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fix: prevent addPPExplicitToExposeDiff
from assigning metavariables
#5276
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kmill
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bb92b53
fix: prevent `addPPExplicitToExposeDiff` from assigning metavariables
kmill d018246
new strategy: just avoid faking higher-order unification; also adds s…
kmill 123c787
pp.mvars for 4405
kmill 2aec78c
be sure to instantiatemvars
kmill 1c1df9b
new strategy: do assignments, but without modifying state
kmill b7e66cf
test
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Original file line number | Diff line number | Diff line change |
---|---|---|
@@ -0,0 +1,65 @@ | ||
/-! | ||
# Tests of `addPPExplicitToExposeDiff` | ||
-/ | ||
set_option pp.mvars false | ||
|
||
/-! | ||
Basic example. | ||
-/ | ||
/-- | ||
error: type mismatch | ||
rfl | ||
has type | ||
?_ = ?_ : Prop | ||
but is expected to have type | ||
1 = 2 : Prop | ||
-/ | ||
#guard_msgs in example : 1 = 2 := by | ||
exact rfl | ||
|
||
|
||
/-! | ||
Error message shouldn't fake a higher-order unification. This next one used to give | ||
``` | ||
type mismatch | ||
test n2 ?_ | ||
has type | ||
(fun x ↦ x * 2) (g2 n2) = n2 : Prop | ||
but is expected to have type | ||
(fun x ↦ x * 2) (g2 n2) = n2 : Prop | ||
``` | ||
It now doesn't for the stronger reason that we don't let `addPPExplicitToExposeDiff` have side effects, | ||
but still it avoids doing incorrect higher-order unifications in its reasoning. | ||
-/ | ||
|
||
theorem test {f g : Nat → Nat} (n : Nat) (hfg : ∀ a, f (g a) = a) : | ||
f (g n) = n := hfg n | ||
|
||
/-- | ||
error: type mismatch | ||
test n2 ?_ | ||
has type | ||
?_ (?_ n2) = n2 : Prop | ||
but is expected to have type | ||
(fun x => x * 2) (g2 n2) = n2 : Prop | ||
-/ | ||
#guard_msgs in | ||
example {g2 : Nat → Nat} (n2 : Nat) : (fun x => x * 2) (g2 n2) = n2 := by | ||
with_reducible refine test n2 ?_ | ||
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||
|
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/-! | ||
Exposes an implicit argument because the explicit arguments can be unified. | ||
-/ | ||
def f {a : Nat} (b : Nat) : Prop := a + b = 0 | ||
/-- | ||
error: type mismatch | ||
sorry | ||
has type | ||
@f 0 ?_ : Prop | ||
but is expected to have type | ||
@f 1 2 : Prop | ||
-/ | ||
#guard_msgs in | ||
example : @f 1 2 := by | ||
exact (sorry : @f 0 _) |
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Hmm, in this instance it’s not really an improvement. As a user I look at this and wonder why these metavariables couldn’t just be instantiated. But maybe this example is somewhat artificial.
Maybe add the test from the issue description to see one where it’s a UX improvement?
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Yeah, this one is a bit obscure. I've added some tests.
I tried making a version that is consistent about partially assigning metavariables to mimic a half-successful isDefEq, but I think in the end that's more confusing, since you don't know what the real type is. It might be nice having a real isDefEq diagnostic function instead.