A statement found in a function body always has specific requirements as to what type the final expression must be:

• Evaluation Statement: Cannot be non-simple type.  A void type is permitted (results from a function call with no return value or structure-to-structure assignment).
• The “return” statement: If a function has a return value, it cannot be a structure type.  A scalar result is automatically cast to a scalar return type and a pointer result is automatically cast to a pointer return type.  It is not possible to cast pointer types to scalar types or vice versa (with 0 being an exception for pointer return types—this represents a null pointer).
• The “if,” “while,” and “do” statements: Cannot be a structure type.  A void type is not permitted.  Either scalar or pointer types are allowed.
• The “for” statement: None of the expressions can result in a structure type.  A void type is permitted for the initialization expression and increment expression, but not for the evaluation expression.  Either scalar or pointer types are allowed for all expressions.
• The “switch” statement: Cannot be a non-simple type.  Only certain scalar integer types are permitted, including char, short, and long.  Automatic typecasting is never performed on the final result.
• Function arguments:  If a function parameter type is scalar, the argument must also be scalar; the argument is automatically cast to the parameter type.  If a function parameter type is pointer, the argument must also be a pointer to the same type if the parameter type is not pointer-to-void, or a pointer to any type if the parameter type is pointer-to-void.

  See also:  [Final expression type result] [L-value status gain and loss]
[Overly complex expressions] [Limitations on pointer usage] [Structure member dereference:  bit scan blocks]