On 16/02/16 22:20, Richard Smith wrote:
--- a/lib/Sema/SemaLookup.cpp
+++ b/lib/Sema/SemaLookup.cpp
@@ -419,6 +419,25 @@ static bool isPreferredLookupResult(Sema &S, 
Sema::LookupNameKind Kind,
      }
    }
+ // VarDecl can have incomplete array types, prefer the one with more complete
+  // array type.
+  if (VarDecl *DVD = dyn_cast<VarDecl>(DUnderlying)) {
+    VarDecl *EVD = cast<VarDecl>(EUnderlying);
+    // Skip local VarDecls with incomplete array type.
+    if ((!DVD->isLocalVarDecl() && DVD->hasExternalStorage()) ||
+        (!EVD->isLocalVarDecl() && EVD->hasExternalStorage())) {
Why do you need this special case?
No reason, I decided to be conservative. All comments should be addressed in the new version of the patch.
+
+      ASTContext &C = S.getASTContext();
+
+      if (const ArrayType *DVDTy = C.getAsArrayType(DVD->getType())) {
+        const ArrayType *EVDTy = C.getAsArrayType(EVD->getType());
+        // Prefer the decl with more complete type if visible.
+        return !DVDTy->isIncompleteArrayType() && 
EVDTy->isIncompleteArrayType()
Checking for an array type here seems unnecessary -- it'd be simpler to check EVD->getType()->isIncompleteType() && !DVD->getType()->isIncompleteType().
+                 && S.isVisible(D);
This seems like a very subtle case: we're performing redeclaration lookup for a variable declaration X, and we find two results D and E. D is hidden but has a complete type. E is visible but has an incomplete type. If X has no array bound, it should not inherit one from D. But if it does have an array bound, and that bound differs from E's bound, we should diagnose the mismatch.

Please add another test to the end of merge-incomplete-array-vars.cpp to check this is working:
int c[2]; // expected-error {{different type: 'int [2]' vs 'int [1]'}}
That said, I think this test will fail if you reorder c1 and c2 in the module map, but that's a pre-existing bug (see the last FIXME in Sema::MergeVarDeclTypes).
+      }
+    }
+  }

On Tue, Feb 16, 2016 at 11:12 AM, Vassil Vassilev <v.g.vassi...@gmail.com <mailto:v.g.vassi...@gmail.com>> wrote:

    ping...

    On 07/02/16 22:33, Vassil Vassilev wrote:
    Improve a comment.
    --Vassil
    On 07/02/16 20:48, Vassil Vassilev wrote:
    Would this patch be any better?
    --Vassil
    On 05/02/16 21:49, Richard Smith wrote:
    This belongs in ASTDeclReader::attachPreviousDecl[Impl]. That's
    where we propagate data that's supposed to be consistent across
    a redeclaration chain from earlier declarations to later ones.

    There's another wrinkle here: we should only be propagating the
    type from a previous declaration that's declared in the same
    scope (in particular, we should skip over declarations declared
    as local extern declarations within a function). This may in
    some cases require walking backwards along the redeclaration
    chain to find the previous declaration that was not a local
    extern declaration. That'd be observable in a case like this:

    modulemap:
    module A { module A1 { header "a1.h" } module A2 { header
    "a2.h" } }
    module B { header "b.h" }

    a1.h:
    int a[];

    b.h:
    void g(int(*)[], int);
    void g(int(*)[1], int) = delete;
    template<typename T> void f(T t) {
      extern int a[];
      g(&a, t);
    }

    a2.h:
    int a[1];

    x.cc:
    #include "a1.h"
    #include "b.h"
    void h() { f(0); } // should not produce an error; type of 'a'
    within 'f' should not have been updated

    That example actually reveals another problem: we really don't
    want the completed type to be visible unless there is a visible
    declaration with the completed type. That suggests that
    propagating the type across the redeclaration chain may be the
    wrong approach, and we should instead handle this by changing
    isPreferredLookupResult (in SemaLookup.cpp) to prefer a VarDecl
    with a complete type over one with an incomplete type.

