Improved error messages for Union index signature params

src/compiler/checker.ts

@@ -25781,6 +25781,10 @@ namespace ts {
                     return grammarErrorOnNode(parameter.name, Diagnostics.An_index_signature_parameter_type_cannot_be_a_type_alias_Use_index_Colon_0_instead, typeToString(type));
                 }
 
+                if (type.flags & TypeFlags.Union && forEach((<UnionType>type).types, t => !!(t.flags & TypeFlags.StringOrNumberLiteral))) {
+                    return grammarErrorOnNode(parameter.name, Diagnostics.An_index_signature_parameter_type_cannot_be_a_union_type_Use_K_in_0_instead, symbolName(type.aliasSymbol));
+                }
+
                 return grammarErrorOnNode(parameter.name, Diagnostics.An_index_signature_parameter_type_must_be_string_or_number);
             }
             if (!node.type) {

src/compiler/diagnosticMessages.json

@@ -943,6 +943,10 @@
         "category": "Error",
         "code": 1336
     },
+    "An index signature parameter type cannot be a union type. Use '[K in {0}]' instead.": {
+        "category": "Error",
+        "code": 1337
+    },
 
     "Duplicate identifier '{0}'.": {
         "category": "Error",

tests/baselines/reference/indexerConstraints2.errors.txt

@@ -4,9 +4,12 @@ tests/cases/compiler/indexerConstraints2.ts(26,5): error TS2413: Numeric index t
 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(52,6): error TS1337: An index signature parameter type cannot be a union type. Use '[K in IndexableUnion]' instead.
+tests/cases/compiler/indexerConstraints2.ts(58,6): error TS1023: An index signature parameter type must be 'string' or 'number'.
+tests/cases/compiler/indexerConstraints2.ts(64,6): error TS1023: An index signature parameter type must be 'string' or 'number'.
 
 
-==== tests/cases/compiler/indexerConstraints2.ts (6 errors) ====
+==== tests/cases/compiler/indexerConstraints2.ts (9 errors) ====
     class A { a: number; }
     class B extends A { b: number; }
     
@@ -64,5 +67,29 @@ tests/cases/compiler/indexerConstraints2.ts(46,6): error TS1023: An index signat
     interface N {
         [b: AliasedBoolean]: A;
          ~
+!!! error TS1023: An index signature parameter type must be 'string' or 'number'.
+    }
+    
+    type IndexableUnion = "foo" | 42;
+    
+    interface O {
+        [u: IndexableUnion]: A;
+         ~
+!!! error TS1337: An index signature parameter type cannot be a union type. Use '[K in IndexableUnion]' instead.
+    }
+    
+    type NonIndexableUnion = boolean | {};
+    
+    interface P {
+        [u: NonIndexableUnion]: A;
+         ~
+!!! error TS1023: An index signature parameter type must be 'string' or 'number'.
+    }
+    
+    type NonIndexableUnion2 = string | number;
+    
+    interface Q {
+        [u: NonIndexableUnion2]: A;
+         ~
 !!! error TS1023: An index signature parameter type must be 'string' or 'number'.
     }

tests/baselines/reference/indexerConstraints2.js

@@ -45,6 +45,24 @@ type AliasedBoolean = boolean;
 
 interface N {
     [b: AliasedBoolean]: A;
+}
+
+type IndexableUnion = "foo" | 42;
+
+interface O {
+    [u: IndexableUnion]: A;
+}
+
+type NonIndexableUnion = boolean | {};
+
+interface P {
+    [u: NonIndexableUnion]: A;
+}
+
+type NonIndexableUnion2 = string | number;
+
+interface Q {
+    [u: NonIndexableUnion2]: A;
 }
 
 //// [indexerConstraints2.js]

tests/baselines/reference/indexerConstraints2.symbols

@@ -99,3 +99,39 @@ interface N {
 >AliasedBoolean : Symbol(AliasedBoolean, Decl(indexerConstraints2.ts, 40, 1))
 >A : Symbol(A, Decl(indexerConstraints2.ts, 0, 0))
 }
+
+type IndexableUnion = "foo" | 42;
+>IndexableUnion : Symbol(IndexableUnion, Decl(indexerConstraints2.ts, 46, 1))
+
+interface O {
+>O : Symbol(O, Decl(indexerConstraints2.ts, 48, 33))
+
+    [u: IndexableUnion]: A;
+>u : Symbol(u, Decl(indexerConstraints2.ts, 51, 5))
+>IndexableUnion : Symbol(IndexableUnion, Decl(indexerConstraints2.ts, 46, 1))
+>A : Symbol(A, Decl(indexerConstraints2.ts, 0, 0))
+}
+
+type NonIndexableUnion = boolean | {};
+>NonIndexableUnion : Symbol(NonIndexableUnion, Decl(indexerConstraints2.ts, 52, 1))
+
+interface P {
+>P : Symbol(P, Decl(indexerConstraints2.ts, 54, 38))
+
+    [u: NonIndexableUnion]: A;
+>u : Symbol(u, Decl(indexerConstraints2.ts, 57, 5))
+>NonIndexableUnion : Symbol(NonIndexableUnion, Decl(indexerConstraints2.ts, 52, 1))
+>A : Symbol(A, Decl(indexerConstraints2.ts, 0, 0))
+}
+
+type NonIndexableUnion2 = string | number;
+>NonIndexableUnion2 : Symbol(NonIndexableUnion2, Decl(indexerConstraints2.ts, 58, 1))
+
+interface Q {
+>Q : Symbol(Q, Decl(indexerConstraints2.ts, 60, 42))
+
+    [u: NonIndexableUnion2]: A;
+>u : Symbol(u, Decl(indexerConstraints2.ts, 63, 5))
+>NonIndexableUnion2 : Symbol(NonIndexableUnion2, Decl(indexerConstraints2.ts, 58, 1))
+>A : Symbol(A, Decl(indexerConstraints2.ts, 0, 0))
+}

tests/baselines/reference/indexerConstraints2.types

@@ -99,3 +99,39 @@ interface N {
 >AliasedBoolean : boolean
 >A : A
 }
+
+type IndexableUnion = "foo" | 42;
+>IndexableUnion : IndexableUnion
+
+interface O {
+>O : O
+
+    [u: IndexableUnion]: A;
+>u : IndexableUnion
+>IndexableUnion : IndexableUnion
+>A : A
+}
+
+type NonIndexableUnion = boolean | {};
+>NonIndexableUnion : NonIndexableUnion
+
+interface P {
+>P : P
+
+    [u: NonIndexableUnion]: A;
+>u : NonIndexableUnion
+>NonIndexableUnion : NonIndexableUnion
+>A : A
+}
+
+type NonIndexableUnion2 = string | number;
+>NonIndexableUnion2 : NonIndexableUnion2
+
+interface Q {
+>Q : Q
+
+    [u: NonIndexableUnion2]: A;
+>u : NonIndexableUnion2
+>NonIndexableUnion2 : NonIndexableUnion2
+>A : A
+}

tests/cases/compiler/indexerConstraints2.ts

@@ -44,4 +44,22 @@ type AliasedBoolean = boolean;
 
 interface N {
     [b: AliasedBoolean]: A;
+}
+
+type IndexableUnion = "foo" | 42;
+
+interface O {
+    [u: IndexableUnion]: A;
+}
+
+type NonIndexableUnion = boolean | {};
+
+interface P {
+    [u: NonIndexableUnion]: A;
+}
+
+type NonIndexableUnion2 = string | number;
+
+interface Q {
+    [u: NonIndexableUnion2]: A;
 }