Add assignability rule relaxing the assignability of partial mapped types

src/compiler/checker.ts

@@ -12672,16 +12672,26 @@ namespace ts {
                             (<IndexedAccessType>template).indexType === getTypeParameterFromMappedType(target)) {
                             return Ternary.True;
                         }
-                        // A source type T is related to a target type { [P in Q]: X } if Q is related to keyof T and T[Q] is related to X.
-                        if (!isGenericMappedType(source) && isRelatedTo(getConstraintTypeFromMappedType(target), getIndexType(source))) {
-                            const indexedAccessType = getIndexedAccessType(source, getTypeParameterFromMappedType(target));
-                            const templateType = getTemplateTypeFromMappedType(target);
-                            if (result = isRelatedTo(indexedAccessType, templateType, reportErrors)) {
-                                return result;
+                        if (!isGenericMappedType(source)) {
+                            const targetConstraint = getConstraintTypeFromMappedType(target);
+                            const sourceKeys = getIndexType(source);
+                            const hasOptionalUnionKeys = modifiers & MappedTypeModifiers.IncludeOptional && targetConstraint.flags & TypeFlags.Union;
+                            const filteredByApplicability = hasOptionalUnionKeys ? filterType(targetConstraint, t => !!isRelatedTo(t, sourceKeys)) : undefined;
+                            // A source type T is related to a target type { [P in Q]: X } if Q is related to keyof T and T[Q] is related to X.
+                            // A source type T is related to a target type { [P in Q]?: X } if some Q = Q' is related to keyof T and T[Q'] is related to X.
+                            if (hasOptionalUnionKeys
+                                    ? !(filteredByApplicability!.flags & TypeFlags.Never)
+                                    : isRelatedTo(targetConstraint, sourceKeys)) {
+                                const indexingType = hasOptionalUnionKeys ? filteredByApplicability! : getTypeParameterFromMappedType(target);
+                                const indexedAccessType = getIndexedAccessType(source, indexingType);
+                                const templateType = getTemplateTypeFromMappedType(target);
+                                if (result = isRelatedTo(indexedAccessType, templateType, reportErrors)) {
+                                    return result;
+                                }
                             }
+                            originalErrorInfo = errorInfo;
+                            errorInfo = saveErrorInfo;
                         }
-                        originalErrorInfo = errorInfo;
-                        errorInfo = saveErrorInfo;
                     }
                 }
 

tests/baselines/reference/mappedTypePartialNonHomomorphicBaseConstraint.js

@@ -0,0 +1,21 @@
+//// [mappedTypePartialNonHomomorphicBaseConstraint.ts]
+export type Errors<D> = { readonly [K in keyof D | "base"]?: string[] };
+
+class Model<D> {
+  getErrors(): Errors<D> {
+    return { base: ["some base error"] };
+  }
+}
+
+
+//// [mappedTypePartialNonHomomorphicBaseConstraint.js]
+"use strict";
+exports.__esModule = true;
+var Model = /** @class */ (function () {
+    function Model() {
+    }
+    Model.prototype.getErrors = function () {
+        return { base: ["some base error"] };
+    };
+    return Model;
+}());

tests/baselines/reference/mappedTypePartialNonHomomorphicBaseConstraint.symbols

@@ -0,0 +1,21 @@
+=== tests/cases/compiler/mappedTypePartialNonHomomorphicBaseConstraint.ts ===
+export type Errors<D> = { readonly [K in keyof D | "base"]?: string[] };
+>Errors : Symbol(Errors, Decl(mappedTypePartialNonHomomorphicBaseConstraint.ts, 0, 0))
+>D : Symbol(D, Decl(mappedTypePartialNonHomomorphicBaseConstraint.ts, 0, 19))
+>K : Symbol(K, Decl(mappedTypePartialNonHomomorphicBaseConstraint.ts, 0, 36))
+>D : Symbol(D, Decl(mappedTypePartialNonHomomorphicBaseConstraint.ts, 0, 19))
+
+class Model<D> {
+>Model : Symbol(Model, Decl(mappedTypePartialNonHomomorphicBaseConstraint.ts, 0, 72))
+>D : Symbol(D, Decl(mappedTypePartialNonHomomorphicBaseConstraint.ts, 2, 12))
+
+  getErrors(): Errors<D> {
+>getErrors : Symbol(Model.getErrors, Decl(mappedTypePartialNonHomomorphicBaseConstraint.ts, 2, 16))
+>Errors : Symbol(Errors, Decl(mappedTypePartialNonHomomorphicBaseConstraint.ts, 0, 0))
+>D : Symbol(D, Decl(mappedTypePartialNonHomomorphicBaseConstraint.ts, 2, 12))
+
+    return { base: ["some base error"] };
+>base : Symbol(base, Decl(mappedTypePartialNonHomomorphicBaseConstraint.ts, 4, 12))
+  }
+}
+

tests/baselines/reference/mappedTypePartialNonHomomorphicBaseConstraint.types

@@ -0,0 +1,18 @@
+=== tests/cases/compiler/mappedTypePartialNonHomomorphicBaseConstraint.ts ===
+export type Errors<D> = { readonly [K in keyof D | "base"]?: string[] };
+>Errors : Errors<D>
+
+class Model<D> {
+>Model : Model<D>
+
+  getErrors(): Errors<D> {
+>getErrors : () => Errors<D>
+
+    return { base: ["some base error"] };
+>{ base: ["some base error"] } : { base: string[]; }
+>base : string[]
+>["some base error"] : string[]
+>"some base error" : "some base error"
+  }
+}
+

tests/cases/compiler/mappedTypePartialNonHomomorphicBaseConstraint.ts

@@ -0,0 +1,7 @@
+export type Errors<D> = { readonly [K in keyof D | "base"]?: string[] };
+
+class Model<D> {
+  getErrors(): Errors<D> {
+    return { base: ["some base error"] };
+  }
+}