Creates a reasonable workaround for microsoft#46939

Author: RyanCavanaugh

src/compiler/checker.ts

@@ -14203,6 +14203,11 @@ namespace ts {
             if (match) {
                 return match;
             }
+            // [] and [T] immediately reduce to [] and [T] respectively
+            if (types.length < 2) {
+                return types;
+            }
+
             // We assume that redundant primitive types have already been removed from the types array and that there
             // are no any and unknown types in the array. Thus, the only possible supertypes for primitive types are empty
             // object types, and if none of those are present we can exclude primitive types from the subtype check.
@@ -14328,6 +14333,7 @@ namespace ts {
             if (types.length === 1) {
                 return types[0];
             }
+
             let typeSet: Type[] | undefined = [];
             const includes = addTypesToUnion(typeSet, 0, types);
             if (unionReduction !== UnionReduction.None) {

tests/baselines/reference/trivialSubtypeReductionNoStructuralCheck.errors.txt

@@ -0,0 +1,19 @@
+tests/cases/compiler/trivialSubtypeReductionNoStructuralCheck.ts(3,7): error TS7023: 'steps' implicitly has return type 'any' because it does not have a return type annotation and is referenced directly or indirectly in one of its return expressions.
+
+
+==== tests/cases/compiler/trivialSubtypeReductionNoStructuralCheck.ts (1 errors) ====
+    declare const props: WizardStepProps;
+    export class Wizard {
+      get steps() {
+          ~~~~~
+!!! error TS7023: 'steps' implicitly has return type 'any' because it does not have a return type annotation and is referenced directly or indirectly in one of its return expressions.
+        return {
+          wizard: this,
+          ...props,
+        } as WizardStepProps;
+      }
+    }
+    
+    export interface WizardStepProps {
+      wizard?: Wizard;
+    }

tests/baselines/reference/trivialSubtypeReductionNoStructuralCheck.js

@@ -0,0 +1,43 @@
+//// [trivialSubtypeReductionNoStructuralCheck.ts]
+declare const props: WizardStepProps;
+export class Wizard {
+  get steps() {
+    return {
+      wizard: this,
+      ...props,
+    } as WizardStepProps;
+  }
+}
+
+export interface WizardStepProps {
+  wizard?: Wizard;
+}
+
+//// [trivialSubtypeReductionNoStructuralCheck.js]
+"use strict";
+var __assign = (this && this.__assign) || function () {
+    __assign = Object.assign || function(t) {
+        for (var s, i = 1, n = arguments.length; i < n; i++) {
+            s = arguments[i];
+            for (var p in s) if (Object.prototype.hasOwnProperty.call(s, p))
+                t[p] = s[p];
+        }
+        return t;
+    };
+    return __assign.apply(this, arguments);
+};
+Object.defineProperty(exports, "__esModule", { value: true });
+exports.Wizard = void 0;
+var Wizard = /** @class */ (function () {
+    function Wizard() {
+    }
+    Object.defineProperty(Wizard.prototype, "steps", {
+        get: function () {
+            return __assign({ wizard: this }, props);
+        },
+        enumerable: false,
+        configurable: true
+    });
+    return Wizard;
+}());
+exports.Wizard = Wizard;

tests/baselines/reference/trivialSubtypeReductionNoStructuralCheck.symbols

@@ -0,0 +1,31 @@
+=== tests/cases/compiler/trivialSubtypeReductionNoStructuralCheck.ts ===
+declare const props: WizardStepProps;
+>props : Symbol(props, Decl(trivialSubtypeReductionNoStructuralCheck.ts, 0, 13))
+>WizardStepProps : Symbol(WizardStepProps, Decl(trivialSubtypeReductionNoStructuralCheck.ts, 8, 1))
+
+export class Wizard {
+>Wizard : Symbol(Wizard, Decl(trivialSubtypeReductionNoStructuralCheck.ts, 0, 37))
+
+  get steps() {
+>steps : Symbol(Wizard.steps, Decl(trivialSubtypeReductionNoStructuralCheck.ts, 1, 21))
+
+    return {
+      wizard: this,
+>wizard : Symbol(wizard, Decl(trivialSubtypeReductionNoStructuralCheck.ts, 3, 12))
+>this : Symbol(Wizard, Decl(trivialSubtypeReductionNoStructuralCheck.ts, 0, 37))
+
+      ...props,
+>props : Symbol(props, Decl(trivialSubtypeReductionNoStructuralCheck.ts, 0, 13))
+
+    } as WizardStepProps;
+>WizardStepProps : Symbol(WizardStepProps, Decl(trivialSubtypeReductionNoStructuralCheck.ts, 8, 1))
+  }
+}
+
+export interface WizardStepProps {
+>WizardStepProps : Symbol(WizardStepProps, Decl(trivialSubtypeReductionNoStructuralCheck.ts, 8, 1))
+
+  wizard?: Wizard;
+>wizard : Symbol(WizardStepProps.wizard, Decl(trivialSubtypeReductionNoStructuralCheck.ts, 10, 34))
+>Wizard : Symbol(Wizard, Decl(trivialSubtypeReductionNoStructuralCheck.ts, 0, 37))
+}

tests/baselines/reference/trivialSubtypeReductionNoStructuralCheck.types

@@ -0,0 +1,29 @@
+=== tests/cases/compiler/trivialSubtypeReductionNoStructuralCheck.ts ===
