Consider inferences between mapped type templates lower priority

src/compiler/checker.ts

@@ -21718,12 +21718,14 @@ namespace ts {
             }
 
             function inferFromIndexTypes(source: Type, target: Type) {
+                // Inferences across mapped type index signatures are pretty much the same a inferences to homomorphic variables
+                const priority = (getObjectFlags(source) & getObjectFlags(target) & ObjectFlags.Mapped) ? InferencePriority.HomomorphicMappedType : 0;
                 const targetStringIndexType = getIndexTypeOfType(target, IndexKind.String);
                 if (targetStringIndexType) {
                     const sourceIndexType = getIndexTypeOfType(source, IndexKind.String) ||
                         getImplicitIndexTypeOfType(source, IndexKind.String);
                     if (sourceIndexType) {
-                        inferFromTypes(sourceIndexType, targetStringIndexType);
+                        inferWithPriority(sourceIndexType, targetStringIndexType, priority);
                     }
                 }
                 const targetNumberIndexType = getIndexTypeOfType(target, IndexKind.Number);
@@ -21732,7 +21734,7 @@ namespace ts {
                         getIndexTypeOfType(source, IndexKind.String) ||
                         getImplicitIndexTypeOfType(source, IndexKind.Number);
                     if (sourceIndexType) {
-                        inferFromTypes(sourceIndexType, targetNumberIndexType);
+                        inferWithPriority(sourceIndexType, targetNumberIndexType, priority);
                     }
                 }
             }

tests/baselines/reference/localTypeParameterInferencePriority.js

@@ -0,0 +1,58 @@
+//// [localTypeParameterInferencePriority.ts]
+export type UnrollOnHover<O extends object> = O extends object ?
+    { [K in keyof O]: O[K]; } :
+    never;
+
+
+export type Schema = Record<string, unknown>;
+class Table<S extends Schema>  {
+    __schema!: S;
+
+    // Removing this line, removes the error 
+    getRows<C extends keyof S>(): Array<UnrollOnHover<Pick<S, C>>> {
+        return null!
+    }
+}
+
+class ColumnSelectViewImp<S extends Schema> extends Table<S> { }
+
+
+const ColumnSelectView1: new <S extends Schema>() => Table<UnrollOnHover<S>> = ColumnSelectViewImp;
+const ColumnSelectView2: new <S extends Schema>() => Table<UnrollOnHover<S>> = Table;
+
+//// [localTypeParameterInferencePriority.js]
+"use strict";
+var __extends = (this && this.__extends) || (function () {
+    var extendStatics = function (d, b) {
+        extendStatics = Object.setPrototypeOf ||
+            ({ __proto__: [] } instanceof Array && function (d, b) { d.__proto__ = b; }) ||
+            function (d, b) { for (var p in b) if (Object.prototype.hasOwnProperty.call(b, p)) d[p] = b[p]; };
+        return extendStatics(d, b);
+    };
+    return function (d, b) {
+        if (typeof b !== "function" && b !== null)
+            throw new TypeError("Class extends value " + String(b) + " is not a constructor or null");
+        extendStatics(d, b);
+        function __() { this.constructor = d; }
+        d.prototype = b === null ? Object.create(b) : (__.prototype = b.prototype, new __());
+    };
+})();
+exports.__esModule = true;
+var Table = /** @class */ (function () {
+    function Table() {
+    }
+    // Removing this line, removes the error 
+    Table.prototype.getRows = function () {
+        return null;
+    };
+    return Table;
+}());
+var ColumnSelectViewImp = /** @class */ (function (_super) {
+    __extends(ColumnSelectViewImp, _super);
+    function ColumnSelectViewImp() {
+        return _super !== null && _super.apply(this, arguments) || this;
+    }
+    return ColumnSelectViewImp;
+}(Table));
+var ColumnSelectView1 = ColumnSelectViewImp;
+var ColumnSelectView2 = Table;

