Extra Parentheses with Different Effects:

Parentheses in C usually do not affect program meaning, except in uncommon situations:

Argument-Dependent Name Lookup Prevention:

Parentheses can prevent argument-dependent name lookup (ADL) in function calls. While the form fun(arg) considers namespace-scope friend functions via ADL, (fun)(arg) does not, allowing precise function selection.

Comma Operator Enablement:

In list contexts (function arguments, initializer lists), the comma operator applies only within parentheses. Parentheses of the form a, (b, c), d enable the comma operator, while a, b, c, d do not.

Ambiguity Resolution in Vexing Parses:

Parentheses can resolve ambiguity in "vexing parse" situations, where a construct could be either a declaration or an expression. By surrounding a function-style type cast in parentheses, it can be clearly identified as an expression, avoiding parse ambiguity.

Referenceness Deduction in decltype:

Extra parentheses can affect the deduced referenceness in decltype expressions. If the operand of decltype(e) is an unevaluated lvalue, enclosing it in parentheses (decltype((e))) deduces a const lvalue reference (&).

Preprocessor Macro Errors:

Parentheses can prevent errors in preprocessor macros by controlling macro expansion. For example, using parentheses around macro parameters avoids operator precedence issues, and enclosing function calls in parentheses protects against unwanted macro substitutions.