Improve apparent type of mapped types

tests/baselines/reference/assignmentToAnyArrayRestParameters.errors.txt

@@ -0,0 +1,30 @@
+assignmentToAnyArrayRestParameters.ts(15,25): error TS2339: Property '0.0' does not exist on type 'string[]'.
+assignmentToAnyArrayRestParameters.ts(18,16): error TS2536: Type '"0.0"' cannot be used to index type 'T'.
+
+
+==== assignmentToAnyArrayRestParameters.ts (2 errors) ====
+    // Repros from #57122
+
+    function foo<T extends string[]>(
+        fa: (s: string, ...args: string[]) => string,
+        fb: (s: string, ...args: T) => string
+    ) {
+        const f1: (...args: any) => string = fa;
+        const f2: (...args: any[]) => string = fa;
+        const f3: (...args: any) => string = fb;
+        const f4: (...args: any[]) => string = fb;
+    }
+
+    function bar<T extends string[], K extends number>() {
+        type T00 = string[]["0"];
+        type T01 = string[]["0.0"];  // Error
+                            ~~~~~
+!!! error TS2339: Property '0.0' does not exist on type 'string[]'.
+        type T02 = string[][K | "0"];
+        type T10 = T["0"];
+        type T11 = T["0.0"];  // Error
+                   ~~~~~~~~
+!!! error TS2536: Type '"0.0"' cannot be used to index type 'T'.
+        type T12 = T[K | "0"];
+    }
+

tests/baselines/reference/assignmentToAnyArrayRestParameters.symbols

@@ -1,7 +1,7 @@
 //// [tests/cases/compiler/assignmentToAnyArrayRestParameters.ts] ////
 
 === assignmentToAnyArrayRestParameters.ts ===
-// Repro from #57122
+// Repros from #57122
 
 function foo<T extends string[]>(
 >foo : Symbol(foo, Decl(assignmentToAnyArrayRestParameters.ts, 0, 0))
@@ -40,3 +40,32 @@ function foo<T extends string[]>(
 >fb : Symbol(fb, Decl(assignmentToAnyArrayRestParameters.ts, 3, 49))
 }
 
+function bar<T extends string[], K extends number>() {
+>bar : Symbol(bar, Decl(assignmentToAnyArrayRestParameters.ts, 10, 1))
+>T : Symbol(T, Decl(assignmentToAnyArrayRestParameters.ts, 12, 13))
+>K : Symbol(K, Decl(assignmentToAnyArrayRestParameters.ts, 12, 32))
+
+    type T00 = string[]["0"];
+>T00 : Symbol(T00, Decl(assignmentToAnyArrayRestParameters.ts, 12, 54))
+
+    type T01 = string[]["0.0"];  // Error
+>T01 : Symbol(T01, Decl(assignmentToAnyArrayRestParameters.ts, 13, 29))
+
+    type T02 = string[][K | "0"];
+>T02 : Symbol(T02, Decl(assignmentToAnyArrayRestParameters.ts, 14, 31))
+>K : Symbol(K, Decl(assignmentToAnyArrayRestParameters.ts, 12, 32))
+
+    type T10 = T["0"];
+>T10 : Symbol(T10, Decl(assignmentToAnyArrayRestParameters.ts, 15, 33))
+>T : Symbol(T, Decl(assignmentToAnyArrayRestParameters.ts, 12, 13))
+
+    type T11 = T["0.0"];  // Error
+>T11 : Symbol(T11, Decl(assignmentToAnyArrayRestParameters.ts, 16, 22))
+>T : Symbol(T, Decl(assignmentToAnyArrayRestParameters.ts, 12, 13))
+
+    type T12 = T[K | "0"];
+>T12 : Symbol(T12, Decl(assignmentToAnyArrayRestParameters.ts, 17, 24))
+>T : Symbol(T, Decl(assignmentToAnyArrayRestParameters.ts, 12, 13))
+>K : Symbol(K, Decl(assignmentToAnyArrayRestParameters.ts, 12, 32))
+}
+

tests/baselines/reference/assignmentToAnyArrayRestParameters.types

@@ -1,7 +1,7 @@
 //// [tests/cases/compiler/assignmentToAnyArrayRestParameters.ts] ////
 
 === assignmentToAnyArrayRestParameters.ts ===
-// Repro from #57122
+// Repros from #57122
 
 function foo<T extends string[]>(
 >foo : <T extends string[]>(fa: (s: string, ...args: string[]) => string, fb: (s: string, ...args: T) => string) => void
@@ -38,3 +38,25 @@ function foo<T extends string[]>(
 >fb : (s: string, ...args: T) => string
 }
 
+function bar<T extends string[], K extends number>() {
+>bar : <T extends string[], K extends number>() => void
+
+    type T00 = string[]["0"];
+>T00 : string
+
+    type T01 = string[]["0.0"];  // Error
+>T01 : any
+
+    type T02 = string[][K | "0"];
+>T02 : string[][K | "0"]
+
+    type T10 = T["0"];
+>T10 : T["0"]
+
+    type T11 = T["0.0"];  // Error
+>T11 : T["0.0"]
+
+    type T12 = T[K | "0"];
+>T12 : T[K | "0"]
+}
+

tests/cases/compiler/assignmentToAnyArrayRestParameters.ts

@@ -1,7 +1,7 @@
 // @strict: true
 // @noEmit: true
 
-// Repro from #57122
+// Repros from #57122
 
 function foo<T extends string[]>(
     fa: (s: string, ...args: string[]) => string,
@@ -12,3 +12,12 @@ function foo<T extends string[]>(
     const f3: (...args: any) => string = fb;
     const f4: (...args: any[]) => string = fb;
 }
+
+function bar<T extends string[], K extends number>() {
+    type T00 = string[]["0"];
+    type T01 = string[]["0.0"];  // Error
+    type T02 = string[][K | "0"];
+    type T10 = T["0"];
+    type T11 = T["0.0"];  // Error
+    type T12 = T[K | "0"];
+}