tests/baselines/reference/localTypeParameterInferencePriority.symbols

@@ -0,0 +1,69 @@
+=== tests/cases/compiler/localTypeParameterInferencePriority.ts ===
+export type UnrollOnHover<O extends object> = O extends object ?
+>UnrollOnHover : Symbol(UnrollOnHover, Decl(localTypeParameterInferencePriority.ts, 0, 0))
+>O : Symbol(O, Decl(localTypeParameterInferencePriority.ts, 0, 26))
+>O : Symbol(O, Decl(localTypeParameterInferencePriority.ts, 0, 26))
+
+    { [K in keyof O]: O[K]; } :
+>K : Symbol(K, Decl(localTypeParameterInferencePriority.ts, 1, 7))
+>O : Symbol(O, Decl(localTypeParameterInferencePriority.ts, 0, 26))
+>O : Symbol(O, Decl(localTypeParameterInferencePriority.ts, 0, 26))
+>K : Symbol(K, Decl(localTypeParameterInferencePriority.ts, 1, 7))
+
+    never;
+
+
+export type Schema = Record<string, unknown>;
+>Schema : Symbol(Schema, Decl(localTypeParameterInferencePriority.ts, 2, 10))
+>Record : Symbol(Record, Decl(lib.es5.d.ts, --, --))
+
+class Table<S extends Schema>  {
+>Table : Symbol(Table, Decl(localTypeParameterInferencePriority.ts, 5, 45))
+>S : Symbol(S, Decl(localTypeParameterInferencePriority.ts, 6, 12))
+>Schema : Symbol(Schema, Decl(localTypeParameterInferencePriority.ts, 2, 10))
+
+    __schema!: S;
+>__schema : Symbol(Table.__schema, Decl(localTypeParameterInferencePriority.ts, 6, 32))
+>S : Symbol(S, Decl(localTypeParameterInferencePriority.ts, 6, 12))
+
+    // Removing this line, removes the error 
+    getRows<C extends keyof S>(): Array<UnrollOnHover<Pick<S, C>>> {
+>getRows : Symbol(Table.getRows, Decl(localTypeParameterInferencePriority.ts, 7, 17))
+>C : Symbol(C, Decl(localTypeParameterInferencePriority.ts, 10, 12))
+>S : Symbol(S, Decl(localTypeParameterInferencePriority.ts, 6, 12))
+>Array : Symbol(Array, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>UnrollOnHover : Symbol(UnrollOnHover, Decl(localTypeParameterInferencePriority.ts, 0, 0))
+>Pick : Symbol(Pick, Decl(lib.es5.d.ts, --, --))
+>S : Symbol(S, Decl(localTypeParameterInferencePriority.ts, 6, 12))
+>C : Symbol(C, Decl(localTypeParameterInferencePriority.ts, 10, 12))
+
+        return null!
+    }
+}
+
+class ColumnSelectViewImp<S extends Schema> extends Table<S> { }
+>ColumnSelectViewImp : Symbol(ColumnSelectViewImp, Decl(localTypeParameterInferencePriority.ts, 13, 1))
+>S : Symbol(S, Decl(localTypeParameterInferencePriority.ts, 15, 26))
+>Schema : Symbol(Schema, Decl(localTypeParameterInferencePriority.ts, 2, 10))
+>Table : Symbol(Table, Decl(localTypeParameterInferencePriority.ts, 5, 45))
+>S : Symbol(S, Decl(localTypeParameterInferencePriority.ts, 15, 26))
+
+
+const ColumnSelectView1: new <S extends Schema>() => Table<UnrollOnHover<S>> = ColumnSelectViewImp;
+>ColumnSelectView1 : Symbol(ColumnSelectView1, Decl(localTypeParameterInferencePriority.ts, 18, 5))
+>S : Symbol(S, Decl(localTypeParameterInferencePriority.ts, 18, 30))
+>Schema : Symbol(Schema, Decl(localTypeParameterInferencePriority.ts, 2, 10))
+>Table : Symbol(Table, Decl(localTypeParameterInferencePriority.ts, 5, 45))
+>UnrollOnHover : Symbol(UnrollOnHover, Decl(localTypeParameterInferencePriority.ts, 0, 0))
+>S : Symbol(S, Decl(localTypeParameterInferencePriority.ts, 18, 30))
+>ColumnSelectViewImp : Symbol(ColumnSelectViewImp, Decl(localTypeParameterInferencePriority.ts, 13, 1))
+
+const ColumnSelectView2: new <S extends Schema>() => Table<UnrollOnHover<S>> = Table;
+>ColumnSelectView2 : Symbol(ColumnSelectView2, Decl(localTypeParameterInferencePriority.ts, 19, 5))
+>S : Symbol(S, Decl(localTypeParameterInferencePriority.ts, 19, 30))
+>Schema : Symbol(Schema, Decl(localTypeParameterInferencePriority.ts, 2, 10))
+>Table : Symbol(Table, Decl(localTypeParameterInferencePriority.ts, 5, 45))
+>UnrollOnHover : Symbol(UnrollOnHover, Decl(localTypeParameterInferencePriority.ts, 0, 0))
+>S : Symbol(S, Decl(localTypeParameterInferencePriority.ts, 19, 30))
+>Table : Symbol(Table, Decl(localTypeParameterInferencePriority.ts, 5, 45))
+

