Improve type inference to allow HKT technique function again

typescript-bot · May 3, 2023 · 33aeffb · 33aeffb
1 parent 94564cf
commit 33aeffb
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Showing 2 changed files with 20 additions and 10 deletions.
diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
@@ -17685,7 +17685,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
         // eagerly using the constraint type of 'this' at the given location.
         if (isGenericIndexType(indexType) || (accessNode && accessNode.kind !== SyntaxKind.IndexedAccessType ?
             isGenericTupleType(objectType) && !indexTypeLessThan(indexType, objectType.target.fixedLength) :
-            isGenericObjectType(objectType) && !(isTupleType(objectType) && indexTypeLessThan(indexType, objectType.target.fixedLength)) || isGenericReducibleType(objectType))) {
+            isGenericObjectType(objectType) && !(isTupleType(objectType) && indexTypeLessThan(indexType, objectType.target.fixedLength)) || !(accessFlags & AccessFlags.ResolveReducibleTypes) && isGenericReducibleType(objectType))) {
             if (objectType.flags & TypeFlags.AnyOrUnknown) {
                 return objectType;
             }
@@ -24511,6 +24511,15 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
                         }
                     }
                 }
+                // The following is a targeted fix to allow higher-kinded types to be emulated using the technique in #53970.
+                // Specifically, when an indexed access type was deferred because it has a reducible object type, here we force
+                // resolution of the type and then infer to the result.
+                if (target.flags & TypeFlags.IndexedAccess && isGenericReducibleType((target as IndexedAccessType).objectType)) {
+                    const instantiated = getIndexedAccessType((target as IndexedAccessType).objectType, (target as IndexedAccessType).indexType, AccessFlags.ResolveReducibleTypes);
+                    if (instantiated && instantiated !== target) {
+                        inferFromTypes(source, instantiated);
+                    }
+                }
             }
             if (getObjectFlags(source) & ObjectFlags.Reference && getObjectFlags(target) & ObjectFlags.Reference && (
                 (source as TypeReference).target === (target as TypeReference).target || isArrayType(source) && isArrayType(target)) &&

diff --git a/src/compiler/types.ts b/src/compiler/types.ts
@@ -6593,15 +6593,16 @@ export interface TypeParameter extends InstantiableType {
 export const enum AccessFlags {
     None = 0,
     IncludeUndefined = 1 << 0,
-    NoIndexSignatures = 1 << 1,
-    Writing = 1 << 2,
-    CacheSymbol = 1 << 3,
-    NoTupleBoundsCheck = 1 << 4,
-    ExpressionPosition = 1 << 5,
-    ReportDeprecated = 1 << 6,
-    SuppressNoImplicitAnyError = 1 << 7,
-    Contextual = 1 << 8,
-    Persistent = IncludeUndefined,
+    ResolveReducibleTypes = 1 << 1,
+    NoIndexSignatures = 1 << 2,
+    Writing = 1 << 3,
+    CacheSymbol = 1 << 4,
+    NoTupleBoundsCheck = 1 << 5,
+    ExpressionPosition = 1 << 6,
+    ReportDeprecated = 1 << 7,
+    SuppressNoImplicitAnyError = 1 << 8,
+    Contextual = 1 << 9,
+    Persistent = IncludeUndefined | ResolveReducibleTypes,
 }
 
 // Indexed access types (TypeFlags.IndexedAccess)