    On Fri, Feb 5, 2016 at 12:27 PM, Vassil Vassilev
    <v.g.vassi...@gmail.com <mailto:v.g.vassi...@gmail.com>> wrote:

        I am not sure where else to put this logic if not in
        isSameEntity... Could you point me to a better place?
        --Vassil

        On 05/02/16 19:56, Richard Smith wrote:
        On Fri, Feb 5, 2016 at 7:04 AM, Vassil Vassilev
        <v.g.vassi...@gmail.com <mailto:v.g.vassi...@gmail.com>>
        wrote:

            Good point. Do you have in mind calling
            'clang::Sema::MergeVarDeclTypes' or we to just
            "duplicate" the logic in
            clang::ASTDeclReader::mergeRedeclarable?


        It's not safe to call into Sema while we're in a
        partially-deserialized state. We know the only interesting
        case here is complete versus incomplete array types, so we
        don't need anything like the complexity of
        MergeVarDeclTypes -- something as easy as "if (new type is
        incomplete and old type is not) set new type to old type"
        should work.

            On 05/02/16 02:50, Richard Smith via cfe-commits wrote:
            I suspect we'll need to do a little more than this:
            when we rebuild the redeclaration chain, we should
            probably propagate the complete type to later
            redeclarations in the same scope. Otherwise, if we
            import a module that has a complete type and one that
            has an incomplete type, the declaration found by name
            lookup might be the one with the incomplete type,
            possibly resulting in rejects-valid on code like this:

            a.h:
            extern int a[5];

            b.h:
            extern int a[];

            x.cc:
            #include "a.h"
            #include "b.h"
            int k = sizeof(a);

            On Fri, Jan 22, 2016 at 11:03 AM, Yaron Keren via
            cfe-commits <cfe-commits@lists.llvm.org
            <mailto:cfe-commits@lists.llvm.org>> wrote:

                Author: yrnkrn
                Date: Fri Jan 22 13:03:27 2016
                New Revision: 258524

                URL:
                http://llvm.org/viewvc/llvm-project?rev=258524&view=rev
                Log:
                Merge templated static member variables, fixes
                http://llvm.org/pr26179.

                Patch by Vassil Vassilev!
                Reviewed by Richard Smith.


                Added:
                cfe/trunk/test/Modules/Inputs/PR26179/
                cfe/trunk/test/Modules/Inputs/PR26179/A.h
                cfe/trunk/test/Modules/Inputs/PR26179/B.h
                cfe/trunk/test/Modules/Inputs/PR26179/basic_string.h
                cfe/trunk/test/Modules/Inputs/PR26179/module.modulemap
                cfe/trunk/test/Modules/pr26179.cpp
                Modified:
                cfe/trunk/lib/Serialization/ASTReaderDecl.cpp

                Modified:
                cfe/trunk/lib/Serialization/ASTReaderDecl.cpp
                URL:
                
http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/Serialization/ASTReaderDecl.cpp?rev=258524&r1=258523&r2=258524&view=diff
                
