Allow preferinfer modifier on type parameters

Author: Andarist

src/compiler/factory/nodeFactory.ts

@@ -1331,6 +1331,7 @@ export function createNodeFactory(flags: NodeFactoryFlags, baseFactory: BaseNode
             case SyntaxKind.InKeyword:
             case SyntaxKind.OutKeyword:
             case SyntaxKind.OverrideKeyword:
+            case SyntaxKind.PreferInferKeyword:
             case SyntaxKind.StringKeyword:
             case SyntaxKind.BooleanKeyword:
             case SyntaxKind.SymbolKeyword:
@@ -1417,6 +1418,7 @@ export function createNodeFactory(flags: NodeFactoryFlags, baseFactory: BaseNode
         if (flags & ModifierFlags.Async) result.push(createModifier(SyntaxKind.AsyncKeyword));
         if (flags & ModifierFlags.In) result.push(createModifier(SyntaxKind.InKeyword));
         if (flags & ModifierFlags.Out) result.push(createModifier(SyntaxKind.OutKeyword));
+        if (flags & ModifierFlags.PreferInfer) result.push(createModifier(SyntaxKind.PreferInferKeyword));
         return result.length ? result : undefined;
     }
 

src/compiler/program.ts

@@ -3016,6 +3016,7 @@ export function createProgram(rootNamesOrOptions: readonly string[] | CreateProg
                         case SyntaxKind.OverrideKeyword:
                         case SyntaxKind.InKeyword:
                         case SyntaxKind.OutKeyword:
+                        case SyntaxKind.PreferInferKeyword:
                             diagnostics.push(createDiagnosticForNode(modifier, Diagnostics.The_0_modifier_can_only_be_used_in_TypeScript_files, tokenToString(modifier.kind)));
                             break;
 

src/compiler/scanner.ts

@@ -156,6 +156,7 @@ export const textToKeywordObj: MapLike<KeywordSyntaxKind> = {
     number: SyntaxKind.NumberKeyword,
     object: SyntaxKind.ObjectKeyword,
     package: SyntaxKind.PackageKeyword,
+    preferinfer: SyntaxKind.PreferInferKeyword,
     private: SyntaxKind.PrivateKeyword,
     protected: SyntaxKind.ProtectedKeyword,
     public: SyntaxKind.PublicKeyword,

src/compiler/transformers/ts.ts

@@ -602,6 +602,7 @@ export function transformTypeScript(context: TransformationContext) {
             case SyntaxKind.ReadonlyKeyword:
             case SyntaxKind.InKeyword:
             case SyntaxKind.OutKeyword:
+            case SyntaxKind.PreferInferKeyword:
             // TypeScript accessibility and readonly modifiers are elided
             // falls through
             case SyntaxKind.ArrayType:

src/compiler/types.ts

@@ -198,6 +198,7 @@ export const enum SyntaxKind {
     NamespaceKeyword,
     NeverKeyword,
     OutKeyword,
+    PreferInferKeyword,
     ReadonlyKeyword,
     RequireKeyword,
     NumberKeyword,
@@ -625,6 +626,7 @@ export type KeywordSyntaxKind =
     | SyntaxKind.ObjectKeyword
     | SyntaxKind.OfKeyword
     | SyntaxKind.PackageKeyword
+    | SyntaxKind.PreferInferKeyword
     | SyntaxKind.PrivateKeyword
     | SyntaxKind.ProtectedKeyword
     | SyntaxKind.PublicKeyword
@@ -665,6 +667,7 @@ export type ModifierSyntaxKind =
     | SyntaxKind.DefaultKeyword
     | SyntaxKind.ExportKeyword
     | SyntaxKind.InKeyword
+    | SyntaxKind.PreferInferKeyword
     | SyntaxKind.PrivateKeyword
     | SyntaxKind.ProtectedKeyword
     | SyntaxKind.PublicKeyword
@@ -858,16 +861,17 @@ export const enum ModifierFlags {
     In =                 1 << 15, // Contravariance modifier
     Out =                1 << 16, // Covariance modifier
     Decorator =          1 << 17, // Contains a decorator.
+    PreferInfer =        1 << 18, // preferinfer modifier on type params to allow partial inference
     HasComputedFlags =   1 << 29, // Modifier flags have been computed
 
     AccessibilityModifier = Public | Private | Protected,
     // Accessibility modifiers and 'readonly' can be attached to a parameter in a constructor to make it a property.
     ParameterPropertyModifier = AccessibilityModifier | Readonly | Override,
     NonPublicAccessibilityModifier = Private | Protected,
 
-    TypeScriptModifier = Ambient | Public | Private | Protected | Readonly | Abstract | Const | Override | In | Out,
+    TypeScriptModifier = Ambient | Public | Private | Protected | Readonly | Abstract | Const | Override | In | Out | PreferInfer,
     ExportDefault = Export | Default,
-    All = Export | Ambient | Public | Private | Protected | Static | Readonly | Abstract | Accessor | Async | Default | Const | Deprecated | Override | In | Out | Decorator,
+    All = ExportDefault | Static | Accessor | Async | Deprecated | Decorator | TypeScriptModifier,
     Modifier = All & ~Decorator
 }
 
@@ -1606,6 +1610,7 @@ export type DeclareKeyword = ModifierToken<SyntaxKind.DeclareKeyword>;
 export type DefaultKeyword = ModifierToken<SyntaxKind.DefaultKeyword>;
 export type ExportKeyword = ModifierToken<SyntaxKind.ExportKeyword>;
 export type InKeyword = ModifierToken<SyntaxKind.InKeyword>;
+export type PreferInferKeyword = ModifierToken<SyntaxKind.PreferInferKeyword>;
 export type PrivateKeyword = ModifierToken<SyntaxKind.PrivateKeyword>;
 export type ProtectedKeyword = ModifierToken<SyntaxKind.ProtectedKeyword>;
 export type PublicKeyword = ModifierToken<SyntaxKind.PublicKeyword>;
@@ -1626,6 +1631,7 @@ export type Modifier =
     | DefaultKeyword
     | ExportKeyword
     | InKeyword
+    | PreferInferKeyword
     | PrivateKeyword
     | ProtectedKeyword
     | PublicKeyword

src/compiler/utilities.ts

@@ -6238,6 +6238,7 @@ export function modifierToFlag(token: SyntaxKind): ModifierFlags {
         case SyntaxKind.OverrideKeyword: return ModifierFlags.Override;
         case SyntaxKind.InKeyword: return ModifierFlags.In;
         case SyntaxKind.OutKeyword: return ModifierFlags.Out;
+        case SyntaxKind.PreferInferKeyword: return ModifierFlags.PreferInfer;
         case SyntaxKind.Decorator: return ModifierFlags.Decorator;
     }
     return ModifierFlags.None;

src/compiler/utilitiesPublic.ts

@@ -1523,6 +1523,7 @@ export function isModifierKind(token: SyntaxKind): token is Modifier["kind"] {
         case SyntaxKind.ExportKeyword:
         case SyntaxKind.InKeyword:
         case SyntaxKind.PublicKeyword:
+        case SyntaxKind.PreferInferKeyword:
         case SyntaxKind.PrivateKeyword:
         case SyntaxKind.ProtectedKeyword:
         case SyntaxKind.ReadonlyKeyword: