Add inference priority level for conditional types in contravariant positions

Author: ahejlsberg

src/compiler/checker.ts

@@ -17552,9 +17552,12 @@ namespace ts {
                     inferFromTypes(getTrueTypeFromConditionalType(<ConditionalType>source), getTrueTypeFromConditionalType(<ConditionalType>target));
                     inferFromTypes(getFalseTypeFromConditionalType(<ConditionalType>source), getFalseTypeFromConditionalType(<ConditionalType>target));
                 }
-                else if (target.flags & TypeFlags.Conditional && !contravariant) {
+                else if (target.flags & TypeFlags.Conditional) {
+                    const savePriority = priority;
+                    priority |= contravariant ? InferencePriority.ContravariantConditional : 0;
                     const targetTypes = [getTrueTypeFromConditionalType(<ConditionalType>target), getFalseTypeFromConditionalType(<ConditionalType>target)];
                     inferToMultipleTypes(source, targetTypes, target.flags);
+                    priority = savePriority;
                 }
                 else if (target.flags & TypeFlags.UnionOrIntersection) {
                     inferToMultipleTypes(source, (<UnionOrIntersectionType>target).types, target.flags);

src/compiler/types.ts

@@ -4740,11 +4740,12 @@ namespace ts {
         HomomorphicMappedType        = 1 << 1,  // Reverse inference for homomorphic mapped type
         PartialHomomorphicMappedType = 1 << 2,  // Partial reverse inference for homomorphic mapped type
         MappedTypeConstraint         = 1 << 3,  // Reverse inference for mapped type
-        ReturnType                   = 1 << 4,  // Inference made from return type of generic function
-        LiteralKeyof                 = 1 << 5,  // Inference made from a string literal to a keyof T
-        NoConstraints                = 1 << 6,  // Don't infer from constraints of instantiable types
-        AlwaysStrict                 = 1 << 7,  // Always use strict rules for contravariant inferences
-        MaxValue                     = 1 << 8,  // Seed for inference priority tracking
+        ContravariantConditional     = 1 << 4,  // Conditional type in contravariant position
+        ReturnType                   = 1 << 5,  // Inference made from return type of generic function
+        LiteralKeyof                 = 1 << 6,  // Inference made from a string literal to a keyof T
+        NoConstraints                = 1 << 7,  // Don't infer from constraints of instantiable types
+        AlwaysStrict                 = 1 << 8,  // Always use strict rules for contravariant inferences
+        MaxValue                     = 1 << 9,  // Seed for inference priority tracking
 
         PriorityImpliesCombination = ReturnType | MappedTypeConstraint | LiteralKeyof,  // These priorities imply that the resulting type should be a combination of all candidates
         Circularity = -1,  // Inference circularity (value less than all other priorities)