==============================================================================
                --- cfe/trunk/lib/Serialization/ASTReaderDecl.cpp
                (original)
                +++ cfe/trunk/lib/Serialization/ASTReaderDecl.cpp
                Fri Jan 22 13:03:27 2016
                @@ -2595,8 +2595,24 @@ static bool
                isSameEntity(NamedDecl *X, N
                   // Variables with the same type and linkage match.
                   if (VarDecl *VarX = dyn_cast<VarDecl>(X)) {
                     VarDecl *VarY = cast<VarDecl>(Y);
                -    return (VarX->getLinkageInternal() ==
                VarY->getLinkageInternal()) &&
                -
                VarX->getASTContext().hasSameType(VarX->getType(), 
VarY->getType());
                +    if (VarX->getLinkageInternal() ==
                VarY->getLinkageInternal()) {
                + ASTContext &C = VarX->getASTContext();
                +      if (C.hasSameType(VarX->getType(),
                VarY->getType()))
                + return true;
                +
                +      // We can get decls with different types
                on the redecl chain. Eg.
                +      // template <typename T> struct S { static
                T Var[]; }; // #1
                +      // template <typename T> T
                S<T>::Var[sizeof(T)]; // #2
                +      // Only? happens when completing an
                incomplete array type. In this case
                +      // when comparing #1 and #2 we should go
                through their elements types.
                +      const ArrayType *VarXTy =
                C.getAsArrayType(VarX->getType());
                +      const ArrayType *VarYTy =
                C.getAsArrayType(VarY->getType());
                +      if (!VarXTy || !VarYTy)
                + return false;
                +      if (VarXTy->isIncompleteArrayType() ||
                VarYTy->isIncompleteArrayType())
                + return C.hasSameType(VarXTy->getElementType(),
                VarYTy->getElementType());
                +    }
                +    return false;
                   }

                   // Namespaces with the same name and
                inlinedness match.

                Added: cfe/trunk/test/Modules/Inputs/PR26179/A.h
                URL:
                
http://llvm.org/viewvc/llvm-project/cfe/trunk/test/Modules/Inputs/PR26179/A.h?rev=258524&view=auto
                
==============================================================================
                --- cfe/trunk/test/Modules/Inputs/PR26179/A.h (added)
                +++ cfe/trunk/test/Modules/Inputs/PR26179/A.h Fri
                Jan 22 13:03:27 2016
                @@ -0,0 +1,4 @@
                +#include "basic_string.h"
                +#include "B.h"
                +
                +int *p = a;

                Added: cfe/trunk/test/Modules/Inputs/PR26179/B.h
                URL:
                
http://llvm.org/viewvc/llvm-project/cfe/trunk/test/Modules/Inputs/PR26179/B.h?rev=258524&view=auto
                
==============================================================================
                --- cfe/trunk/test/Modules/Inputs/PR26179/B.h (added)
                +++ cfe/trunk/test/Modules/Inputs/PR26179/B.h Fri
                Jan 22 13:03:27 2016
                @@ -0,0 +1,2 @@
                +#include "basic_string.h"
                +extern int a[5];

                Added:
                cfe/trunk/test/Modules/Inputs/PR26179/basic_string.h
                URL:
                
http://llvm.org/viewvc/llvm-project/cfe/trunk/test/Modules/Inputs/PR26179/basic_string.h?rev=258524&view=auto
                
==============================================================================
                ---
                cfe/trunk/test/Modules/Inputs/PR26179/basic_string.h
                (added)
                +++
                cfe/trunk/test/Modules/Inputs/PR26179/basic_string.h
                Fri Jan 22 13:03:27 2016
                @@ -0,0 +1,14 @@
                +#ifndef _GLIBCXX_STRING
                +#define _GLIBCXX_STRING 1
                +
                +template<typename T>
                +struct basic_string {
                +  static T _S_empty_rep_storage[];
                +};
                +
                +template<typename T>
                +T basic_string<T>::_S_empty_rep_storage[sizeof(T)];
                +
                +extern int a[];
                +
                +#endif

                Added:
                cfe/trunk/test/Modules/Inputs/PR26179/module.modulemap
                URL:
                
http://llvm.org/viewvc/llvm-project/cfe/trunk/test/Modules/Inputs/PR26179/module.modulemap?rev=258524&view=auto
                
==============================================================================
                ---
                cfe/trunk/test/Modules/Inputs/PR26179/module.modulemap
                (added)
                +++
                cfe/trunk/test/Modules/Inputs/PR26179/module.modulemap
                Fri Jan 22 13:03:27 2016
                @@ -0,0 +1,9 @@
                +module A {
                +  header "A.h"
                +  export *
                +}
                +
                +module B {
                +  header "B.h"
                +  export *
                +}

