Make lower priority inference when source is completely matched in target

src/compiler/checker.ts

@@ -15568,24 +15568,30 @@ namespace ts {
                     return;
                 }
                 if (target.flags & TypeFlags.Union) {
-                    if (source.flags & TypeFlags.Union) {
-                        // First, infer between identically matching source and target constituents and remove the
-                        // matching types.
-                        const [tempSources, tempTargets] = inferFromMatchingTypes((<UnionType>source).types, (<UnionType>target).types, isTypeOrBaseIdenticalTo);
-                        // Next, infer between closely matching source and target constituents and remove
-                        // the matching types. Types closely match when they are instantiations of the same
-                        // object type or instantiations of the same type alias.
-                        const [sources, targets] = inferFromMatchingTypes(tempSources, tempTargets, isTypeCloselyMatchedBy);
-                        if (sources.length === 0 || targets.length === 0) {
-                            return;
-                        }
-                        source = getUnionType(sources);
-                        target = getUnionType(targets);
+                    // First, infer between identically matching source and target constituents and remove the
+                    // matching types.
+                    const [tempSources, tempTargets] = inferFromMatchingTypes(source.flags & TypeFlags.Union ? (<UnionType>source).types : [source], (<UnionType>target).types, isTypeOrBaseIdenticalTo);
+                    // Next, infer between closely matching source and target constituents and remove
+                    // the matching types. Types closely match when they are instantiations of the same
+                    // object type or instantiations of the same type alias.
+                    const [sources, targets] = inferFromMatchingTypes(tempSources, tempTargets, isTypeCloselyMatchedBy);
+                    if (targets.length === 0) {
+                        return;
                     }
-                    else {
-                        if (inferFromMatchingType(source, (<UnionType>target).types, isTypeOrBaseIdenticalTo)) return;
-                        if (inferFromMatchingType(source, (<UnionType>target).types, isTypeCloselyMatchedBy)) return;
+                    target = getUnionType(targets);
+                    if (sources.length === 0) {
+                        // All source constituents have been matched and there is nothing further to infer from.
+                        // However, simply making no inferences is undesirable because it could ultimately mean
+                        // inferring a type parameter constraint. Instead, make a lower priority inference from
+                        // the full source to whatever remains in the target. For example, when inferring from
+                        // string to 'string | T', make a lower priority inference of string for T.
+                        const savePriority = priority;
+                        priority |= InferencePriority.NakedTypeVariable;
+                        inferFromTypes(source, target);
+                        priority = savePriority;
+                        return;
                     }
+                    source = getUnionType(sources);
                 }
                 else if (target.flags & TypeFlags.Intersection && some((<IntersectionType>target).types,
                     t => !!getInferenceInfoForType(t) || (isGenericMappedType(t) && !!getInferenceInfoForType(getHomomorphicTypeVariable(t) || neverType)))) {
@@ -15595,18 +15601,15 @@ namespace ts {
                     // string[] on the source side and infer string for T.
                     // Likewise, we consider a homomorphic mapped type constrainted to the target type parameter as similar to a "naked type variable"
                     // in such scenarios.
-                    if (source.flags & TypeFlags.Intersection) {
+                    if (!(source.flags & TypeFlags.Union)) {
                         // Infer between identically matching source and target constituents and remove the matching types.
-                        const [sources, targets] = inferFromMatchingTypes((<IntersectionType>source).types, (<IntersectionType>target).types, isTypeIdenticalTo);
+                        const [sources, targets] = inferFromMatchingTypes(source.flags & TypeFlags.Intersection ? (<IntersectionType>source).types : [source], (<IntersectionType>target).types, isTypeIdenticalTo);
                         if (sources.length === 0 || targets.length === 0) {
                             return;
                         }
                         source = getIntersectionType(sources);
                         target = getIntersectionType(targets);
                     }
-                    else if (!(source.flags & TypeFlags.Union)) {
-                        if (inferFromMatchingType(source, (<IntersectionType>target).types, isTypeIdenticalTo)) return;
-                    }
                 }
                 else if (target.flags & (TypeFlags.IndexedAccess | TypeFlags.Substitution)) {
                     target = getActualTypeVariable(target);
@@ -15756,17 +15759,6 @@ namespace ts {
                 inferencePriority = Math.min(inferencePriority, saveInferencePriority);
             }
 
