invocation_of_non_function_expression

The expression doesn't evaluate to a function, so it can't be invoked.

The analyzer produces this diagnostic when a function invocation is found, but the name being referenced isn't the name of a function, or when the expression computing the function doesn't compute a function.

The following code produces this diagnostic because x isn't a function:

int x = 0;
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int f() => x;
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var y = x();

The following code produces this diagnostic because f() doesn't return a function:

int x = 0;
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int f() => x;
​
var y = f()();

If you need to invoke a function, then replace the code before the argument list with the name of a function or with an expression that computes a function:

int x = 0;
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int f() => x;
​
var y = f();