isAssignableFromType throws null check error for Function types

[leonsenft]

The implementation null asserts DartType.element2:

source_gen/source_gen/lib/src/type_checker.dart

Lines 170 to 171 in 0d4c54d

	bool isAssignableFromType(DartType staticType) =>
	isAssignableFrom(staticType.element2!);

Which is always null for Function types:
https://github.com/dart-lang/sdk/blob/d6ff1193c5eb1d4b98e232f545cce49c63492b5c/pkg/analyzer/lib/src/dart/element/type.dart#L95-L96

[leonsenft]

@grouma this has been the cause of several unique unhandled exceptions in the Angular compiler.

[grouma]

cc @natebosch

[natebosch]

Making an implementation that works for structural types is not trivial.

I think that these types also don't work in the angular compiler though - you can't inject a Function type right? Is there some safeguard you can move earlier in the compilation to prevent this?

[leonsenft]

I understand that, but perhaps it would make sense to throw an explicit error instead of knowingly failing a null check.

For now, I'm creating an extension method to wrap this method and throw such an error if the underlying type is a FunctionType.

[natebosch]

Yeah, I think we can throw some more informative errors, and perhaps even more eagerly in some cases.