Allow infer type variables to have constraints inferred

src/compiler/checker.ts

@@ -12333,6 +12333,13 @@ namespace ts {
             }
         }
 
+        function unwrapSubstitution(type: Type): Type {
+            if (type.flags & TypeFlags.Substitution) {
+                return (type as SubstitutionType).substitute;
+            }
+            return type;
+        }
+
         // Transform an indexed access to a simpler form, if possible. Return the simpler form, or return
         // the type itself if no transformation is possible. The writing flag indicates that the type is
         // the target of an assignment.
@@ -12344,7 +12351,7 @@ namespace ts {
             type[cache] = circularConstraintType;
             // We recursively simplify the object type as it may in turn be an indexed access type. For example, with
             // '{ [P in T]: { [Q in U]: number } }[T][U]' we want to first simplify the inner indexed access type.
-            const objectType = getSimplifiedType(type.objectType, writing);
+            const objectType = unwrapSubstitution(getSimplifiedType(type.objectType, writing));
             const indexType = getSimplifiedType(type.indexType, writing);
             // T[A | B] -> T[A] | T[B] (reading)
             // T[A | B] -> T[A] & T[B] (writing)
@@ -12522,7 +12529,11 @@ namespace ts {
             let combinedMapper: TypeMapper | undefined;
             if (root.inferTypeParameters) {
                 const context = createInferenceContext(root.inferTypeParameters, /*signature*/ undefined, InferenceFlags.None);
-                if (!checkTypeInstantiable) {
+                // We skip inference of the possible `infer` types unles the `extendsType` _is_ an infer type
+                // if it was, it's trivial to say that extendsType = checkType, however such a pattern is used to
+                // "reset" the type being build up during constraint calculation and avoid making an apparently "infinite" constraint
+                // so in those cases we refain from performing inference and retain the uninfered type parameter
+                if (!checkTypeInstantiable || !some(root.inferTypeParameters, t => t === extendsType)) {
                     // We don't want inferences from constraints as they may cause us to eagerly resolve the
                     // conditional type instead of deferring resolution. Also, we always want strict function
                     // types rules (i.e. proper contravariance) for inferences.
@@ -17854,6 +17865,12 @@ namespace ts {
                         invokeOnce(source, target, inferFromObjectTypes);
                     }
                 }
+                if (source.flags & TypeFlags.Simplifiable) {
+                    const simplified = getSimplifiedType(source, contravariant);
+                    if (simplified !== source) {
+                        inferFromTypes(simplified, target);
+                    }
+                }
             }
 
             function inferWithPriority(source: Type, target: Type, newPriority: InferencePriority) {

tests/baselines/reference/ramdaToolsNoInfinite.js

@@ -0,0 +1,123 @@
+//// [ramdaToolsNoInfinite.ts]
+// All the following types are explained here:
+// https://medium.freecodecamp.org/typescript-curry-ramda-types-f747e99744ab
+// https://github.com/pirix-gh/medium/blob/master/types-curry-ramda/src/index.ts
+declare namespace Tools {
+    type Head<T extends any[]> =
+        T extends [any, ...any[]]
+        ? T[0]
+        : never;
+
+    type Tail<T extends any[]> =
+        ((...t: T) => any) extends ((_: any, ...tail: infer TT) => any)
+        ? TT
+        : [];
+
+    type HasTail<T extends any[]> =
+        T extends ([] | [any])
+        ? false
+        : true;
+
+    type Last<T extends any[]> = {
+        0: Last<Tail<T>>;
+        1: Head<T>;
+    }[
+        HasTail<T> extends true
+        ? 0
+        : 1
+    ];
+
+    type Length<T extends any[]> =
+        T['length'];
+
+    type Prepend<E, T extends any[]> =
+        ((head: E, ...args: T) => any) extends ((...args: infer U) => any)
+        ? U
+        : T;
+
+    type Drop<N extends number, T extends any[], I extends any[] = []> = {
+        0: Drop<N, Tail<T>, Prepend<any, I>>;
+        1: T;
+    }[
+        Length<I> extends N
+        ? 1
+        : 0
+    ];
+
+    type Cast<X, Y> = X extends Y ? X : Y;
+
+    type Pos<I extends any[]> =
+        Length<I>;
+
+    type Next<I extends any[]> =
+        Prepend<any, I>;
+
+    type Prev<I extends any[]> =
+        Tail<I>;
+
+    type Iterator<Index extends number = 0, From extends any[] = [], I extends any[] = []> = {
+        0: Iterator<Index, Next<From>, Next<I>>;
+        1: From;
+    }[
+        Pos<I> extends Index
+        ? 1
+        : 0
+    ];
+
+    type Reverse<T extends any[], R extends any[] = [], I extends any[] = []> = {
+        0: Reverse<T, Prepend<T[Pos<I>], R>, Next<I>>;
+        1: R;
+    }[
+        Pos<I> extends Length<T>
+        ? 1
+        : 0
+    ];
+
+    type Concat<T1 extends any[], T2 extends any[]> =
+        Reverse<Reverse<T1> extends infer R ? Cast<R, any[]> : never, T2>;
+
+    type Append<E, T extends any[]> =
+        Concat<T, [E]>;
+
+    type ValueOfRecord<R> = R extends Record<any, infer T> ? T : never;
+}
+
+declare namespace R {
+    export type Placeholder = { __placeholder: void };
+}
+
+declare namespace Curry {
+    type GapOf<T1 extends any[], T2 extends any[], TN extends any[], I extends any[]> =
+        T1[Tools.Pos<I>] extends R.Placeholder
+        ? Tools.Append<T2[Tools.Pos<I>], TN>
+        : TN;
+
+    interface GapsOfWorker<T1 extends any[], T2 extends any[], TN extends any[] = [], I extends any[] = []> {
+        0: GapsOf<T1, T2, GapOf<T1, T2, TN, I> extends infer G ? Tools.Cast<G, any[]> : never, Tools.Next<I>>;
+        1: Tools.Concat<TN, Tools.Drop<Tools.Pos<I>, T2> extends infer D ? Tools.Cast<D, any[]> : never>;
+    }
+    type GapsOf<T1 extends any[], T2 extends any[], TN extends any[] = [], I extends any[] = []> = GapsOfWorker<T1, T2, TN, I>[
+        Tools.Pos<I> extends Tools.Length<T1>
+        ? 1
+        : 0
+    ];
+
+    type PartialGaps<T extends any[]> = {
+        [K in keyof T]?: T[K] | R.Placeholder
+    };
+
+    type CleanedGaps<T extends any[]> = {
+        [K in keyof T]: NonNullable<T[K]>
+    };
+
+    type Gaps<T extends any[]> = CleanedGaps<PartialGaps<T>>;
+
+    type Curry<F extends ((...args: any) => any)> =
+        <T extends any[]>(...args: Tools.Cast<Tools.Cast<T, Gaps<Parameters<F>>>, any[]>) =>
+            GapsOf<T, Parameters<F>> extends [any, ...any[]]
+            ? Curry<(...args: GapsOf<T, Parameters<F>> extends infer G ? Tools.Cast<G, any[]> : never) => ReturnType<F>>
+            : ReturnType<F>;
+}
+
+
+//// [ramdaToolsNoInfinite.js]