diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index 581434d33c5c9..beb78eb0a4ef4 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -11152,6 +11152,8 @@ def err_omp_loop_diff_cxx : Error<
   "upper and lower loop bounds">;
 def err_omp_loop_cannot_use_stmt : Error<
   "'%0' statement cannot be used in OpenMP for loop">;
+def err_omp_loop_bad_collapse_var : Error<
+  "cannot use variable %1 in collapsed imperfectly-nested loop %select{init|condition|increment}0 statement">;
 def err_omp_simd_region_cannot_use_stmt : Error<
   "'%0' statement cannot be used in OpenMP simd region">;
 def warn_omp_loop_64_bit_var : Warning<
diff --git a/clang/lib/Sema/SemaOpenMP.cpp b/clang/lib/Sema/SemaOpenMP.cpp
index 4f50efda155fb..f52a5715aa19e 100644
--- a/clang/lib/Sema/SemaOpenMP.cpp
+++ b/clang/lib/Sema/SemaOpenMP.cpp
@@ -21,6 +21,7 @@
 #include "clang/AST/DeclCXX.h"
 #include "clang/AST/DeclOpenMP.h"
 #include "clang/AST/OpenMPClause.h"
+#include "clang/AST/RecursiveASTVisitor.h"
 #include "clang/AST/StmtCXX.h"
 #include "clang/AST/StmtOpenMP.h"
 #include "clang/AST/StmtVisitor.h"
@@ -7668,6 +7669,52 @@ struct LoopIterationSpace final {
   Expr *FinalCondition = nullptr;
 };
 
+/// Scan an AST subtree, checking that no decls in the CollapsedLoopVarDecls
+/// set are referenced.  Used for verifying loop nest structure before
+/// performing a loop collapse operation.
+class ForSubExprChecker final : public RecursiveASTVisitor<ForSubExprChecker> {
+  const llvm::SmallPtrSetImpl<const Decl *> &CollapsedLoopVarDecls;
+  VarDecl *ForbiddenVar = nullptr;
+  SourceRange ErrLoc;
+
+public:
+  explicit ForSubExprChecker(
+      const llvm::SmallPtrSetImpl<const Decl *> &CollapsedLoopVarDecls)
+      : CollapsedLoopVarDecls(CollapsedLoopVarDecls) {}
+
+  // We want to visit implicit code, i.e. synthetic initialisation statements
+  // created during range-for lowering.
+  bool shouldVisitImplicitCode() const { return true; }
+
+  bool VisitDeclRefExpr(DeclRefExpr *E) {
+    ValueDecl *VD = E->getDecl();
+    if (!isa<VarDecl, BindingDecl>(VD))
+      return true;
+    VarDecl *V = VD->getPotentiallyDecomposedVarDecl();
+    if (V->getType()->isReferenceType()) {
+      VarDecl *VD = V->getDefinition();
+      if (VD->hasInit()) {
+        Expr *I = VD->getInit();
+        DeclRefExpr *DRE = dyn_cast<DeclRefExpr>(I);
+        if (!DRE)
+          return true;
+        V = DRE->getDecl()->getPotentiallyDecomposedVarDecl();
+      }
+    }
+    Decl *Canon = V->getCanonicalDecl();
+    if (CollapsedLoopVarDecls.contains(Canon)) {
+      ForbiddenVar = V;
+      ErrLoc = E->getSourceRange();
+      return false;
+    }
+
+    return true;
+  }
+
+  VarDecl *getForbiddenVar() const { return ForbiddenVar; }
+  SourceRange getErrRange() const { return ErrLoc; }
+};
+
 /// Helper class for checking canonical form of the OpenMP loops and
 /// extracting iteration space of each loop in the loop nest, that will be used
 /// for IR generation.
@@ -7682,6 +7729,8 @@ class OpenMPIterationSpaceChecker {
   SourceLocation DefaultLoc;
   /// A location for diagnostics (when increment is not compatible).
   SourceLocation ConditionLoc;
+  /// The set of variables declared within the (to be collapsed) loop nest.
+  const llvm::SmallPtrSetImpl<const Decl *> &CollapsedLoopVarDecls;
   /// A source location for referring to loop init later.
   SourceRange InitSrcRange;
   /// A source location for referring to condition later.
@@ -7725,10 +7774,13 @@ class OpenMPIterationSpaceChecker {
   Expr *Condition = nullptr;
 
