Add lambda/method group defaults proposal #6274
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Let's add some pointers to relevant background:
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This is a breaking change, right? Currently, the following code compiles: void M(int i = 1) {}
var m = M;
Action<int> a = m;But with this proposal, this will be an error, since |
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@svick yes this proposal would amount to a level of breaking change around lambdas and |
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I have now revised the proposal based on LDM, PR, and other feedback so I think it is ready for a second look |
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Excellent write up @adamperlin! |
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| Del del1 = (int x = 1) => x; // Allowed, because default parameter value in lambda matches default parameter value in delegate | ||
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| Del del2 = D; // This behavior does not change and compiles as before as per the method group conversion rules |
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It looks like it's covered below:
d = M; // Allowed with warning.
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Yep the case is covered there
| delegate void M2(long i = 2); | ||
| M2 m = (x = 3.0) => ...; //Error: cannot convert implicitly from long to double | ||
| ``` | ||
| This inference leads to some tricky conversion issues which would require more discussion. |
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It's not just inference that is a problem, it would increase the look ahead for our parser. Consider the state of the parser when it's looking at the expression that starts with a (. It has to decide if that is a lambda expression or not. Today the parser can bail out the moment it gets to (x = as that disqualifies it from being a lambda. If we allowed defaults on implicit then it has to parse all the way through the =>.
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Ah thanks for pointing this out. I will add a note about that as well
Championed Issue: #6051. This is a draft for a proposal to add default parameters to lambda functions and method groups, based heavily on the description in the championed issue. This topic has been discussed several times, but the proposal has yet to be merged.