                Added: cfe/trunk/test/Modules/pr26179.cpp
                URL:
                
http://llvm.org/viewvc/llvm-project/cfe/trunk/test/Modules/pr26179.cpp?rev=258524&view=auto
                
==============================================================================
                --- cfe/trunk/test/Modules/pr26179.cpp (added)
                +++ cfe/trunk/test/Modules/pr26179.cpp Fri Jan 22
                13:03:27 2016
                @@ -0,0 +1,7 @@
                +// RUN: rm -rf %t
                +// RUN: %clang_cc1 -I%S/Inputs/PR26179 -verify %s
                +// RUN: %clang_cc1 -fmodules
                -fmodule-map-file=%S/Inputs/PR26179/module.modulemap
                -fmodules-cache-path=%t -I%S/Inputs/PR26179
                -verify %s
                +
                +#include "A.h"
                +
                +// expected-no-diagnostics


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From 9d32e9590135345c2d3e3fdea31c1850720244b2 Mon Sep 17 00:00:00 2001
From: Vassil Vassilev <v.g.vassi...@gmail.com>
Date: Fri, 5 Feb 2016 21:15:05 +0100
Subject: [PATCH] [modules] Prefer more complete variable types.

---
 lib/Sema/SemaDecl.cpp                              | 74 +++++++++++++++-------
 lib/Sema/SemaLookup.cpp                            | 12 ++++
 lib/Serialization/ASTReaderDecl.cpp                |  2 +-
 test/Modules/Inputs/PR26179/A.h                    |  2 -
 test/Modules/Inputs/PR26179/B.h                    |  1 -
 test/Modules/Inputs/PR26179/basic_string.h         |  2 -
 .../Modules/Inputs/merge-incomplete-array-vars/a.h |  4 ++
 .../Modules/Inputs/merge-incomplete-array-vars/b.h |  4 ++
 .../Inputs/merge-incomplete-array-vars/c1.h        |  1 +
 .../Inputs/merge-incomplete-array-vars/c2.h        |  1 +
 .../Modules/Inputs/merge-incomplete-array-vars/d.h |  7 ++
 .../merge-incomplete-array-vars/module.modulemap   |  5 ++
 test/Modules/merge-incomplete-array-vars.cpp       | 21 ++++++
 13 files changed, 106 insertions(+), 30 deletions(-)
 create mode 100644 test/Modules/Inputs/merge-incomplete-array-vars/a.h
 create mode 100644 test/Modules/Inputs/merge-incomplete-array-vars/b.h
 create mode 100644 test/Modules/Inputs/merge-incomplete-array-vars/c1.h
 create mode 100644 test/Modules/Inputs/merge-incomplete-array-vars/c2.h
 create mode 100644 test/Modules/Inputs/merge-incomplete-array-vars/d.h
 create mode 100644 
test/Modules/Inputs/merge-incomplete-array-vars/module.modulemap
 create mode 100644 test/Modules/merge-incomplete-array-vars.cpp

diff --git a/lib/Sema/SemaDecl.cpp b/lib/Sema/SemaDecl.cpp
index 66c0e05..4e999d3 100644
--- a/lib/Sema/SemaDecl.cpp
+++ b/lib/Sema/SemaDecl.cpp
@@ -3245,6 +3245,22 @@ void Sema::mergeObjCMethodDecls(ObjCMethodDecl 
*newMethod,
   CheckObjCMethodOverride(newMethod, oldMethod);
 }
 
