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Disallow type predicate as type assertion #5936

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a74a457
Error on type-predicate type in a type assertion
sandersn Dec 4, 2015
00f4c88
Add tests and accept baselines
sandersn Dec 4, 2015
e92a5b7
Use checkTypePredicate instead of new, custom error
sandersn Dec 7, 2015
55de051
Remove trailing comma from diagnosticMessages.json
sandersn Dec 7, 2015
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20 changes: 13 additions & 7 deletions src/compiler/checker.ts
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Expand Up @@ -9656,16 +9656,22 @@ namespace ts {
function checkAssertion(node: AssertionExpression) {
const exprType = getRegularTypeOfObjectLiteral(checkExpression(node.expression));
const targetType = getTypeFromTypeNode(node.type);
if (produceDiagnostics && targetType !== unknownType) {
const widenedType = getWidenedType(exprType);
if (produceDiagnostics) {
if (node.type.kind === SyntaxKind.TypePredicate) {
error(node.type, Diagnostics.A_type_predicate_is_not_allowed_as_part_of_a_type_assertion_expression);
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Rather than creating a new error message, why not call checkTypePredicate, which aught to error here?

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Thanks, I didn't know about that.

}
else if (targetType !== unknownType) {
const widenedType = getWidenedType(exprType);

// Permit 'number[] | "foo"' to be asserted to 'string'.
const bothAreStringLike =
someConstituentTypeHasKind(targetType, TypeFlags.StringLike) &&
// Permit 'number[] | "foo"' to be asserted to 'string'.
const bothAreStringLike =
someConstituentTypeHasKind(targetType, TypeFlags.StringLike) &&
someConstituentTypeHasKind(widenedType, TypeFlags.StringLike);
if (!bothAreStringLike && !(isTypeAssignableTo(targetType, widenedType))) {
checkTypeAssignableTo(exprType, targetType, node, Diagnostics.Neither_type_0_nor_type_1_is_assignable_to_the_other);
if (!bothAreStringLike && !(isTypeAssignableTo(targetType, widenedType))) {
checkTypeAssignableTo(exprType, targetType, node, Diagnostics.Neither_type_0_nor_type_1_is_assignable_to_the_other);
}
}

}
return targetType;
}
Expand Down
4 changes: 4 additions & 0 deletions src/compiler/diagnosticMessages.json
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Expand Up @@ -2551,5 +2551,9 @@
"A type assertion expression is not allowed in the left-hand side of an exponentiation expression. Consider enclosing the expression in parentheses.": {
"category": "Error",
"code": 17007
},
"A type predicate is not allowed as part of a type assertion expression.": {
"category": "Error",
"code": 17008
}
}
22 changes: 21 additions & 1 deletion tests/baselines/reference/typeAssertions.errors.txt
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Expand Up @@ -7,9 +7,13 @@ tests/cases/conformance/expressions/typeAssertions/typeAssertions.ts(37,13): err
Property 'q' is missing in type 'SomeDerived'.
tests/cases/conformance/expressions/typeAssertions/typeAssertions.ts(38,13): error TS2352: Neither type 'SomeBase' nor type 'SomeOther' is assignable to the other.
Property 'q' is missing in type 'SomeBase'.
tests/cases/conformance/expressions/typeAssertions/typeAssertions.ts(44,5): error TS17008: A type predicate is not allowed as part of a type assertion expression.
tests/cases/conformance/expressions/typeAssertions/typeAssertions.ts(45,2): error TS2322: Type 'number | string' is not assignable to type 'string'.
Type 'number' is not assignable to type 'string'.
tests/cases/conformance/expressions/typeAssertions/typeAssertions.ts(48,32): error TS17008: A type predicate is not allowed as part of a type assertion expression.


==== tests/cases/conformance/expressions/typeAssertions/typeAssertions.ts (5 errors) ====
==== tests/cases/conformance/expressions/typeAssertions/typeAssertions.ts (8 errors) ====
// Function call whose argument is a 1 arg generic function call with explicit type arguments
function fn1<T>(t: T) { }
function fn2(t: any) { }
Expand Down Expand Up @@ -64,5 +68,21 @@ tests/cases/conformance/expressions/typeAssertions/typeAssertions.ts(38,13): err
!!! error TS2352: Property 'q' is missing in type 'SomeBase'.
someOther = <SomeOther>someOther;

// Type assertion cannot be a type-predicate type
var numOrStr: number | string;
var str: string;
if(<numOrStr is string>(numOrStr === undefined)) { // Error
~~~~~~~~~~~~~~~~~~
!!! error TS17008: A type predicate is not allowed as part of a type assertion expression.
str = numOrStr; // Error, no narrowing occurred
~~~
!!! error TS2322: Type 'number | string' is not assignable to type 'string'.
!!! error TS2322: Type 'number' is not assignable to type 'string'.
}

if((numOrStr === undefined) as numOrStr is string) { // Error
~~~~~~~~~~~~~~~~~~
!!! error TS17008: A type predicate is not allowed as part of a type assertion expression.
}


17 changes: 17 additions & 0 deletions tests/baselines/reference/typeAssertions.js
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Expand Up @@ -39,6 +39,15 @@ someOther = <SomeOther>someDerived; // Error
someOther = <SomeOther>someBase; // Error
someOther = <SomeOther>someOther;

// Type assertion cannot be a type-predicate type
var numOrStr: number | string;
var str: string;
if(<numOrStr is string>(numOrStr === undefined)) { // Error
str = numOrStr; // Error, no narrowing occurred
}

if((numOrStr === undefined) as numOrStr is string) { // Error
}



Expand Down Expand Up @@ -87,3 +96,11 @@ someDerived = someOther; // Error
someOther = someDerived; // Error
someOther = someBase; // Error
someOther = someOther;
// Type assertion cannot be a type-predicate type
var numOrStr;
var str;
if ((numOrStr === undefined)) {
str = numOrStr; // Error, no narrowing occurred
}
if ((numOrStr === undefined)) {
}
9 changes: 9 additions & 0 deletions tests/cases/conformance/expressions/typeAssertions/typeAssertions.ts
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Expand Up @@ -38,4 +38,13 @@ someOther = <SomeOther>someDerived; // Error
someOther = <SomeOther>someBase; // Error
someOther = <SomeOther>someOther;

// Type assertion cannot be a type-predicate type
var numOrStr: number | string;
var str: string;
if(<numOrStr is string>(numOrStr === undefined)) { // Error
str = numOrStr; // Error, no narrowing occurred
}

if((numOrStr === undefined) as numOrStr is string) { // Error
}