Add union check to improved error message (#23027)

Author: weswigham

src/compiler/checker.ts

@@ -26626,7 +26626,7 @@ namespace ts {
                                               typeToString(getTypeFromTypeNode(node.type)));
                 }
 
-                if (allTypesAssignableToKind(type, TypeFlags.StringLiteral, /*strict*/ true)) {
+                if (type.flags & TypeFlags.Union && allTypesAssignableToKind(type, TypeFlags.StringLiteral, /*strict*/ true)) {
                     return grammarErrorOnNode(parameter.name,
                                               Diagnostics.An_index_signature_parameter_type_cannot_be_a_union_type_Consider_using_a_mapped_object_type_instead);
                 }

tests/baselines/reference/genericIndexTypeHasSensibleErrorMessage.errors.txt

@@ -0,0 +1,7 @@
+tests/cases/compiler/genericIndexTypeHasSensibleErrorMessage.ts(1,33): error TS1023: An index signature parameter type must be 'string' or 'number'.
+
+
+==== tests/cases/compiler/genericIndexTypeHasSensibleErrorMessage.ts (1 errors) ====
+    type Wat<T extends string> = { [x: T]: string };
+                                    ~
+!!! error TS1023: An index signature parameter type must be 'string' or 'number'.

tests/baselines/reference/genericIndexTypeHasSensibleErrorMessage.js

@@ -0,0 +1,4 @@
+//// [genericIndexTypeHasSensibleErrorMessage.ts]
+type Wat<T extends string> = { [x: T]: string };
+
+//// [genericIndexTypeHasSensibleErrorMessage.js]

tests/baselines/reference/genericIndexTypeHasSensibleErrorMessage.symbols

@@ -0,0 +1,7 @@
+=== tests/cases/compiler/genericIndexTypeHasSensibleErrorMessage.ts ===
+type Wat<T extends string> = { [x: T]: string };
+>Wat : Symbol(Wat, Decl(genericIndexTypeHasSensibleErrorMessage.ts, 0, 0))
+>T : Symbol(T, Decl(genericIndexTypeHasSensibleErrorMessage.ts, 0, 9))
+>x : Symbol(x, Decl(genericIndexTypeHasSensibleErrorMessage.ts, 0, 32))
+>T : Symbol(T, Decl(genericIndexTypeHasSensibleErrorMessage.ts, 0, 9))
+

tests/baselines/reference/genericIndexTypeHasSensibleErrorMessage.types

@@ -0,0 +1,7 @@
+=== tests/cases/compiler/genericIndexTypeHasSensibleErrorMessage.ts ===
+type Wat<T extends string> = { [x: T]: string };
+>Wat : Wat<T>
+>T : T
+>x : T
+>T : T
+

tests/cases/compiler/genericIndexTypeHasSensibleErrorMessage.ts

@@ -0,0 +1 @@
+type Wat<T extends string> = { [x: T]: string };