+declare const props: WizardStepProps;
+>props : WizardStepProps
+
+export class Wizard {
+>Wizard : Wizard
+
+  get steps() {
+>steps : any
+
+    return {
+>{      wizard: this,      ...props,    } as WizardStepProps : WizardStepProps
+>{      wizard: this,      ...props,    } : { wizard: Wizard; }
+
+      wizard: this,
+>wizard : this
+>this : this
+
+      ...props,
+>props : WizardStepProps
+
+    } as WizardStepProps;
+  }
+}
+
+export interface WizardStepProps {
+  wizard?: Wizard;
+>wizard : Wizard | undefined
+}

tests/baselines/reference/trivialSubtypeReductionNoStructuralCheck2.js

@@ -0,0 +1,43 @@
+//// [trivialSubtypeReductionNoStructuralCheck2.ts]
+declare const props: WizardStepProps;
+export class Wizard {
+  get steps() {
+    return {
+      wizard: this as Wizard,
+      ...props,
+    } as WizardStepProps;
+  }
+}
+
+export interface WizardStepProps {
+  wizard?: Wizard;
+}
+
+//// [trivialSubtypeReductionNoStructuralCheck2.js]
+"use strict";
+var __assign = (this && this.__assign) || function () {
+    __assign = Object.assign || function(t) {
+        for (var s, i = 1, n = arguments.length; i < n; i++) {
+            s = arguments[i];
+            for (var p in s) if (Object.prototype.hasOwnProperty.call(s, p))
+                t[p] = s[p];
+        }
+        return t;
+    };
+    return __assign.apply(this, arguments);
+};
+Object.defineProperty(exports, "__esModule", { value: true });
+exports.Wizard = void 0;
+var Wizard = /** @class */ (function () {
+    function Wizard() {
+    }
+    Object.defineProperty(Wizard.prototype, "steps", {
+        get: function () {
+            return __assign({ wizard: this }, props);
+        },
+        enumerable: false,
+        configurable: true
+    });
+    return Wizard;
+}());
+exports.Wizard = Wizard;

tests/baselines/reference/trivialSubtypeReductionNoStructuralCheck2.symbols

@@ -0,0 +1,32 @@
+=== tests/cases/compiler/trivialSubtypeReductionNoStructuralCheck2.ts ===
+declare const props: WizardStepProps;
+>props : Symbol(props, Decl(trivialSubtypeReductionNoStructuralCheck2.ts, 0, 13))
+>WizardStepProps : Symbol(WizardStepProps, Decl(trivialSubtypeReductionNoStructuralCheck2.ts, 8, 1))
+
+export class Wizard {
+>Wizard : Symbol(Wizard, Decl(trivialSubtypeReductionNoStructuralCheck2.ts, 0, 37))
+
+  get steps() {
+>steps : Symbol(Wizard.steps, Decl(trivialSubtypeReductionNoStructuralCheck2.ts, 1, 21))
+
+    return {
+      wizard: this as Wizard,
+>wizard : Symbol(wizard, Decl(trivialSubtypeReductionNoStructuralCheck2.ts, 3, 12))
+>this : Symbol(Wizard, Decl(trivialSubtypeReductionNoStructuralCheck2.ts, 0, 37))
+>Wizard : Symbol(Wizard, Decl(trivialSubtypeReductionNoStructuralCheck2.ts, 0, 37))
+
+      ...props,
+>props : Symbol(props, Decl(trivialSubtypeReductionNoStructuralCheck2.ts, 0, 13))
+
+    } as WizardStepProps;
+>WizardStepProps : Symbol(WizardStepProps, Decl(trivialSubtypeReductionNoStructuralCheck2.ts, 8, 1))
+  }
+}
+
+export interface WizardStepProps {
+>WizardStepProps : Symbol(WizardStepProps, Decl(trivialSubtypeReductionNoStructuralCheck2.ts, 8, 1))
+
+  wizard?: Wizard;
+>wizard : Symbol(WizardStepProps.wizard, Decl(trivialSubtypeReductionNoStructuralCheck2.ts, 10, 34))
+>Wizard : Symbol(Wizard, Decl(trivialSubtypeReductionNoStructuralCheck2.ts, 0, 37))
+}

tests/baselines/reference/trivialSubtypeReductionNoStructuralCheck2.types

@@ -0,0 +1,30 @@
+=== tests/cases/compiler/trivialSubtypeReductionNoStructuralCheck2.ts ===
+declare const props: WizardStepProps;
+>props : WizardStepProps
+
+export class Wizard {
+>Wizard : Wizard
+
+  get steps() {
+>steps : WizardStepProps
+
+    return {
+>{      wizard: this as Wizard,      ...props,    } as WizardStepProps : WizardStepProps
+>{      wizard: this as Wizard,      ...props,    } : { wizard: Wizard; }
+
+      wizard: this as Wizard,
+>wizard : Wizard
+>this as Wizard : Wizard
+>this : this
+
+      ...props,
+>props : WizardStepProps
+
+    } as WizardStepProps;
+  }
+}
+
+export interface WizardStepProps {
+  wizard?: Wizard;
+>wizard : Wizard | undefined
+}

tests/cases/compiler/trivialSubtypeReductionNoStructuralCheck.ts

@@ -0,0 +1,16 @@
+// @strict: true
+// @target: es5
+
+declare const props: WizardStepProps;
+export class Wizard {
+  get steps() {
+    return {
+      wizard: this,
+      ...props,
+    } as WizardStepProps;
+  }
+}
+
+export interface WizardStepProps {
+  wizard?: Wizard;
+}

tests/cases/compiler/trivialSubtypeReductionNoStructuralCheck2.ts

@@ -0,0 +1,16 @@
+// @strict: true
+// @target: es5
+
+declare const props: WizardStepProps;
+export class Wizard {
+  get steps() {
+    return {
+      wizard: this as Wizard,
+      ...props,
+    } as WizardStepProps;
+  }
+}
+
+export interface WizardStepProps {
+  wizard?: Wizard;
+}