[mlir][Parser] Fix crash in DenseElementsAttr parser when no elements are parsed

[rriddle]

Summary

This fixes a crash when no elements are parsed, but the type expects at least one.

Fixes PR#47763

mlir/lib/Parser/AttributeParser.cpp

DenseElementsAttr TensorLiteralParser::getAttr(llvm::SMLoc loc,
   // Check that the parsed storage size has the same number of elements to the
   // type, or is a known splat.
   if (!shape.empty() && getShape() != type.getShape()) {
     p.emitError(loc) << "inferred shape of elements literal ([" << getShape()
                      << "]) does not match type ([" << type.getShape() << "])";
     return nullptr;
   }
 
+  // Handle the case where no elements were parsed.
+  if (!hexStorage.hasValue() && storage.empty() && type.getNumElements()) {
+    p.emitError(loc) << "parsed zero elements, but type (" << type
+                     << ") expected at least 1";
+    return nullptr;
+  }
+
   // Handle complex types in the specific element type cases below.
   bool isComplex = false;
   if (ComplexType complexTy = eltType.dyn_cast<ComplexType>()) {
     eltType = complexTy.getElementType();
     isComplex = true;
   }
 
   // Handle integer and index types.

mlir/test/IR/invalid.mlir

 
 func @elementsattr_toolarge1() -> () {
 ^bb0:
   "foo"(){bar = dense<[777]> : tensor<1xi8>} : () -> () // expected-error {{integer constant out of range}}
 }
 
 // -----
 
+// expected-error@+1 {{parsed zero elements, but type ('tensor<i64>') expected at least 1}}
+#attr = dense<> : tensor<i64>
+
+// -----
+
 func @elementsattr_toolarge2() -> () {
 ^bb0:
   "foo"(){bar = dense<[-777]> : tensor<1xi8>} : () -> () // expected-error {{integer constant out of range}}
 }
 
 // -----
 
 "foo"(){bar = dense<[()]> : tensor<complex<i64>>} : () -> () // expected-error {{expected element literal of primitive type}}