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Add more commentary to getSpreadType

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This commit is contained in:
Nathan Shively-Sanders 2016-10-24 10:02:04 -07:00
parent a94fb2f5a3
commit 35be8c351c
1 changed files with 17 additions and 4 deletions
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21
src/compiler/checker.ts
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@ -5794,23 +5794,29 @@ namespace ts {
return spread;
}
/**
* Since the source of spread types are object literals and type literals, which are not binary,
* this function should be called in a left folding style, with left = previous result of getSpreadType
* and right = the new element to be spread.
*/
function getSpreadType(left: Type, right: Type, symbol: Symbol, aliasSymbol?: Symbol, aliasTypeArguments?: Type[]): Type {
// TODO: Reorder everything now that it's order independent, to be easier to read
const id = getTypeListId([left, right]);
if (id in spreadTypes) {
return spreadTypes[id];
}
// non-spreadable types
// any spreads to any
if (left.flags & TypeFlags.Any || right.flags & TypeFlags.Any) {
return anyType;
}
// flatten intersections to objects if all member types are objects
if (left.flags & TypeFlags.Intersection) {
left = resolveObjectIntersection(left as IntersectionType);
}
if (right.flags & TypeFlags.Intersection) {
right = resolveObjectIntersection(right as IntersectionType);
}
// distribute unions
if (left.flags & TypeFlags.Union) {
const spreads = map((left as UnionType).types,
t => getSpreadType(t, right, symbol, aliasSymbol, aliasTypeArguments));
@ -5821,7 +5827,14 @@ namespace ts {
t => getSpreadType(left, t, symbol, aliasSymbol, aliasTypeArguments));
return getUnionType(spreads, /*subtypeReduction*/ false, aliasSymbol, aliasTypeArguments);
}
if (right.flags & TypeFlags.Primitive) {
// skip primitives
if (left.flags & TypeFlags.Primitive && right.flags & TypeFlags.Primitive) {
return emptyObjectType;
}
else if (left.flags & TypeFlags.Primitive) {
return right;
}
else if (right.flags & TypeFlags.Primitive) {
return left;
}
@ -5845,7 +5858,7 @@ namespace ts {
// (T ... U) ... V to T ... (U ... V)
const rspread = right as SpreadType;
if (rspread.left === emptyObjectType) {
// U ... ({} ... T) => (U ... T)
// ... U ... ({} ... T) => ... U ... T
return getSpreadType(left, rspread.right, symbol, aliasSymbol, aliasTypeArguments);
}
return getSpreadType(getSpreadType(left, rspread.left, symbol, aliasSymbol, aliasTypeArguments),