mark length of readonly tuple as readonly

src/compiler/checker.ts

@@ -14036,7 +14036,7 @@ namespace ts {
                 }
             }
             const fixedLength = properties.length;
-            const lengthSymbol = createSymbol(SymbolFlags.Property, "length" as __String);
+            const lengthSymbol = createSymbol(SymbolFlags.Property, "length" as __String, readonly ? CheckFlags.Readonly : 0);
             if (combinedFlags & ElementFlags.Variable) {
                 lengthSymbol.type = numberType;
             }

tests/baselines/reference/tupleTypes.errors.txt

@@ -23,9 +23,13 @@ tests/cases/compiler/tupleTypes.ts(50,1): error TS2322: Type '[number, number]'
 tests/cases/compiler/tupleTypes.ts(51,1): error TS2322: Type '[number, {}]' is not assignable to type '[number, string]'.
   Type at position 1 in source is not compatible with type at position 1 in target.
     Type '{}' is not assignable to type 'string'.
+tests/cases/compiler/tupleTypes.ts(57,1): error TS2322: Type '0' is not assignable to type '1'.
+tests/cases/compiler/tupleTypes.ts(59,4): error TS2540: Cannot assign to 'length' because it is a read-only property.
+tests/cases/compiler/tupleTypes.ts(61,4): error TS2540: Cannot assign to 'length' because it is a read-only property.
+tests/cases/compiler/tupleTypes.ts(63,4): error TS2540: Cannot assign to 'length' because it is a read-only property.
 
 
-==== tests/cases/compiler/tupleTypes.ts (14 errors) ====
+==== tests/cases/compiler/tupleTypes.ts (18 errors) ====
     var v1: [];  // Error
     var v2: [number];
     var v3: [number, string];
@@ -120,4 +124,24 @@ tests/cases/compiler/tupleTypes.ts(51,1): error TS2322: Type '[number, {}]' is n
 !!! error TS2322:     Type '{}' is not assignable to type 'string'.
     a3 = a1;
     a3 = a2;
+
+    type B = Pick<[number], 'length'>;
+    declare const b: B;
+    b.length = 0; // Error
+    ~~~~~~~~
+!!! error TS2322: Type '0' is not assignable to type '1'.
+    declare const b1: readonly [number?];
+    b1.length = 0; // Error
+       ~~~~~~
+!!! error TS2540: Cannot assign to 'length' because it is a read-only property.
+    declare const b2: readonly [number, ...number[]];
+    b2.length = 0; // Error
+       ~~~~~~
+!!! error TS2540: Cannot assign to 'length' because it is a read-only property.
+    declare const b3: readonly number[];
+    b3.length = 0; // Error
+       ~~~~~~
+!!! error TS2540: Cannot assign to 'length' because it is a read-only property.
+    declare const b4: [number?];
+    b4.length = 0;
 

tests/baselines/reference/tupleTypes.js

@@ -52,6 +52,18 @@ a1 = a2;  // Error
 a1 = a3;  // Error
 a3 = a1;
 a3 = a2;
+
+type B = Pick<[number], 'length'>;
+declare const b: B;
+b.length = 0; // Error
+declare const b1: readonly [number?];
+b1.length = 0; // Error
+declare const b2: readonly [number, ...number[]];
+b2.length = 0; // Error
+declare const b3: readonly number[];
+b3.length = 0; // Error
+declare const b4: [number?];
+b4.length = 0;
 
 
 //// [tupleTypes.js]
@@ -99,3 +111,8 @@ a1 = a2; // Error
 a1 = a3; // Error
 a3 = a1;
 a3 = a2;
+b.length = 0; // Error
+b1.length = 0; // Error
+b2.length = 0; // Error
+b3.length = 0; // Error
+b4.length = 0;

tests/baselines/reference/tupleTypes.symbols

@@ -184,3 +184,48 @@ a3 = a2;
 >a3 : Symbol(a3, Decl(tupleTypes.ts, 45, 3))
 >a2 : Symbol(a2, Decl(tupleTypes.ts, 44, 3))
 
