Using B.call gives a warning to check the return status, while b.foo() does not. Why?

Question

Using .call to invoke a function on another contract,

address B = 0x4a915089aeC328Dac1F0b197Aba7EB8C1853882d;
bytes4 callSig = bytes4(sha3("foo()"));
B.call(callSig);

gives a warning,

Warning: Return value of low-level calls not used.

using

B public b;

b = B(0x4a915089aeC328Dac1F0b197Aba7EB8C1853882d);

b.foo();

gives no warning.

In what ways does b.foo() know that the call is safe, while B.call(callSig) does not ?

Answer 1

When you use b.foo, the contract checks the return status, and if the call was unsuccessful it throws an exception. When you use the lower level .call, the call will simply return true if the call was successful and false otherwise. The compiler noticed that you never check the return value, and thus could be making the incorrect assumption that the call completed successfully.