 public:
-  OpenMPIterationSpaceChecker(Sema &SemaRef, bool SupportsNonRectangular,
-                              DSAStackTy &Stack, SourceLocation DefaultLoc)
+  OpenMPIterationSpaceChecker(
+      Sema &SemaRef, bool SupportsNonRectangular, DSAStackTy &Stack,
+      SourceLocation DefaultLoc,
+      const llvm::SmallPtrSetImpl<const Decl *> &CollapsedLoopDecls)
       : SemaRef(SemaRef), SupportsNonRectangular(SupportsNonRectangular),
-        Stack(Stack), DefaultLoc(DefaultLoc), ConditionLoc(DefaultLoc) {}
+        Stack(Stack), DefaultLoc(DefaultLoc), ConditionLoc(DefaultLoc),
+        CollapsedLoopVarDecls(CollapsedLoopDecls) {}
   /// Check init-expr for canonical loop form and save loop counter
   /// variable - #Var and its initialization value - #LB.
   bool checkAndSetInit(Stmt *S, bool EmitDiags = true);
@@ -8049,6 +8101,16 @@ bool OpenMPIterationSpaceChecker::checkAndSetInit(Stmt *S, bool EmitDiags) {
     if (!ExprTemp->cleanupsHaveSideEffects())
       S = ExprTemp->getSubExpr();
 
+  if (!CollapsedLoopVarDecls.empty()) {
+    ForSubExprChecker FSEC{CollapsedLoopVarDecls};
+    if (!FSEC.TraverseStmt(S)) {
+      SourceRange Range = FSEC.getErrRange();
+      SemaRef.Diag(Range.getBegin(), diag::err_omp_loop_bad_collapse_var)
+          << Range.getEnd() << 0 << FSEC.getForbiddenVar();
+      return true;
+    }
+  }
+
   InitSrcRange = S->getSourceRange();
   if (Expr *E = dyn_cast<Expr>(S))
     S = E->IgnoreParens();
@@ -8152,6 +8214,17 @@ bool OpenMPIterationSpaceChecker::checkAndSetCond(Expr *S) {
   }
   Condition = S;
   S = getExprAsWritten(S);
+
+  if (!CollapsedLoopVarDecls.empty()) {
+    ForSubExprChecker FSEC{CollapsedLoopVarDecls};
+    if (!FSEC.TraverseStmt(S)) {
+      SourceRange Range = FSEC.getErrRange();
+      SemaRef.Diag(Range.getBegin(), diag::err_omp_loop_bad_collapse_var)
+          << Range.getEnd() << 1 << FSEC.getForbiddenVar();
+      return true;
+    }
+  }
+
   SourceLocation CondLoc = S->getBeginLoc();
   auto &&CheckAndSetCond =
       [this, IneqCondIsCanonical](BinaryOperatorKind Opcode, const Expr *LHS,
@@ -8250,6 +8323,16 @@ bool OpenMPIterationSpaceChecker::checkAndSetInc(Expr *S) {
     if (!ExprTemp->cleanupsHaveSideEffects())
       S = ExprTemp->getSubExpr();
 
+  if (!CollapsedLoopVarDecls.empty()) {
+    ForSubExprChecker FSEC{CollapsedLoopVarDecls};
+    if (!FSEC.TraverseStmt(S)) {
+      SourceRange Range = FSEC.getErrRange();
+      SemaRef.Diag(Range.getBegin(), diag::err_omp_loop_bad_collapse_var)
+          << Range.getEnd() << 2 << FSEC.getForbiddenVar();
+      return true;
+    }
+  }
+
   IncrementSrcRange = S->getSourceRange();
   S = S->IgnoreParens();
   if (auto *UO = dyn_cast<UnaryOperator>(S)) {
@@ -8971,8 +9054,9 @@ void SemaOpenMP::ActOnOpenMPLoopInitialization(SourceLocation ForLoc,
     return;
 
   DSAStack->loopStart();
+  llvm::SmallPtrSet<const Decl *, 1> EmptyDeclSet;
   OpenMPIterationSpaceChecker ISC(SemaRef, /*SupportsNonRectangular=*/true,
-                                  *DSAStack, ForLoc);
+                                  *DSAStack, ForLoc, EmptyDeclSet);
   if (!ISC.checkAndSetInit(Init, /*EmitDiags=*/false)) {
     if (ValueDecl *D = ISC.getLoopDecl()) {
       auto *VD = dyn_cast<VarDecl>(D);
@@ -9069,7 +9153,8 @@ static bool checkOpenMPIterationSpace(
     Expr *OrderedLoopCountExpr,
     SemaOpenMP::VarsWithInheritedDSAType &VarsWithImplicitDSA,
     llvm::MutableArrayRef<LoopIterationSpace> ResultIterSpaces,
-    llvm::MapVector<const Expr *, DeclRefExpr *> &Captures) {
+    llvm::MapVector<const Expr *, DeclRefExpr *> &Captures,
+    const llvm::SmallPtrSetImpl<const Decl *> &CollapsedLoopVarDecls) {
   bool SupportsNonRectangular = !isOpenMPLoopTransformationDirective(DKind);
   // OpenMP [2.9.1, Canonical Loop Form]
   //   for (init-expr; test-expr; incr-expr) structured-block
@@ -9108,7 +9193,8 @@ static bool checkOpenMPIterationSpace(
     return false;
 
