Better error messages for aliased index types

src/compiler/checker.ts

@@ -25775,6 +25775,12 @@ namespace ts {
                 return grammarErrorOnNode(parameter.name, Diagnostics.An_index_signature_parameter_must_have_a_type_annotation);
             }
             if (parameter.type.kind !== SyntaxKind.StringKeyword && parameter.type.kind !== SyntaxKind.NumberKeyword) {
+                const type = getTypeFromTypeNode(parameter.type);
+
+                if (type.flags & TypeFlags.String || type.flags & TypeFlags.Number) {
+                    return grammarErrorOnNode(parameter.name, Diagnostics.An_index_signature_parameter_type_cannot_be_a_type_alias_Use_index_Colon_0_instead, typeToString(type));
+                }
+
                 return grammarErrorOnNode(parameter.name, Diagnostics.An_index_signature_parameter_type_must_be_string_or_number);
             }
             if (!node.type) {

src/compiler/diagnosticMessages.json

@@ -939,6 +939,10 @@
         "category": "Error",
         "code": 1335
     },
+    "An index signature parameter type cannot be a type alias. Use '[index: {0}]' instead.": {
+        "category": "Error",
+        "code": 1336
+    },
 
     "Duplicate identifier '{0}'.": {
         "category": "Error",

tests/baselines/reference/indexerConstraints2.errors.txt

@@ -1,9 +1,12 @@
 tests/cases/compiler/indexerConstraints2.ts(9,5): error TS2413: Numeric index type 'A' is not assignable to string index type 'B'.
 tests/cases/compiler/indexerConstraints2.ts(17,5): error TS2413: Numeric index type 'A' is not assignable to string index type 'B'.
 tests/cases/compiler/indexerConstraints2.ts(26,5): error TS2413: Numeric index type 'A' is not assignable to string index type 'B'.
+tests/cases/compiler/indexerConstraints2.ts(34,6): error TS1336: An index signature parameter type cannot be a type alias. Use '[index: number]' instead.
+tests/cases/compiler/indexerConstraints2.ts(40,6): error TS1336: An index signature parameter type cannot be a type alias. Use '[index: string]' instead.
+tests/cases/compiler/indexerConstraints2.ts(46,6): error TS1023: An index signature parameter type must be 'string' or 'number'.
 
 
-==== tests/cases/compiler/indexerConstraints2.ts (3 errors) ====
+==== tests/cases/compiler/indexerConstraints2.ts (6 errors) ====
     class A { a: number; }
     class B extends A { b: number; }
     
@@ -37,4 +40,29 @@ tests/cases/compiler/indexerConstraints2.ts(26,5): error TS2413: Numeric index t
         ~~~~~~~~~~~~~~~
 !!! error TS2413: Numeric index type 'A' is not assignable to string index type 'B'.
         [s: string]: B;
+    }
+    
+    
+    type AliasedNumber = number;
+    
+    interface L {
+        [n: AliasedNumber]: A;
+         ~
+!!! error TS1336: An index signature parameter type cannot be a type alias. Use '[index: number]' instead.
+    }
+    
+    type AliasedString = string;
+    
+    interface M {
+        [s: AliasedString]: A;
+         ~
+!!! error TS1336: An index signature parameter type cannot be a type alias. Use '[index: string]' instead.
+    }
+    
+    type AliasedBoolean = boolean;
+    
+    interface N {
+        [b: AliasedBoolean]: A;
+         ~
+!!! error TS1023: An index signature parameter type must be 'string' or 'number'.
     }

tests/baselines/reference/indexerConstraints2.js

@@ -26,6 +26,25 @@ class J {
 class K extends J {
     [n: number]: A;
     [s: string]: B;
+}
+
+
+type AliasedNumber = number;
+
+interface L {
+    [n: AliasedNumber]: A;
+}
+
+type AliasedString = string;
+
+interface M {
+    [s: AliasedString]: A;
+}
+
+type AliasedBoolean = boolean;
+
+interface N {
+    [b: AliasedBoolean]: A;
 }
 
 //// [indexerConstraints2.js]

tests/baselines/reference/indexerConstraints2.symbols

@@ -62,3 +62,40 @@ class K extends J {
 >s : Symbol(s, Decl(indexerConstraints2.ts, 26, 5))
 >B : Symbol(B, Decl(indexerConstraints2.ts, 0, 22))
 }
+
+
+type AliasedNumber = number;
+>AliasedNumber : Symbol(AliasedNumber, Decl(indexerConstraints2.ts, 27, 1))
+
+interface L {
+>L : Symbol(L, Decl(indexerConstraints2.ts, 30, 28))
+
+    [n: AliasedNumber]: A;
+>n : Symbol(n, Decl(indexerConstraints2.ts, 33, 5))
+>AliasedNumber : Symbol(AliasedNumber, Decl(indexerConstraints2.ts, 27, 1))
+>A : Symbol(A, Decl(indexerConstraints2.ts, 0, 0))
+}
+
+type AliasedString = string;
+>AliasedString : Symbol(AliasedString, Decl(indexerConstraints2.ts, 34, 1))
+
+interface M {
+>M : Symbol(M, Decl(indexerConstraints2.ts, 36, 28))
+
+    [s: AliasedString]: A;
+>s : Symbol(s, Decl(indexerConstraints2.ts, 39, 5))
+>AliasedString : Symbol(AliasedString, Decl(indexerConstraints2.ts, 34, 1))
+>A : Symbol(A, Decl(indexerConstraints2.ts, 0, 0))
+}
+
+type AliasedBoolean = boolean;
+>AliasedBoolean : Symbol(AliasedBoolean, Decl(indexerConstraints2.ts, 40, 1))
+
+interface N {
+>N : Symbol(N, Decl(indexerConstraints2.ts, 42, 30))
+
+    [b: AliasedBoolean]: A;
+>b : Symbol(b, Decl(indexerConstraints2.ts, 45, 5))
+>AliasedBoolean : Symbol(AliasedBoolean, Decl(indexerConstraints2.ts, 40, 1))
+>A : Symbol(A, Decl(indexerConstraints2.ts, 0, 0))
+}

tests/baselines/reference/indexerConstraints2.types

@@ -62,3 +62,40 @@ class K extends J {
 >s : string
 >B : B
 }
+
+
+type AliasedNumber = number;
+>AliasedNumber : number
+
+interface L {
+>L : L
+
+    [n: AliasedNumber]: A;
+>n : number
+>AliasedNumber : number
+>A : A
+}
+
+type AliasedString = string;
+>AliasedString : string
+
+interface M {
+>M : M
+
+    [s: AliasedString]: A;
+>s : string
+>AliasedString : string
+>A : A
+}
+
+type AliasedBoolean = boolean;
+>AliasedBoolean : boolean
+
+interface N {
+>N : N
+
+    [b: AliasedBoolean]: A;
+>b : boolean
+>AliasedBoolean : boolean
+>A : A
+}

tests/cases/compiler/indexerConstraints2.ts

@@ -25,4 +25,23 @@ class J {
 class K extends J {
     [n: number]: A;
     [s: string]: B;
+}
+
+
+type AliasedNumber = number;
+
+interface L {
+    [n: AliasedNumber]: A;
+}
+
+type AliasedString = string;
+
+interface M {
+    [s: AliasedString]: A;
+}
+
+type AliasedBoolean = boolean;
+
+interface N {
+    [b: AliasedBoolean]: A;
 }