-PEDANTIC: 66:19: error: expected function body after function declarator
+CHECK-C11: 66:19: error: expected function body after function declarator
+CHECK-C11-PEDANTIC: 66:19: error: expected function body after function declarator
+CHECK-C17: 66:19: error: expected function body after function declarator
+CHECK-C17-PEDANTIC: 66:19: error: expected function body after function declarator
+
+CHECK-GNU89-NOT: 66:1: warning: duplicate 'const' declaration specifier
+            ^ NOTE: special case
+CHECK-GNU89-PEDANTIC: 66:1: warning: duplicate 'const' declaration specifier
+            ^ NOTE: special case
+CHECK-GNU99-NOT: 66:1: warning: duplicate 'const' declaration specifier
+CHECK-GNU99-PEDANTIC-NOT: 66:1: warning: duplicate 'const' declaration specifier
+CHECK-GNU11-NOT: 66:1: warning: duplicate 'const' declaration specifier
+CHECK-GNU11-PEDANTIC-NOT: 66:1: warning: duplicate 'const' declaration specifier
+CHECK-GNU17-NOT: 66:1: warning: duplicate 'const' declaration specifier
+CHECK-GNU17-PEDANTIC-NOT: 66:1: warning: duplicate 'const' declaration specifier
+*/