   OpenMPIterationSpaceChecker ISC(SemaRef, SupportsNonRectangular, DSA,
-                                  For ? For->getForLoc() : CXXFor->getForLoc());
+                                  For ? For->getForLoc() : CXXFor->getForLoc(),
+                                  CollapsedLoopVarDecls);
 
   // Check init.
   Stmt *Init = For ? For->getInit() : CXXFor->getBeginStmt();
@@ -9475,6 +9561,39 @@ static Expr *buildPostUpdate(Sema &S, ArrayRef<Expr *> PostUpdates) {
   return PostUpdate;
 }
 
+/// Look for variables declared in the body parts of a for-loop nest.  Used
+/// for verifying loop nest structure before performing a loop collapse
+/// operation.
+class ForVarDeclFinder final : public RecursiveASTVisitor<ForVarDeclFinder> {
+  int NestingDepth = 0;
+  llvm::SmallPtrSetImpl<const Decl *> &VarDecls;
+
+public:
+  explicit ForVarDeclFinder(llvm::SmallPtrSetImpl<const Decl *> &VD)
+      : VarDecls(VD) {}
+
+  bool VisitForStmt(ForStmt *F) {
+    ++NestingDepth;
+    TraverseStmt(F->getBody());
+    --NestingDepth;
+    return false;
+  }
+
+  bool VisitCXXForRangeStmt(CXXForRangeStmt *RF) {
+    ++NestingDepth;
+    TraverseStmt(RF->getBody());
+    --NestingDepth;
+    return false;
+  }
+
+  bool VisitVarDecl(VarDecl *D) {
+    Decl *C = D->getCanonicalDecl();
+    if (NestingDepth > 0)
+      VarDecls.insert(C);
+    return true;
+  }
+};
+
 /// Called on a for stmt to check itself and nested loops (if any).
 /// \return Returns 0 if one of the collapsed stmts is not canonical for loop,
 /// number of collapsed loops otherwise.
@@ -9487,6 +9606,7 @@ checkOpenMPLoop(OpenMPDirectiveKind DKind, Expr *CollapseLoopCountExpr,
   unsigned NestedLoopCount = 1;
   bool SupportsNonPerfectlyNested = (SemaRef.LangOpts.OpenMP >= 50) &&
                                     !isOpenMPLoopTransformationDirective(DKind);
+  llvm::SmallPtrSet<const Decl *, 4> CollapsedLoopVarDecls;
 
