Improve type ordering (#1284)

Author: ahejlsberg

internal/checker/types.go

@@ -408,6 +408,12 @@ type SignatureLinks struct {
 
 type TypeFlags uint32
 
+// Note that for types of different kinds, the numeric values of TypeFlags determine the order
+// computed by the CompareTypes function and therefore the order of constituent types in union types.
+// Since union type processing often bails out early when a result is known, it is important to order
+// TypeFlags in increasing order of potential type complexity. In particular, indexed access and
+// conditional types should sort last as those types are potentially recursive and possibly infinite.
+
 const (
 	TypeFlagsNone            TypeFlags = 0
 	TypeFlagsAny             TypeFlags = 1 << 0
@@ -432,11 +438,11 @@ const (
 	TypeFlagsTypeParameter   TypeFlags = 1 << 19 // Type parameter
 	TypeFlagsObject          TypeFlags = 1 << 20 // Object type
 	TypeFlagsIndex           TypeFlags = 1 << 21 // keyof T
-	TypeFlagsIndexedAccess   TypeFlags = 1 << 22 // T[K]
-	TypeFlagsConditional     TypeFlags = 1 << 23 // T extends U ? X : Y
+	TypeFlagsTemplateLiteral TypeFlags = 1 << 22 // Template literal type
+	TypeFlagsStringMapping   TypeFlags = 1 << 23 // Uppercase/Lowercase type
 	TypeFlagsSubstitution    TypeFlags = 1 << 24 // Type parameter substitution
-	TypeFlagsTemplateLiteral TypeFlags = 1 << 25 // Template literal type
-	TypeFlagsStringMapping   TypeFlags = 1 << 26 // Uppercase/Lowercase type
+	TypeFlagsIndexedAccess   TypeFlags = 1 << 25 // T[K]
+	TypeFlagsConditional     TypeFlags = 1 << 26 // T extends U ? X : Y
 	TypeFlagsUnion           TypeFlags = 1 << 27 // Union (T | U)
 	TypeFlagsIntersection    TypeFlags = 1 << 28 // Intersection (T & U)
 	TypeFlagsReserved1       TypeFlags = 1 << 29 // Used by union/intersection type construction

testdata/baselines/reference/submodule/compiler/awaitedTypeNoLib.errors.txt

@@ -10,7 +10,7 @@ error TS2318: Cannot find global type 'String'.
 awaitedTypeNoLib.ts(3,15): error TS2304: Cannot find name 'PromiseLike'.
 awaitedTypeNoLib.ts(18,27): error TS2345: Argument of type 'Thenable<NotPromise<TResult>> | NotPromise<TResult>' is not assignable to parameter of type 'Thenable<TResult>'.
   Type 'NotPromise<TResult>' is not assignable to type 'Thenable<TResult>'.
-    Type '(TResult extends PromiseLike<unknown> ? never : TResult) | TResult' is not assignable to type 'Thenable<TResult>'.
+    Type 'TResult | (TResult extends PromiseLike<unknown> ? never : TResult)' is not assignable to type 'Thenable<TResult>'.
       Type 'Thenable<unknown> & TResult' is not assignable to type 'Thenable<TResult>'.
         Type 'unknown' is not assignable to type 'TResult'.
           'TResult' could be instantiated with an arbitrary type which could be unrelated to 'unknown'.
@@ -49,7 +49,7 @@ awaitedTypeNoLib.ts(18,27): error TS2345: Argument of type 'Thenable<NotPromise<
                               ~~~~~~
 !!! error TS2345: Argument of type 'Thenable<NotPromise<TResult>> | NotPromise<TResult>' is not assignable to parameter of type 'Thenable<TResult>'.
 !!! error TS2345:   Type 'NotPromise<TResult>' is not assignable to type 'Thenable<TResult>'.
-!!! error TS2345:     Type '(TResult extends PromiseLike<unknown> ? never : TResult) | TResult' is not assignable to type 'Thenable<TResult>'.
+!!! error TS2345:     Type 'TResult | (TResult extends PromiseLike<unknown> ? never : TResult)' is not assignable to type 'Thenable<TResult>'.
 !!! error TS2345:       Type 'Thenable<unknown> & TResult' is not assignable to type 'Thenable<TResult>'.
 !!! error TS2345:         Type 'unknown' is not assignable to type 'TResult'.
 !!! error TS2345:           'TResult' could be instantiated with an arbitrary type which could be unrelated to 'unknown'.

testdata/baselines/reference/submodule/compiler/awaitedTypeNoLib.errors.txt.diff

@@ -0,0 +1,20 @@
+--- old.awaitedTypeNoLib.errors.txt
++++ new.awaitedTypeNoLib.errors.txt
+@@= skipped -9, +9 lines =@@
+ awaitedTypeNoLib.ts(3,15): error TS2304: Cannot find name 'PromiseLike'.
+ awaitedTypeNoLib.ts(18,27): error TS2345: Argument of type 'Thenable<NotPromise<TResult>> | NotPromise<TResult>' is not assignable to parameter of type 'Thenable<TResult>'.
+   Type 'NotPromise<TResult>' is not assignable to type 'Thenable<TResult>'.
+-    Type '(TResult extends PromiseLike<unknown> ? never : TResult) | TResult' is not assignable to type 'Thenable<TResult>'.
++    Type 'TResult | (TResult extends PromiseLike<unknown> ? never : TResult)' is not assignable to type 'Thenable<TResult>'.
+       Type 'Thenable<unknown> & TResult' is not assignable to type 'Thenable<TResult>'.
+         Type 'unknown' is not assignable to type 'TResult'.
+           'TResult' could be instantiated with an arbitrary type which could be unrelated to 'unknown'.
+@@= skipped -39, +39 lines =@@
+                               ~~~~~~
+ !!! error TS2345: Argument of type 'Thenable<NotPromise<TResult>> | NotPromise<TResult>' is not assignable to parameter of type 'Thenable<TResult>'.
+ !!! error TS2345:   Type 'NotPromise<TResult>' is not assignable to type 'Thenable<TResult>'.
+-!!! error TS2345:     Type '(TResult extends PromiseLike<unknown> ? never : TResult) | TResult' is not assignable to type 'Thenable<TResult>'.
++!!! error TS2345:     Type 'TResult | (TResult extends PromiseLike<unknown> ? never : TResult)' is not assignable to type 'Thenable<TResult>'.
+ !!! error TS2345:       Type 'Thenable<unknown> & TResult' is not assignable to type 'Thenable<TResult>'.
+ !!! error TS2345:         Type 'unknown' is not assignable to type 'TResult'.
+ !!! error TS2345:           'TResult' could be instantiated with an arbitrary type which could be unrelated to 'unknown'.