+static void diagnoseVarDeclTypeMismatch(Sema &S, VarDecl *New, VarDecl* Old) {
+  assert(!S.Context.hasSameType(New->getType(), Old->getType()));
+
+  S.Diag(New->getLocation(), New->isThisDeclarationADefinition()
+         ? diag::err_redefinition_different_type
+         : diag::err_redeclaration_different_type)
+    << New->getDeclName() << New->getType() << Old->getType();
+
+  diag::kind PrevDiag;
+  SourceLocation OldLocation;
+  std::tie(PrevDiag, OldLocation)
+    = getNoteDiagForInvalidRedeclaration(Old, New);
+  S.Diag(OldLocation, PrevDiag);
+  New->setInvalidDecl();
+}
+
 /// MergeVarDeclTypes - We parsed a variable 'New' which has the same name and
 /// scope as a previous declaration 'Old'.  Figure out how to merge their 
types,
 /// emitting diagnostics as appropriate.
@@ -3271,21 +3287,40 @@ void Sema::MergeVarDeclTypes(VarDecl *New, VarDecl *Old,
     //   object or function shall be identical, except that declarations for an
     //   array object can specify array types that differ by the presence or
     //   absence of a major array bound (8.3.4).
-    else if (Old->getType()->isIncompleteArrayType() &&
-             New->getType()->isArrayType()) {
-      const ArrayType *OldArray = Context.getAsArrayType(Old->getType());
-      const ArrayType *NewArray = Context.getAsArrayType(New->getType());
-      if (Context.hasSameType(OldArray->getElementType(),
-                              NewArray->getElementType()))
-        MergedT = New->getType();
-    } else if (Old->getType()->isArrayType() &&
-               New->getType()->isIncompleteArrayType()) {
+    else if (Old->getType()->isArrayType() && New->getType()->isArrayType()) {
       const ArrayType *OldArray = Context.getAsArrayType(Old->getType());
       const ArrayType *NewArray = Context.getAsArrayType(New->getType());
-      if (Context.hasSameType(OldArray->getElementType(),
-                              NewArray->getElementType()))
-        MergedT = Old->getType();
-    } else if (New->getType()->isObjCObjectPointerType() &&
+
+      // We are merging a variable declaration New into Old. If it has an array
+      // bound, and that bound differs from Old's bound, we should diagnose the
+      // mismatch.
+      // TODO: Check visibility. New is hidden but has a complete type. If New
+      // has no array bound, it should not inherit one from Old, if Old is not
+      // visible.
+      if (!NewArray->isIncompleteArrayType()) {
+        for (VarDecl *PrevVD = Old->getMostRecentDecl(); PrevVD;
+             PrevVD = PrevVD->getPreviousDecl()) {
+          const ArrayType *PrevVDTy = 
Context.getAsArrayType(PrevVD->getType());
+          if (PrevVDTy->isIncompleteArrayType())
+            continue;
+
+          if (!Context.hasSameType(NewArray, PrevVDTy))
+            return diagnoseVarDeclTypeMismatch(*this, New, PrevVD);
+        }
+      }
+
+      if (OldArray->isIncompleteArrayType() && NewArray->isArrayType()) {
+        if (Context.hasSameType(OldArray->getElementType(),
+                                NewArray->getElementType()))
+          MergedT = New->getType();
+      }
+      else if (OldArray->isArrayType() && NewArray->isIncompleteArrayType()) {
+        if (Context.hasSameType(OldArray->getElementType(),
+                                NewArray->getElementType()))
+          MergedT = Old->getType();
+      }
+    }
+    else if (New->getType()->isObjCObjectPointerType() &&
                Old->getType()->isObjCObjectPointerType()) {
       MergedT = Context.mergeObjCGCQualifiers(New->getType(),
                                               Old->getType());
@@ -3321,17 +3356,8 @@ void Sema::MergeVarDeclTypes(VarDecl *New, VarDecl *Old,
     //
     // Neither C nor C++ requires a diagnostic for this, but we should still 
try
     // to diagnose it.
-    Diag(New->getLocation(), New->isThisDeclarationADefinition()
-                                 ? diag::err_redefinition_different_type
-                                 : diag::err_redeclaration_different_type)
-        << New->getDeclName() << New->getType() << Old->getType();
-
-    diag::kind PrevDiag;
-    SourceLocation OldLocation;
-    std::tie(PrevDiag, OldLocation) =
-        getNoteDiagForInvalidRedeclaration(Old, New);
-    Diag(OldLocation, PrevDiag);
-    return New->setInvalidDecl();
+
+    return diagnoseVarDeclTypeMismatch(*this, New, Old);
   }
 
