Handle literals and literal unions

Author: Andy Hanson

src/compiler/checker.ts

@@ -21997,23 +21997,29 @@ namespace ts {
             readonly side: "left" | "right";
             readonly conversionFunctionName: string;
         }
+
         function getExplicitConversion(left: Type, right: Type): ExplicitConversion | undefined {
-            const convertLeft = getFunctionNameToConvertToType(right);
+            const convertLeft = getFunctionNameToConvertToType(right, left);
             if (convertLeft) return { side: "left", conversionFunctionName: convertLeft };
-            const convertRight = getFunctionNameToConvertToType(left);
+            const convertRight = getFunctionNameToConvertToType(left, right);
             if (convertRight) return { side: "right", conversionFunctionName: convertRight };
             return undefined;
         }
-        function getFunctionNameToConvertToType(type: Type): string | undefined {
-            return type.flags & TypeFlags.String
+
+        function getFunctionNameToConvertToType(to: Type, from: Type): string | undefined {
+            return typeOrTypesHaveFlags(to, TypeFlags.StringLike) && !typeOrTypesHaveFlags(from, TypeFlags.StringLike)
                 ? "String"
-                : type.flags & TypeFlags.Number
+                : typeOrTypesHaveFlags(to, TypeFlags.NumberLike) && !typeOrTypesHaveFlags(from, TypeFlags.NumberLike)
                 ? "Number"
-                : type.flags & TypeFlags.Boolean
+                : typeOrTypesHaveFlags(to, TypeFlags.BooleanLike) && !typeOrTypesHaveFlags(from, TypeFlags.BooleanLike)
                 ? "Boolean"
                 : undefined;
         }
 
+        function typeOrTypesHaveFlags(type: Type, flags: TypeFlags): boolean {
+            return (type.flags & TypeFlags.Union ? (type as UnionType).types : [type]).every(t => !!(t.flags & flags));
+        }
+
         function isYieldExpressionInClass(node: YieldExpression): boolean {
             let current: Node = node;
             let parent = node.parent;

tests/baselines/reference/expr.errors.txt

@@ -2,9 +2,9 @@ tests/cases/compiler/expr.ts(87,5): error TS2587: Types 'number' and 'string' ha
 tests/cases/compiler/expr.ts(88,5): error TS2587: Types 'number' and 'boolean' have no overlap. Consider explicitly converting the left side using `Boolean(x)`.
 tests/cases/compiler/expr.ts(94,5): error TS2587: Types 'string' and 'number' have no overlap. Consider explicitly converting the left side using `Number(x)`.
 tests/cases/compiler/expr.ts(95,5): error TS2587: Types 'string' and 'boolean' have no overlap. Consider explicitly converting the left side using `Boolean(x)`.
-tests/cases/compiler/expr.ts(98,5): error TS2588: Types 'string' and 'E' have no overlap. Consider explicitly converting the right side using `String(x)`.
+tests/cases/compiler/expr.ts(98,5): error TS2587: Types 'string' and 'E' have no overlap. Consider explicitly converting the left side using `Number(x)`.
 tests/cases/compiler/expr.ts(115,5): error TS2587: Types 'E' and 'string' have no overlap. Consider explicitly converting the left side using `String(x)`.
-tests/cases/compiler/expr.ts(116,5): error TS2367: This condition will always return 'false' since the types 'E' and 'false' have no overlap.
+tests/cases/compiler/expr.ts(116,5): error TS2587: Types 'E' and 'false' have no overlap. Consider explicitly converting the left side using `Boolean(x)`.
 tests/cases/compiler/expr.ts(142,5): error TS2365: Operator '+' cannot be applied to types 'number' and 'false'.
 tests/cases/compiler/expr.ts(143,5): error TS2365: Operator '+' cannot be applied to types 'number' and 'I'.
 tests/cases/compiler/expr.ts(161,5): error TS2365: Operator '+' cannot be applied to types 'I' and 'number'.
@@ -177,7 +177,7 @@ tests/cases/compiler/expr.ts(242,7): error TS2363: The right-hand side of an ari
         s==s;
         s==e;
         ~~~~
-!!! error TS2588: Types 'string' and 'E' have no overlap. Consider explicitly converting the right side using `String(x)`.
+!!! error TS2587: Types 'string' and 'E' have no overlap. Consider explicitly converting the left side using `Number(x)`.
 
         a==n;
         a==s;
@@ -199,7 +199,7 @@ tests/cases/compiler/expr.ts(242,7): error TS2363: The right-hand side of an ari
 !!! error TS2587: Types 'E' and 'string' have no overlap. Consider explicitly converting the left side using `String(x)`.
         e==b;
         ~~~~
-!!! error TS2367: This condition will always return 'false' since the types 'E' and 'false' have no overlap.
+!!! error TS2587: Types 'E' and 'false' have no overlap. Consider explicitly converting the left side using `Boolean(x)`.
         e==a;
         e==i;
         e==e;