This PR improves the error message encountered in the case of a type
class instance resolution failure, and adds an error explanation that
discusses the common new-user case of binary operation overloading and
points to the `trace.Meta.synthInstance` option for advanced debugging.
## Example
```lean4
def f (x : String) := x + x
```
Before:
```
failed to synthesize
HAdd String String ?m.5
Hint: Additional diagnostic information may be available using the `set_option diagnostics true` command.
```
After:
```
failed to synthesize instance of type class
HAdd String String ?m.5
Hint: Type class instance resolution failures can be inspected with the `set_option trace.Meta.synthInstance true` command.
Error code: lean.failedToSynthesizeTypeclassInstance
[View explanation](https://lean-lang.org/doc/reference/latest/find/?domain=Manual.errorExplanation&name=lean.failedToSynthesizeTypeclassInstance)
```
The error message is changed in three important ways:
* Explains *what* failed to synthesize, using the "type class"
terminology that's more likely to be recognized than the "instance"
terminology
* Points to the `trace.Meta.synthInstance` option which is otherwise
nearly undiscoverable but is quite powerful (see also
leanprover/reference-manual#663 which is adding commentary on this
option)
* Gives an error explanation link (which won't actually work until the
next release after this is merged) which prioritizes the common-case
explanation of using the wrong binary operation
This PR updates the formatting of, and adds explanations for, "unknown
identifier" errors as well as "failed to infer type" errors for binders
and definitions.
It attempts to ameliorate some of the confusion encountered in #1592 by
modifying the wording of the "header is elaborated before body is
processed" note and adding further discussion and examples of this
behavior in the corresponding error explanation.
This PR improves the “expected type mismatch” error message by omitting
the type's types when they are defeq, and putting them into separate
lines when not.
I found it rather tediuos to parse the error message when the expected
type is long, because I had to find the `:` in the middle of a large
expression somewhere. Also, when both are of sort `Prop` or `Type` it
doesn't add much value to print the sort (and it’s only one hover away
anyways).
This PR refines the new wording of the "application type mismatch" error
message to avoid ambiguity in references to the "final" argument in a
subexpression that may be followed by additional arguments.
It does so by replacing "final" with "last," rephrasing the message so
that this adjective modifies the argument itself rather than the word
"argument," and only displaying this wording when two arguments could be
confused (determined by expression equality).
These changes were motivated by a report that in cases where a function
application `f a b c` fails to elaborate because `b` is incorrectly
typed, the existing error message's reference to `b` being the "final"
argument in the application `f a b` may create confusion because it is
not the final argument in the full application expression.
This PR rewords the `application type mismatch` error message by more
specifically mentioning that the problem is with the final argument.
This is useful when the same argument is passed to the function multiple
times.
We decided against using a wording which specifically mentions the
"function expression", because users who are not used to currying might
not think of the `f a` in `f a b` as a function.
This PR enables the elaboration of theorem bodies, i.e. proofs, to
happen in parallel to each other as well as to other elaboration tasks.
Specifically, to be eligible for parallel proof elaboration,
* the theorem must not be in a `mutual` block
* `deprecated.oldSectionVars` must not be set
* `Elab.async` must be set (currently defaults to `true` in the language
server, `false` on the cmdline)
To be activated for downstream projects (i.e. in stage 1) pending
further Mathlib validation.
This PR adds an addition newline before the "Additional diagnostic
information may be available using the `set_option ... true` command."
messages, to provide better visual separation from the main error
message.