   // Don't actually update the type on the new declaration if the old
diff --git a/lib/Sema/SemaLookup.cpp b/lib/Sema/SemaLookup.cpp
index 1d7b15c..7405032 100644
--- a/lib/Sema/SemaLookup.cpp
+++ b/lib/Sema/SemaLookup.cpp
@@ -419,6 +419,18 @@ static bool isPreferredLookupResult(Sema &S, 
Sema::LookupNameKind Kind,
     }
   }
 
+  // VarDecl can have incomplete array types, prefer the one with more complete
+  // array type.
+  if (VarDecl *DVD = dyn_cast<VarDecl>(DUnderlying)) {
+    VarDecl *EVD = cast<VarDecl>(EUnderlying);
+    if (EVD->getType()->isIncompleteType() &&
+        !DVD->getType()->isIncompleteType()) {
+      // Prefer the decl with a more complete type if visible.
+      return S.isVisible(DVD);
+    }
+    return false; // Avoid picking up a newer decl, just because it was newer.
+  }
+
   // For most kinds of declaration, it doesn't really matter which one we pick.
   if (!isa<FunctionDecl>(DUnderlying) && !isa<VarDecl>(DUnderlying)) {
     // If the existing declaration is hidden, prefer the new one. Otherwise,
diff --git a/lib/Serialization/ASTReaderDecl.cpp 
b/lib/Serialization/ASTReaderDecl.cpp
index 9bcd9a0..feb9d12 100644
--- a/lib/Serialization/ASTReaderDecl.cpp
+++ b/lib/Serialization/ASTReaderDecl.cpp
@@ -2604,7 +2604,7 @@ static bool isSameEntity(NamedDecl *X, NamedDecl *Y) {
       // template <typename T> struct S { static T Var[]; }; // #1
       // template <typename T> T S<T>::Var[sizeof(T)]; // #2
       // Only? happens when completing an incomplete array type. In this case
-      // when comparing #1 and #2 we should go through their elements types.
+      // when comparing #1 and #2 we should go through their element type.
       const ArrayType *VarXTy = C.getAsArrayType(VarX->getType());
       const ArrayType *VarYTy = C.getAsArrayType(VarY->getType());
       if (!VarXTy || !VarYTy)
diff --git a/test/Modules/Inputs/PR26179/A.h b/test/Modules/Inputs/PR26179/A.h
index ce95faf..c264f4c 100644
--- a/test/Modules/Inputs/PR26179/A.h
+++ b/test/Modules/Inputs/PR26179/A.h
@@ -1,4 +1,2 @@
 #include "basic_string.h"
 #include "B.h"
-
-int *p = a;
diff --git a/test/Modules/Inputs/PR26179/B.h b/test/Modules/Inputs/PR26179/B.h
index eb8d1c2..46a109e 100644
--- a/test/Modules/Inputs/PR26179/B.h
+++ b/test/Modules/Inputs/PR26179/B.h
@@ -1,2 +1 @@
 #include "basic_string.h"
-extern int a[5];
diff --git a/test/Modules/Inputs/PR26179/basic_string.h 
b/test/Modules/Inputs/PR26179/basic_string.h
index afd1e0d..653ce07 100644
--- a/test/Modules/Inputs/PR26179/basic_string.h
+++ b/test/Modules/Inputs/PR26179/basic_string.h
@@ -9,6 +9,4 @@ struct basic_string {
 template<typename T>
 T basic_string<T>::_S_empty_rep_storage[sizeof(T)];
 
