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#### *`imag-if-needed`* <a id="linalg.helpers.imag">[[linalg.helpers.imag]]</a>
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The name *`imag-if-needed`* denotes an exposition-only function object.
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The expression `imag-if-needed(E)` for a subexpression `E` whose type is
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`T` is expression-equivalent to:
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- `imag(E)`, if `T` is not an arithmetic type and the expression
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`imag(E)` is valid, with overload resolution performed in a context
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that includes the declaration
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``` cpp
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template<class U> U imag(const U&) = delete;
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```
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If the function selected by overload resolution does not return the
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imaginary part of its input, the program is ill-formed, no diagnostic
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required;
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- otherwise, `((void)E, T{})`.
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