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10.4.4.5 if

Conditional handling of statements is supported by the if, which can have an optional else: 

if ::= ("if" "(" bool-expression ")" imperative-statement)
         | ("if" "(" bool-expression ")" imperative-statement
              "else" imperative-statement)

For example 

if (idle)
  {world; idle = false;}
else
  cruel;

Since 2.14.0, a typed call may be used in an if-expression.

For example: 

if (bool_function ()) …;
if (!bool_function ()) …;
if (!bool_function () && b) …;
if (enum_function () == result.TRUE) …;
if (enum_function () != result.ERROR || b) …;

Since 2.16.0, arbitrarily complex expressions may be used.

Note that nested ifs are allowed: 

if (b1) if (b2) then-statement else else-statement

is interpreted as 

if (b1)
{ if (b2) then-statement else else-statement }

In other words: else binds to the closest if.

Note: In an interface, an illegal is not allowed as a then-statement or an else-statement, however the same can be expressed using a guard.