Per-file strictNullChecks support

Author: weswigham

src/compiler/checker.ts

@@ -4576,9 +4576,9 @@ namespace ts {
         /* @internal */ getParameterType(signature: Signature, parameterIndex: number): Type;
         /* @internal */ getParameterIdentifierNameAtPosition(signature: Signature, parameterIndex: number): [parameterName: __String, isRestParameter: boolean] | undefined;
         getNullableType(type: Type, flags: TypeFlags): Type;
-        getNonNullableType(type: Type): Type;
+        getNonNullableType(type: Type, context?: Node | undefined): Type;
         /* @internal */ getNonOptionalType(type: Type): Type;
-        /* @internal */ isNullableType(type: Type): boolean;
+        /* @internal */ isNullableType(type: Type, context?: Node | undefined): boolean;
         getTypeArguments(type: TypeReference): readonly Type[];
 
         // TODO: GH#18217 `xToDeclaration` calls are frequently asserted as defined.
@@ -4710,8 +4710,8 @@ namespace ts {
         /* @internal */ getFalseType(fresh?: boolean): Type;
         /* @internal */ getTrueType(fresh?: boolean): Type;
         /* @internal */ getVoidType(): Type;
-        /* @internal */ getUndefinedType(): Type;
-        /* @internal */ getNullType(): Type;
+        /* @internal */ getUndefinedType(widening?: boolean): Type;
+        /* @internal */ getNullType(widening?: boolean): Type;
         /* @internal */ getESSymbolType(): Type;
         /* @internal */ getNeverType(): Type;
         /* @internal */ getOptionalType(): Type;
@@ -5606,12 +5606,12 @@ namespace ts {
         Narrowable = Any | Unknown | StructuredOrInstantiable | StringLike | NumberLike | BigIntLike | BooleanLike | ESSymbol | UniqueESSymbol | NonPrimitive,
         // The following flags are aggregated during union and intersection type construction
         /* @internal */
-        IncludesMask = Any | Unknown | Primitive | Never | Object | Union | Intersection | NonPrimitive | TemplateLiteral,
+        IncludesMask = Any | Unknown | Primitive | Never | Object | Union | Intersection | NonPrimitive | TemplateLiteral | Index | StringMapping,
         // The following flags are used for different purposes during union and intersection type construction
         /* @internal */
         IncludesMissingType = TypeParameter,
         /* @internal */
-        IncludesNonWideningType = Index,
+        IncludesNonWideningType = 1 << 29, // repurpose for true typeflag when needed
         /* @internal */
         IncludesWildcard = IndexedAccess,
         /* @internal */

src/compiler/types.ts

@@ -2323,7 +2323,7 @@ declare namespace ts {
         getWidenedType(type: Type): Type;
         getReturnTypeOfSignature(signature: Signature): Type;
         getNullableType(type: Type, flags: TypeFlags): Type;
-        getNonNullableType(type: Type): Type;
+        getNonNullableType(type: Type, context?: Node | undefined): Type;
         getTypeArguments(type: TypeReference): readonly Type[];
         /** Note that the resulting nodes cannot be checked. */
         typeToTypeNode(type: Type, enclosingDeclaration: Node | undefined, flags: NodeBuilderFlags | undefined): TypeNode | undefined;

src/services/codefixes/inferFromUsage.ts

@@ -2323,7 +2323,7 @@ declare namespace ts {
         getWidenedType(type: Type): Type;
         getReturnTypeOfSignature(signature: Signature): Type;
         getNullableType(type: Type, flags: TypeFlags): Type;
-        getNonNullableType(type: Type): Type;
+        getNonNullableType(type: Type, context?: Node | undefined): Type;
         getTypeArguments(type: TypeReference): readonly Type[];
         /** Note that the resulting nodes cannot be checked. */
         typeToTypeNode(type: Type, enclosingDeclaration: Node | undefined, flags: NodeBuilderFlags | undefined): TypeNode | undefined;

tests/baselines/reference/api/tsserverlibrary.d.ts

@@ -23,7 +23,7 @@ function fetchMsg<V extends Message>(protoCtor: MsgConstructor<V>): V {
 >protoCtor : MsgConstructor<V>
 
   return null!;
->null! : null
+>null! : never
 >null : null
 }
 

tests/baselines/reference/api/typescript.d.ts

@@ -148,7 +148,6 @@ tests/cases/conformance/types/conditional/conditionalTypes2.ts(75,12): error TS2
 !!! error TS2345:   Property 'bat' is missing in type 'Bar & Foo' but required in type '{ foo: string; bat: string; }'.
 !!! error TS2345:     Type 'Extract<T, Bar>' is not assignable to type '{ foo: string; bat: string; }'.
 !!! error TS2345:       Property 'bat' is missing in type 'Bar & Foo' but required in type '{ foo: string; bat: string; }'.
-!!! related TS2728 tests/cases/conformance/types/conditional/conditionalTypes2.ts:62:43: 'bat' is declared here.
 !!! related TS2728 tests/cases/conformance/types/conditional/conditionalTypes2.ts:62:43: 'bat' is declared here.
         fooBat(y);  // Error
                ~

tests/baselines/reference/circularlyConstrainedMappedTypeContainingConditionalNoInfiniteInstantiationDepth.errors.txt

@@ -15,7 +15,7 @@ function fn<T extends { x: Map<T['x']> }>(sliceIndex: T): AllArg<T['x']> {
 >sliceIndex : T
 
     return null!;
->null! : null
+>null! : never
 >null : null
 }
 

tests/baselines/reference/collectionPatternNoError.types

@@ -80,7 +80,7 @@ class HelloWorld {
   handleEvent3(event: C3): T1 { return undefined! } // Ok, Error
 >handleEvent3 : (event: C3) => T1
 >event : C3
->undefined! : undefined
+>undefined! : never
 >undefined : undefined
 }