Emit declaration file which inferred type alias with its value

Author: Yui T

src/compiler/checker.ts

@@ -2150,8 +2150,13 @@ namespace ts {
                         buildSymbolDisplay(type.symbol, writer, enclosingDeclaration, SymbolFlags.Type, SymbolFormatFlags.None, nextFlags);
                     }
                     else if (!(flags & TypeFormatFlags.InTypeAlias) && type.flags & (TypeFlags.Anonymous | TypeFlags.UnionOrIntersection) && type.aliasSymbol) {
-                        const typeArguments = type.aliasTypeArguments;
-                        writeSymbolTypeReference(type.aliasSymbol, typeArguments, 0, typeArguments ? typeArguments.length : 0, nextFlags);
+                        if (type.flags & TypeFlags.Anonymous || !(flags & TypeFormatFlags.UseTypeAliasValue)) {
+                            const typeArguments = type.aliasTypeArguments;
+                            writeSymbolTypeReference(type.aliasSymbol, typeArguments, 0, typeArguments ? typeArguments.length : 0, nextFlags);
+                        }
+                        else {
+                            writeUnionOrIntersectionType(<UnionOrIntersectionType>type, nextFlags);
+                        }
                     }
                     else if (type.flags & TypeFlags.UnionOrIntersection) {
                         writeUnionOrIntersectionType(<UnionOrIntersectionType>type, nextFlags);

src/compiler/declarationEmitter.ts

@@ -327,7 +327,7 @@ namespace ts {
             }
             else {
                 errorNameNode = declaration.name;
-                resolver.writeTypeOfDeclaration(declaration, enclosingDeclaration, TypeFormatFlags.UseTypeOfFunction, writer);
+                resolver.writeTypeOfDeclaration(declaration, enclosingDeclaration, TypeFormatFlags.UseTypeOfFunction | TypeFormatFlags.UseTypeAliasValue, writer);
                 errorNameNode = undefined;
             }
         }

src/compiler/types.ts

@@ -1946,6 +1946,7 @@ namespace ts {
         UseFullyQualifiedType           = 0x00000080,  // Write out the fully qualified type name (eg. Module.Type, instead of Type)
         InFirstTypeArgument             = 0x00000100,  // Writing first type argument of the instantiated type
         InTypeAlias                     = 0x00000200,  // Writing type in type alias declaration
+        UseTypeAliasValue               = 0x00000400,  // Writing infered type alias as its value. This is used when we emit declaration file
     }
 
     export const enum SymbolFormatFlags {