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Fixed a false positive "await has no effect on the type" diagnostic with mixed generic union #50833

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merged 1 commit into from Oct 18, 2022
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Fixed a false positive "await has no effect on the type" diagnostic with mixed generic union #50833

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2 changes: 1 addition & 1 deletion src/compiler/checker.ts
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Expand Up @@ -36923,7 +36923,7 @@ namespace ts {
// We only need `Awaited<T>` if `T` is a type variable that has no base constraint, or the base constraint of `T` is `any`, `unknown`, `{}`, `object`,
// or is promise-like.
if (baseConstraint ?
baseConstraint.flags & TypeFlags.AnyOrUnknown || isEmptyObjectType(baseConstraint) || isThenableType(baseConstraint) :
baseConstraint.flags & TypeFlags.AnyOrUnknown || isEmptyObjectType(baseConstraint) || someType(baseConstraint, isThenableType) :
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@Andarist Andarist Sep 18, 2022 •
edited

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An alternative fix would be to move someType to isThenableType:

patch 
diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 088ba325ec..89cfb6bcae 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -36884,8 +36884,10 @@ namespace ts {
                 return false;
             }
 
-            const thenFunction = getTypeOfPropertyOfType(type, "then" as __String);
-            return !!thenFunction && getSignaturesOfType(getTypeWithFacts(thenFunction, TypeFacts.NEUndefinedOrNull), SignatureKind.Call).length > 0;
+            return someType(type, t => {
+                const thenFunction = getTypeOfPropertyOfType(t, "then" as __String);
+                return !!thenFunction && getSignaturesOfType(getTypeWithFacts(thenFunction, TypeFacts.NEUndefinedOrNull), SignatureKind.Call).length > 0;
+            });
         }
 
         interface AwaitedTypeInstantiation extends Type {
@@ -36923,7 +36925,7 @@ namespace ts {
                 // We only need `Awaited<T>` if `T` is a type variable that has no base constraint, or the base constraint of `T` is `any`, `unknown`, `{}`, `object`,
                 // or is promise-like.
                 if (baseConstraint ?
-                    baseConstraint.flags & TypeFlags.AnyOrUnknown || isEmptyObjectType(baseConstraint) || someType(baseConstraint, isThenableType) :
+                    baseConstraint.flags & TypeFlags.AnyOrUnknown || isEmptyObjectType(baseConstraint) || isThenableType(baseConstraint) :
                     maybeTypeOfKind(type, TypeFlags.TypeVariable)) {
                     return true;
                 }

That would in turn impact the other call site to this util, here, and I'm not sure if that's desirable.

maybeTypeOfKind(type, TypeFlags.TypeVariable)) {
return true;
}
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12 changes: 12 additions & 0 deletions tests/cases/fourslash/codeFixRemoveUnnecessaryAwait_mixedUnion.ts
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@@ -0,0 +1,12 @@
/// <reference path="fourslash.ts" />

// @target: esnext
//// async function fn1(a: Promise<void> | void) {
//// await a;
//// }
////
//// async function fn2<T extends Promise<void> | void>(a: T) {
//// await a;
//// }

verify.getSuggestionDiagnostics([]);