   if (CollapseLoopCountExpr) {
     // Found 'collapse' clause - calculate collapse number.
@@ -9494,6 +9614,9 @@ checkOpenMPLoop(OpenMPDirectiveKind DKind, Expr *CollapseLoopCountExpr,
     if (!CollapseLoopCountExpr->isValueDependent() &&
         CollapseLoopCountExpr->EvaluateAsInt(Result, SemaRef.getASTContext())) {
       NestedLoopCount = Result.Val.getInt().getLimitedValue();
+
+      ForVarDeclFinder FVDF{CollapsedLoopVarDecls};
+      FVDF.TraverseStmt(AStmt);
     } else {
       Built.clear(/*Size=*/1);
       return 1;
@@ -9531,11 +9654,13 @@ checkOpenMPLoop(OpenMPDirectiveKind DKind, Expr *CollapseLoopCountExpr,
           SupportsNonPerfectlyNested, NumLoops,
           [DKind, &SemaRef, &DSA, NumLoops, NestedLoopCount,
            CollapseLoopCountExpr, OrderedLoopCountExpr, &VarsWithImplicitDSA,
-           &IterSpaces, &Captures](unsigned Cnt, Stmt *CurStmt) {
+           &IterSpaces, &Captures,
+           &CollapsedLoopVarDecls](unsigned Cnt, Stmt *CurStmt) {
             if (checkOpenMPIterationSpace(
                     DKind, CurStmt, SemaRef, DSA, Cnt, NestedLoopCount,
                     NumLoops, CollapseLoopCountExpr, OrderedLoopCountExpr,
-                    VarsWithImplicitDSA, IterSpaces, Captures))
+                    VarsWithImplicitDSA, IterSpaces, Captures,
+                    CollapsedLoopVarDecls))
               return true;
             if (Cnt > 0 && Cnt >= NestedLoopCount &&
                 IterSpaces[Cnt].CounterVar) {
diff --git a/clang/test/OpenMP/loop_collapse_1.c b/clang/test/OpenMP/loop_collapse_1.c
new file mode 100644
index 0000000000000..c9877419223dd
--- /dev/null
+++ b/clang/test/OpenMP/loop_collapse_1.c
@@ -0,0 +1,40 @@
+// RUN: %clang_cc1 -fopenmp -fopenmp-version=50 -verify %s
+
+void func( double *A, int N, int M, int NB ) {
+#pragma omp parallel
+  {
+    int nblks = (N-1)/NB;
+    int lnb = ((N-1)/NB)*NB;
+
+#pragma omp for collapse(2)
+    for (int jblk = 0 ; jblk < nblks ; jblk++ ) {
+      int jb = (jblk == nblks - 1 ? lnb : NB);
+      for (int jk = 0; jk < N; jk+=jb) {  // expected-error{{cannot use variable 'jb' in collapsed imperfectly-nested loop increment statement}}
+      }
+    }
+
+#pragma omp for collapse(2)
+    for (int a = 0; a < N; a++) {
+      for (int b = 0; b < M; b++) {
+        int cx = a+b < NB ? a : b;
+        for (int c = 0; c < cx; c++) {
+        }
+      }
+    }
+
+#pragma omp for collapse(3)
+    for (int a = 0; a < N; a++) {
+      for (int b = 0; b < M; b++) {
+        int cx = a+b < NB ? a : b;
+        for (int c = 0; c < cx; c++) {  // expected-error{{cannot use variable 'cx' in collapsed imperfectly-nested loop condition statement}}
+        }
+      }
+    }
+  }
+}
+
+int main(void) {
+  double arr[256];
+  func (arr, 16, 16, 16);
+  return 0;
+}
diff --git a/clang/test/OpenMP/loop_collapse_2.cpp b/clang/test/OpenMP/loop_collapse_2.cpp
new file mode 100644
index 0000000000000..59deddf65e37b
--- /dev/null
+++ b/clang/test/OpenMP/loop_collapse_2.cpp
@@ -0,0 +1,80 @@
+// RUN: %clang_cc1 -fopenmp -fopenmp-version=50 -verify %s
+
+// We just want to try out a range for statement... this seems a bit OTT.
+template<typename T>
+class fakevector {
+  T *contents;
+  long size;
+public:
+  fakevector(long sz) : size(sz) {
+    contents = new T[sz];
+  }
+  ~fakevector() {
+    delete[] contents;
+  }
+  T& operator[](long x) { return contents[x]; }
+  typedef T *iterator;
+  fakevector<T>::iterator begin() {
+    return &contents[0];
+  }
+  fakevector<T>::iterator end() {
+    return &contents[size];
+  }
+};
+
+void func( double *A, int N, int M, int NB ) {
+#pragma omp parallel
+  {
+    int nblks = (N-1)/NB;
+    int lnb = ((N-1)/NB)*NB;
+#pragma omp for collapse(2)
+    for (int jblk = 0 ; jblk < nblks ; jblk++ ) {
+      int jb = (jblk == nblks - 1 ? lnb : NB);
+      for (int jk = 0; jk < N; jk+=jb) {  // expected-error{{cannot use variable 'jb' in collapsed imperfectly-nested loop increment statement}}
+      }
+    }
+
+#pragma omp for collapse(2)
+    for (int a = 0; a < N; a++) {
+        for (int b = 0; b < M; b++) {
+          int cx = a+b < NB ? a : b;
+          for (int c = 0; c < cx; c++) {
+          }
+        }
+    }
+
+    fakevector<float> myvec{N};
+#pragma omp for collapse(2)
+    for (auto &a : myvec) {
+      fakevector<float> myvec3{M};
+      for (auto &b : myvec3) {  // expected-error{{cannot use variable 'myvec3' in collapsed imperfectly-nested loop init statement}}
+      }
+    }
+
+    fakevector<float> myvec2{M};
+
+#pragma omp for collapse(3)
+    for (auto &a : myvec) {
+      for (auto &b : myvec2) {
+        int cx = a < b ? N : M;
+        for (int c = 0; c < cx; c++) {  // expected-error {{cannot use variable 'cx' in collapsed imperfectly-nested loop condition statement}}
+        }
+      }
+    }
+
+#pragma omp for collapse(3)
+    for (auto &a : myvec) {
+      int cx = a < 5 ? M : N;
+      for (auto &b : myvec2) {
+        for (int c = 0; c < cx; c++) {  // expected-error{{cannot use variable 'cx' in collapsed imperfectly-nested loop condition statement}}
+        }
+      }
+    }
+  }
+}
+
+int main(void) {
+  double arr[256];
+  func (arr, 16, 16, 16);
+  return 0;
+}