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Improve error elaboration for invariant generic types

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This commit is contained in:
Anders Hejlsberg 2017-10-02 13:39:55 -07:00
parent bf75a3f4ac
commit bff843a9c9
1 changed files with 10 additions and 1 deletions
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11
src/compiler/checker.ts
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@ -9415,7 +9415,16 @@ namespace ts {
// with a structural comparison. Otherwise, we know for certain the instantiations aren't
// related and we can return here.
if (variances !== emptyArray && !hasCovariantVoidArgument(<TypeReference>target, variances)) {
return Ternary.False;
// In some cases generic types that are covariant in regular type checking mode become
// invariant in --strictFunctionTypes mode because one or more type parameters are used in
// both co- and contravariant positions. In order to make it easier to diagnose *why* such
// types are invariant, if any of the type parameters are invariant we reset the reported
// errors and instead force a structural comparison (which will include elaborations that
// reveal the reason).
if (!(reportErrors && some(variances, v => v === Variance.Invariant))) {
return Ternary.False;
}
errorInfo = saveErrorInfo;
}
}
// Even if relationship doesn't hold for unions, intersections, or generic type references,