Use base constraint when testing whether a type needs an Awaited<T> wrapper

src/compiler/checker.ts

@@ -36985,7 +36985,7 @@ namespace ts {
             }
 
             // primitives with a `{ then() }` won't be unwrapped/adopted.
-            if (allTypesAssignableToKind(type, TypeFlags.Primitive | TypeFlags.Never)) {
+            if (allTypesAssignableToKind(getBaseConstraintOrType(type), TypeFlags.Primitive | TypeFlags.Never)) {
                 return undefined;
             }
 
@@ -37060,7 +37060,7 @@ namespace ts {
          * Determines whether a type is an object with a callable `then` member.
          */
         function isThenableType(type: Type): boolean {
-            if (allTypesAssignableToKind(type, TypeFlags.Primitive | TypeFlags.Never)) {
+            if (allTypesAssignableToKind(getBaseConstraintOrType(type), TypeFlags.Primitive | TypeFlags.Never)) {
                 // primitive types cannot be considered "thenable" since they are not objects.
                 return false;
             }

tests/baselines/reference/asyncFunctionReturnType.2.js

@@ -0,0 +1,11 @@
+//// [asyncFunctionReturnType.2.ts]
+// https://github.com/microsoft/TypeScript/issues/47291
+class X {
+    f = async (): Promise<this> => this;
+}
+
+//// [asyncFunctionReturnType.2.js]
+// https://github.com/microsoft/TypeScript/issues/47291
+class X {
+    f = async () => this;
+}

tests/baselines/reference/asyncFunctionReturnType.2.symbols

@@ -0,0 +1,10 @@
+=== tests/cases/compiler/asyncFunctionReturnType.2.ts ===
+// https://github.com/microsoft/TypeScript/issues/47291
+class X {
+>X : Symbol(X, Decl(asyncFunctionReturnType.2.ts, 0, 0))
+
+    f = async (): Promise<this> => this;
+>f : Symbol(X.f, Decl(asyncFunctionReturnType.2.ts, 1, 9))
+>Promise : Symbol(Promise, Decl(lib.es5.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.promise.d.ts, --, --), Decl(lib.es2015.symbol.wellknown.d.ts, --, --), Decl(lib.es2018.promise.d.ts, --, --))
+>this : Symbol(X, Decl(asyncFunctionReturnType.2.ts, 0, 0))
+}

tests/baselines/reference/asyncFunctionReturnType.2.types

@@ -0,0 +1,10 @@
+=== tests/cases/compiler/asyncFunctionReturnType.2.ts ===
+// https://github.com/microsoft/TypeScript/issues/47291
+class X {
+>X : X
+
+    f = async (): Promise<this> => this;
+>f : () => Promise<this>
+>async (): Promise<this> => this : () => Promise<this>
+>this : this
+}

tests/cases/compiler/asyncFunctionReturnType.2.ts

@@ -0,0 +1,6 @@
+// @target: esnext
+
+// https://github.com/microsoft/TypeScript/issues/47291
+class X {
+    f = async (): Promise<this> => this;
+}