tests/baselines/reference/localTypeParameterInferencePriority.types

@@ -0,0 +1,40 @@
+=== tests/cases/compiler/localTypeParameterInferencePriority.ts ===
+export type UnrollOnHover<O extends object> = O extends object ?
+>UnrollOnHover : UnrollOnHover<O>
+
+    { [K in keyof O]: O[K]; } :
+    never;
+
+
+export type Schema = Record<string, unknown>;
+>Schema : Schema
+
+class Table<S extends Schema>  {
+>Table : Table<S>
+
+    __schema!: S;
+>__schema : S
+
+    // Removing this line, removes the error 
+    getRows<C extends keyof S>(): Array<UnrollOnHover<Pick<S, C>>> {
+>getRows : <C extends keyof S>() => Array<UnrollOnHover<Pick<S, C>>>
+
+        return null!
+>null! : null
+>null : null
+    }
+}
+
+class ColumnSelectViewImp<S extends Schema> extends Table<S> { }
+>ColumnSelectViewImp : ColumnSelectViewImp<S>
+>Table : Table<S>
+
+
+const ColumnSelectView1: new <S extends Schema>() => Table<UnrollOnHover<S>> = ColumnSelectViewImp;
+>ColumnSelectView1 : new <S extends Schema>() => Table<UnrollOnHover<S>>
+>ColumnSelectViewImp : typeof ColumnSelectViewImp
+
+const ColumnSelectView2: new <S extends Schema>() => Table<UnrollOnHover<S>> = Table;
+>ColumnSelectView2 : new <S extends Schema>() => Table<UnrollOnHover<S>>
+>Table : typeof Table
+

tests/cases/compiler/localTypeParameterInferencePriority.ts

@@ -0,0 +1,20 @@
+export type UnrollOnHover<O extends object> = O extends object ?
+    { [K in keyof O]: O[K]; } :
+    never;
+
+
+export type Schema = Record<string, unknown>;
+class Table<S extends Schema>  {
+    __schema!: S;
+
+    // Removing this line, removes the error 
+    getRows<C extends keyof S>(): Array<UnrollOnHover<Pick<S, C>>> {
+        return null!
+    }
+}
+
+class ColumnSelectViewImp<S extends Schema> extends Table<S> { }
+
+
+const ColumnSelectView1: new <S extends Schema>() => Table<UnrollOnHover<S>> = ColumnSelectViewImp;
+const ColumnSelectView2: new <S extends Schema>() => Table<UnrollOnHover<S>> = Table;