In JS, constructor functions infer from call+construct (#28353)

* constructor functions infer from call+construct

Also fix an incorrect combining of inferences for rest parameters: the
inferred types will be arrays in the body of the function and the
arguments from outside the function will be the element type.

* All functions infer from call+construct contexts

src/compiler/checker.ts

@@ -283,6 +283,7 @@ namespace ts {
             getNumberType: () => numberType,
             createPromiseType,
             createArrayType,
+            getElementTypeOfArrayType,
             getBooleanType: () => booleanType,
             getFalseType: (fresh?) => fresh ? falseType : regularFalseType,
             getTrueType: (fresh?) => fresh ? trueType : regularTrueType,
@@ -13222,6 +13223,10 @@ namespace ts {
             return !!(getObjectFlags(type) & ObjectFlags.Reference) && (<TypeReference>type).target === globalReadonlyArrayType;
         }
 
+        function getElementTypeOfArrayType(type: Type): Type | undefined {
+            return isArrayType(type) && (type as TypeReference).typeArguments ? (type as TypeReference).typeArguments![0] : undefined;
+        }
+
         function isArrayLikeType(type: Type): boolean {
             // A type is array-like if it is a reference to the global Array or global ReadonlyArray type,
             // or if it is not the undefined or null type and if it is assignable to ReadonlyArray<any>

src/compiler/types.ts

@@ -3141,6 +3141,7 @@ namespace ts {
         /* @internal */ getNeverType(): Type;
         /* @internal */ getUnionType(types: Type[], subtypeReduction?: UnionReduction): Type;
         /* @internal */ createArrayType(elementType: Type): Type;
+        /* @internal */ getElementTypeOfArrayType(arrayType: Type): Type | undefined;
         /* @internal */ createPromiseType(type: Type): Type;
 
         /* @internal */ createAnonymousType(symbol: Symbol, members: SymbolTable, callSignatures: Signature[], constructSignatures: Signature[], stringIndexInfo: IndexInfo | undefined, numberIndexInfo: IndexInfo | undefined): Type;