[Matrix] Preserve signedness when extending matrix index expression.

As per [1] the indices for a matrix element access operator shall have integral
or unscoped enumeration types and be non-negative. At the moment, the
index expression is converted to SizeType irrespective of the signedness
of the index expression. This causes implicit sign conversion warnings
if any of the indices is signed.

As per the spec, using signed types as indices is allowed and should not
cause any warnings. If the index expression is signed, extend to
SignedSizeType to avoid the warning.

[1] https://clang.llvm.org/docs/MatrixTypes.html#matrix-type-element-access-operator

Author: fhahn

clang/lib/Sema/SemaExpr.cpp

@@ -5025,8 +5025,10 @@ ExprResult Sema::CreateBuiltinMatrixSubscriptExpr(Expr *Base, Expr *RowIdx,
       }
     }
 
-    ExprResult ConvExpr =
-        tryConvertExprToType(IndexExpr, Context.getSizeType());
+    ExprResult ConvExpr = tryConvertExprToType(
+        IndexExpr, IndexExpr->getType()->isSignedIntegerType()
+                       ? Context.getSignedSizeType()
+                       : Context.getSizeType());
     assert(!ConvExpr.isInvalid() &&
            "should be able to convert any integer type to size type");
     return ConvExpr.get();

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