diff --git a/clang/lib/CodeGen/CGExpr.cpp b/clang/lib/CodeGen/CGExpr.cpp
index 48d9a3b8a5acb..426fccb721db8 100644
--- a/clang/lib/CodeGen/CGExpr.cpp
+++ b/clang/lib/CodeGen/CGExpr.cpp
@@ -4392,13 +4392,24 @@ LValue CodeGenFunction::EmitArraySubscriptExpr(const ArraySubscriptExpr *E,
   return LV;
 }
 
+llvm::Value *CodeGenFunction::EmitMatrixIndexExpr(const Expr *E) {
+  llvm::Value *Idx = EmitScalarExpr(E);
+  if (Idx->getType() == IntPtrTy)
+    return Idx;
+  bool IsSigned = E->getType()->isSignedIntegerOrEnumerationType();
+  return Builder.CreateIntCast(Idx, IntPtrTy, IsSigned);
+}
+
 LValue CodeGenFunction::EmitMatrixSubscriptExpr(const MatrixSubscriptExpr *E) {
   assert(
       !E->isIncomplete() &&
       "incomplete matrix subscript expressions should be rejected during Sema");
   LValue Base = EmitLValue(E->getBase());
-  llvm::Value *RowIdx = EmitScalarExpr(E->getRowIdx());
-  llvm::Value *ColIdx = EmitScalarExpr(E->getColumnIdx());
+
+  // Extend or truncate the index type to 32 or 64-bits if needed.
+  llvm::Value *RowIdx = EmitMatrixIndexExpr(E->getRowIdx());
+  llvm::Value *ColIdx = EmitMatrixIndexExpr(E->getColumnIdx());
+
   llvm::Value *NumRows = Builder.getIntN(
       RowIdx->getType()->getScalarSizeInBits(),
       E->getBase()->getType()->castAs<ConstantMatrixType>()->getNumRows());
diff --git a/clang/lib/CodeGen/CGExprScalar.cpp b/clang/lib/CodeGen/CGExprScalar.cpp
index 3bda254c86adf..2a726bba2dd30 100644
--- a/clang/lib/CodeGen/CGExprScalar.cpp
+++ b/clang/lib/CodeGen/CGExprScalar.cpp
@@ -2007,8 +2007,8 @@ Value *ScalarExprEmitter::VisitMatrixSubscriptExpr(MatrixSubscriptExpr *E) {
 
   // Handle the vector case.  The base must be a vector, the index must be an
   // integer value.
-  Value *RowIdx = Visit(E->getRowIdx());
-  Value *ColumnIdx = Visit(E->getColumnIdx());
+  Value *RowIdx = CGF.EmitMatrixIndexExpr(E->getRowIdx());
+  Value *ColumnIdx = CGF.EmitMatrixIndexExpr(E->getColumnIdx());
 
   const auto *MatrixTy = E->getBase()->getType()->castAs<ConstantMatrixType>();
   unsigned NumRows = MatrixTy->getNumRows();
diff --git a/clang/lib/CodeGen/CodeGenFunction.h b/clang/lib/CodeGen/CodeGenFunction.h
index e1b9ada3c1e1f..05f85f8b95bfa 100644
--- a/clang/lib/CodeGen/CodeGenFunction.h
+++ b/clang/lib/CodeGen/CodeGenFunction.h
@@ -4280,6 +4280,7 @@ class CodeGenFunction : public CodeGenTypeCache {
   LValue EmitUnaryOpLValue(const UnaryOperator *E);
   LValue EmitArraySubscriptExpr(const ArraySubscriptExpr *E,
                                 bool Accessed = false);
+  llvm::Value *EmitMatrixIndexExpr(const Expr *E);
   LValue EmitMatrixSubscriptExpr(const MatrixSubscriptExpr *E);
   LValue EmitArraySectionExpr(const ArraySectionExpr *E,
                               bool IsLowerBound = true);
diff --git a/clang/lib/Sema/SemaExpr.cpp b/clang/lib/Sema/SemaExpr.cpp
index c67183df335dd..ea57316ad8014 100644
--- a/clang/lib/Sema/SemaExpr.cpp
+++ b/clang/lib/Sema/SemaExpr.cpp
@@ -5033,8 +5033,7 @@ ExprResult Sema::CreateBuiltinMatrixSubscriptExpr(Expr *Base, Expr *RowIdx,
       }
     }
 
-    ExprResult ConvExpr =
-        tryConvertExprToType(IndexExpr, Context.getSizeType());
+    ExprResult ConvExpr = IndexExpr;
     assert(!ConvExpr.isInvalid() &&
            "should be able to convert any integer type to size type");
     return ConvExpr.get();
diff --git a/clang/test/SemaCXX/matrix-index-operator-sign-conversion.cpp b/clang/test/SemaCXX/matrix-index-operator-sign-conversion.cpp
index 4254780651c5f..e6fe4a6c57ff2 100644
--- a/clang/test/SemaCXX/matrix-index-operator-sign-conversion.cpp
+++ b/clang/test/SemaCXX/matrix-index-operator-sign-conversion.cpp
@@ -2,19 +2,16 @@
 
 template <typename T, int R, int C> using m __attribute__((__matrix_type__(R,C))) = T;
 
-// FIXME: should not warn here.
 double index1(m<double,3,1> X, int      i) { return X[i][0]; }
-// expected-warning@-1 {{implicit conversion changes signedness: 'int' to 'unsigned long'}}
 
 double index2(m<double,3,1> X, unsigned i) { return X[i][0]; }
 
 double index3(m<double,3,1> X, char     i) { return X[i][0]; }
-// expected-warning@-1 {{implicit conversion changes signedness: 'char' to 'unsigned long'}}
 
 double index4(m<double,3,1> X, int      i) { return X[0][i]; }
-// expected-warning@-1 {{implicit conversion changes signedness: 'int' to 'unsigned long'}}
 
 double index5(m<double,3,1> X, unsigned i) { return X[0][i]; }
 
 double index6(m<double,3,1> X, char     i) { return X[0][i]; }
-// expected-warning@-1 {{implicit conversion changes signedness: 'char' to 'unsigned long'}}
+
+// expected-no-diagnostics