+type B = Pick<[number], 'length'>;
+>B : Symbol(B, Decl(tupleTypes.ts, 52, 8))
+>Pick : Symbol(Pick, Decl(lib.es5.d.ts, --, --))
+
+declare const b: B;
+>b : Symbol(b, Decl(tupleTypes.ts, 55, 13))
+>B : Symbol(B, Decl(tupleTypes.ts, 52, 8))
+
+b.length = 0; // Error
+>b.length : Symbol(length)
+>b : Symbol(b, Decl(tupleTypes.ts, 55, 13))
+>length : Symbol(length)
+
+declare const b1: readonly [number?];
+>b1 : Symbol(b1, Decl(tupleTypes.ts, 57, 13))
+
+b1.length = 0; // Error
+>b1.length : Symbol(length)
+>b1 : Symbol(b1, Decl(tupleTypes.ts, 57, 13))
+>length : Symbol(length)
+
+declare const b2: readonly [number, ...number[]];
+>b2 : Symbol(b2, Decl(tupleTypes.ts, 59, 13))
+
+b2.length = 0; // Error
+>b2.length : Symbol(length)
+>b2 : Symbol(b2, Decl(tupleTypes.ts, 59, 13))
+>length : Symbol(length)
+
+declare const b3: readonly number[];
+>b3 : Symbol(b3, Decl(tupleTypes.ts, 61, 13))
+
+b3.length = 0; // Error
+>b3.length : Symbol(ReadonlyArray.length, Decl(lib.es5.d.ts, --, --))
+>b3 : Symbol(b3, Decl(tupleTypes.ts, 61, 13))
+>length : Symbol(ReadonlyArray.length, Decl(lib.es5.d.ts, --, --))
+
+declare const b4: [number?];
+>b4 : Symbol(b4, Decl(tupleTypes.ts, 63, 13))
+
+b4.length = 0;
+>b4.length : Symbol(length)
+>b4 : Symbol(b4, Decl(tupleTypes.ts, 63, 13))
+>length : Symbol(length)
+

tests/baselines/reference/tupleTypes.types

@@ -226,3 +226,56 @@ a3 = a2;
 >a3 : [number, {}]
 >a2 : [number, number]
 
+type B = Pick<[number], 'length'>;
+>B : B
+
+declare const b: B;
+>b : B
+
+b.length = 0; // Error
+>b.length = 0 : 0
+>b.length : 1
+>b : B
+>length : 1
+>0 : 0
+
+declare const b1: readonly [number?];
+>b1 : readonly [number?]
+
+b1.length = 0; // Error
+>b1.length = 0 : 0
+>b1.length : any
+>b1 : readonly [number?]
+>length : any
+>0 : 0
+
+declare const b2: readonly [number, ...number[]];
+>b2 : readonly [number, ...number[]]
+
+b2.length = 0; // Error
+>b2.length = 0 : 0
+>b2.length : any
+>b2 : readonly [number, ...number[]]
+>length : any
+>0 : 0
+
+declare const b3: readonly number[];
+>b3 : readonly number[]
+
+b3.length = 0; // Error
+>b3.length = 0 : 0
+>b3.length : any
+>b3 : readonly number[]
+>length : any
+>0 : 0
+
+declare const b4: [number?];
+>b4 : [number?]
+
+b4.length = 0;
+>b4.length = 0 : 0
+>b4.length : 0 | 1
+>b4 : [number?]
+>length : 0 | 1
+>0 : 0
+

tests/cases/compiler/tupleTypes.ts

@@ -51,3 +51,15 @@ a1 = a2;  // Error
 a1 = a3;  // Error
 a3 = a1;
 a3 = a2;
+
+type B = Pick<[number], 'length'>;
+declare const b: B;
+b.length = 0; // Error
+declare const b1: readonly [number?];
+b1.length = 0; // Error
+declare const b2: readonly [number, ...number[]];
+b2.length = 0; // Error
+declare const b3: readonly number[];
+b3.length = 0; // Error
+declare const b4: [number?];
+b4.length = 0;