-extern int a[];
-
 #endif
diff --git a/test/Modules/Inputs/merge-incomplete-array-vars/a.h 
b/test/Modules/Inputs/merge-incomplete-array-vars/a.h
new file mode 100644
index 0000000..4dcf7de
--- /dev/null
+++ b/test/Modules/Inputs/merge-incomplete-array-vars/a.h
@@ -0,0 +1,4 @@
+extern int a[5];
+extern int aa[55];
+extern int b[];
+extern int bb[];
diff --git a/test/Modules/Inputs/merge-incomplete-array-vars/b.h 
b/test/Modules/Inputs/merge-incomplete-array-vars/b.h
new file mode 100644
index 0000000..8486136
--- /dev/null
+++ b/test/Modules/Inputs/merge-incomplete-array-vars/b.h
@@ -0,0 +1,4 @@
+extern int a[];
+extern int aa[];
+extern int b[6];
+extern int bb[65];
diff --git a/test/Modules/Inputs/merge-incomplete-array-vars/c1.h 
b/test/Modules/Inputs/merge-incomplete-array-vars/c1.h
new file mode 100644
index 0000000..99be230
--- /dev/null
+++ b/test/Modules/Inputs/merge-incomplete-array-vars/c1.h
@@ -0,0 +1 @@
+extern int c[];
diff --git a/test/Modules/Inputs/merge-incomplete-array-vars/c2.h 
b/test/Modules/Inputs/merge-incomplete-array-vars/c2.h
new file mode 100644
index 0000000..3fee8c7
--- /dev/null
+++ b/test/Modules/Inputs/merge-incomplete-array-vars/c2.h
@@ -0,0 +1 @@
+extern int c[1];
diff --git a/test/Modules/Inputs/merge-incomplete-array-vars/d.h 
b/test/Modules/Inputs/merge-incomplete-array-vars/d.h
new file mode 100644
index 0000000..e24cf76
--- /dev/null
+++ b/test/Modules/Inputs/merge-incomplete-array-vars/d.h
@@ -0,0 +1,7 @@
+extern int c[];
+void g(int(*)[], int);
+void g(int(*)[1], int) = delete;
+template<typename T> void f(T t) {
+  extern int c[];
+  g(&c, t);
+}
diff --git a/test/Modules/Inputs/merge-incomplete-array-vars/module.modulemap 
b/test/Modules/Inputs/merge-incomplete-array-vars/module.modulemap
new file mode 100644
index 0000000..d2b467d
--- /dev/null
+++ b/test/Modules/Inputs/merge-incomplete-array-vars/module.modulemap
@@ -0,0 +1,5 @@
+module A { header "a.h" export * }
+module B { header "b.h" export * }
+
+module C { module C1 { header "c1.h" } module C2 { header "c2.h" } }
+module D { header "d.h" }
diff --git a/test/Modules/merge-incomplete-array-vars.cpp 
b/test/Modules/merge-incomplete-array-vars.cpp
new file mode 100644
index 0000000..dd08b64
--- /dev/null
+++ b/test/Modules/merge-incomplete-array-vars.cpp
@@ -0,0 +1,21 @@
+// RUN: rm -rf %t
+// RUN: %clang_cc1 -std=c++11 -fmodules -fimplicit-module-maps 
-fmodules-cache-path=%t -I%S/Inputs/merge-incomplete-array-vars -verify %s
+
+extern int aa[];
+extern int bb[];
+
+#include "a.h"
+#include "b.h"
+
+// Import the complete type first and the incomplete second, forcing the
+// most recent decl to have an incomplete type.
+int k = sizeof(a);
+int l = sizeof(aa);
+int m = sizeof(b);
+int n = sizeof(bb);
+
+#include "c1.h"
+#include "d.h"
+int size = sizeof(c); // expected-error{{invalid application of 'sizeof' to an 
incomplete type 'int []'}}
+void h() { f(0); } // should not produce an error; type of 'c' within 'f' 
should not have been updated
+int c[2]; // expected-error {{different type: 'int [2]' vs 'int [1]'}} 
//expected-note@c2.h:1 {{previous declaration is here}}
-- 
2.3.8 (Apple Git-58)

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