Lower priority for inferences made from partial reverse mapped types

Author: ahejlsberg

src/compiler/checker.ts

@@ -14898,11 +14898,8 @@ namespace ts {
 
         function createReverseMappedType(source: Type, target: MappedType, constraint: IndexType) {
             // We consider a source type reverse mappable if it has a string index signature or if
-            // it has one or more properties and all properties have inferable types.
-            const properties = getPropertiesOfType(source);
-            const isReverseMappable = getIndexInfoOfType(source, IndexKind.String) ||
-                properties.length !== 0 && every(properties, prop => isPartiallyInferableType(getTypeOfSymbol(prop)));
-            if (!isReverseMappable) {
+            // it has one or more properties and is of a partially inferable type.
+            if (!(getIndexInfoOfType(source, IndexKind.String) || getPropertiesOfType(source).length !== 0 && isPartiallyInferableType(source))) {
                 return undefined;
             }
             // For arrays and tuples we infer new arrays and tuples where the reverse mapping has been
@@ -15287,7 +15284,11 @@ namespace ts {
                         const inferredType = inferTypeForHomomorphicMappedType(source, target, <IndexType>constraintType);
                         if (inferredType) {
                             const savePriority = priority;
-                            priority |= InferencePriority.HomomorphicMappedType;
+                            // We assign a lower priority to inferences made from types containing non-inferrable
+                            // types because we may only have a partial result (i.e. we may have failed to make
+                            // reverse inferences for some properties).
+                            priority |= getObjectFlags(source) & ObjectFlags.NonInferrableType ?
+                                InferencePriority.PartialHomomorphicMappedType : InferencePriority.HomomorphicMappedType;
                             inferFromTypes(inferredType, inference.typeParameter);
                             priority = savePriority;
                         }

src/compiler/types.ts

@@ -4405,15 +4405,16 @@ namespace ts {
     export type TypeMapper = (t: TypeParameter) => Type;
 
     export const enum InferencePriority {
-        NakedTypeVariable           = 1 << 0,  // Naked type variable in union or intersection type
-        HomomorphicMappedType       = 1 << 1,  // Reverse inference for homomorphic mapped type
-        MappedTypeConstraint        = 1 << 2,  // Reverse inference for mapped type
-        ReturnType                  = 1 << 3,  // Inference made from return type of generic function
-        LiteralKeyof                = 1 << 4,  // Inference made from a string literal to a keyof T
-        NoConstraints               = 1 << 5,  // Don't infer from constraints of instantiable types
-        AlwaysStrict                = 1 << 6,  // Always use strict rules for contravariant inferences
-
-        PriorityImpliesCombination  = ReturnType | MappedTypeConstraint | LiteralKeyof,  // These priorities imply that the resulting type should be a combination of all candidates
+        NakedTypeVariable            = 1 << 0,  // Naked type variable in union or intersection type
+        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
+
+        PriorityImpliesCombination = ReturnType | MappedTypeConstraint | LiteralKeyof,  // These priorities imply that the resulting type should be a combination of all candidates
     }
 
     /* @internal */