* string|number inferences are low priority
Also, refactor unifyFromContext to explicitly handle priorities
* string/number/strnum are not mutually exclusive
* Assert that high/low can't apply to same element
* Parameters infer from body usage and call sites
* Function expressions infer from variable decl usages
If the function expression is the initialiser of a variable declaration.
* Update isApplicableFunctionForInference too
* Update baseline
* infer from usage's unification uses multiple passes
Previously, the unification step of infer-from-usage codefix would stop
as soon an answer was found. Now it continues if the result is
*incomplete*, with the idea that later passes may provide a better
inference.
Currently, an *incomplete* inference is
1. The type any.
2. The empty object type `{}` or a union or intersection that contains
`{}`.
In the checker, any takes priority over other types since it basically
shuts down type checking. For type inference, however, any is one of the least
useful inferences.
`{}` is not a good inference for a similar reason; as a parameter
inference, it doesn't tell the caller much about what is expected, and
it doesn't allow the function author to use an object as expected. But
currently it's inferred whenever there's an initialisation with the
value `{}`. With this change, subsequent property assignments to the
same parameter will replace the `{}` with a specific anonymous type. For
example:
```js
function C(config) {
if (config === undefined) config = {};
this.x = config.x;
this.y = config.y;
this.z = config.z;
}
```
* Unify all passes of inference from usage
In the previous commit, I changed inference from usage to continue
inference if a the result was *incomplete*. This commit now runs all 4
inference passes and combines them in a unification step. Currently the
unification step is simple, it:
1. Gathers all inferences in a list.
2. Makes properties of anonymous types optional if there is an empty
object in the inference list.
3. Removes *vacuous* inferences.
4. Combines the type in a union.
An inference is *vacuous* if it:
1. Is any or void, when a non-any, non-void type is also inferred.
2. Is the empty object type, when an object type that is not empty is
also inferred.
3. Is an anonymous type, when a non-nullable, non-any, non-void,
non-anonymous type is also inferred.
I think I might eventually want a concept of priorities, like the
compiler's type parameter inference, but I don't have enough examples to
be sure yet.
Eventually, unification should have an additional step that examines the
whole inference list to see if its contents are collectively
meaningless. A good example is `null | undefined`, which is not useful.
* Remove isNumberOrString
* Unify anonymous types
@andy-ms pointed out that my empty object code was a special case of
merging all anonymous types from an inference and making properties
optional that are not in all the anonymous type. So I did that instead.
* Use getTypeOfSymbolAtLocation instead of Symbol.type!
* Unify parameter call-site inferences too
Because they still have a separate code path, they didn't use the new
unification code.
Also some cleanup from PR comments.
* Add object type unification test
Also remove dead code.
* Only use fallback if no inferences were found
Instead of relying on the unification code to remove the fallback.