-            function inferFromMatchingType(source: Type, targets: Type[], matches: (s: Type, t: Type) => boolean) {
-                let matched = false;
-                for (const t of targets) {
-                    if (matches(source, t)) {
-                        inferFromTypes(source, t);
-                        matched = true;
-                    }
-                }
-                return matched;
-            }
-
             function inferFromMatchingTypes(sources: Type[], targets: Type[], matches: (s: Type, t: Type) => boolean): [Type[], Type[]] {
                 let matchedSources: Type[] | undefined;
                 let matchedTargets: Type[] | undefined;

tests/baselines/reference/observableInferenceCanBeMade.js

@@ -0,0 +1,29 @@
+//// [observableInferenceCanBeMade.ts]
+// Repro from #33131
+
+declare function of<T>(a: T): Observable<T>;
+declare function from<O extends ObservableInput<any>>(input: O): Observable<ObservedValueOf<O>>;
+
+type ObservedValueOf<O> = O extends ObservableInput<infer T> ? T : never;
+
+interface Subscribable<T> {
+    subscribe(next?: (value: T) => void, error?: (error: any) => void, complete?: () => void): void;
+}
+type ObservableInput<T> = Subscribable<T> | Subscribable<never>;
+
+
+declare class Observable<T> implements Subscribable<T> {
+    subscribe(next?: (value: T) => void, error?: (error: any) => void, complete?: () => void): void;
+}
+
+function asObservable(input: string | ObservableInput<string>): Observable<string> {
+    return typeof input === 'string' ? of(input) : from(input)
+}
+
+
+//// [observableInferenceCanBeMade.js]
+"use strict";
+// Repro from #33131
+function asObservable(input) {
+    return typeof input === 'string' ? of(input) : from(input);
+}

tests/baselines/reference/observableInferenceCanBeMade.symbols

@@ -0,0 +1,80 @@
+=== tests/cases/compiler/observableInferenceCanBeMade.ts ===
+// Repro from #33131
+
+declare function of<T>(a: T): Observable<T>;
+>of : Symbol(of, Decl(observableInferenceCanBeMade.ts, 0, 0))
+>T : Symbol(T, Decl(observableInferenceCanBeMade.ts, 2, 20))
+>a : Symbol(a, Decl(observableInferenceCanBeMade.ts, 2, 23))
+>T : Symbol(T, Decl(observableInferenceCanBeMade.ts, 2, 20))
+>Observable : Symbol(Observable, Decl(observableInferenceCanBeMade.ts, 10, 64))
+>T : Symbol(T, Decl(observableInferenceCanBeMade.ts, 2, 20))
+
+declare function from<O extends ObservableInput<any>>(input: O): Observable<ObservedValueOf<O>>;
+>from : Symbol(from, Decl(observableInferenceCanBeMade.ts, 2, 44))
+>O : Symbol(O, Decl(observableInferenceCanBeMade.ts, 3, 22))
+>ObservableInput : Symbol(ObservableInput, Decl(observableInferenceCanBeMade.ts, 9, 1))
+>input : Symbol(input, Decl(observableInferenceCanBeMade.ts, 3, 54))
+>O : Symbol(O, Decl(observableInferenceCanBeMade.ts, 3, 22))
+>Observable : Symbol(Observable, Decl(observableInferenceCanBeMade.ts, 10, 64))
+>ObservedValueOf : Symbol(ObservedValueOf, Decl(observableInferenceCanBeMade.ts, 3, 96))
+>O : Symbol(O, Decl(observableInferenceCanBeMade.ts, 3, 22))
+
+type ObservedValueOf<O> = O extends ObservableInput<infer T> ? T : never;
+>ObservedValueOf : Symbol(ObservedValueOf, Decl(observableInferenceCanBeMade.ts, 3, 96))
+>O : Symbol(O, Decl(observableInferenceCanBeMade.ts, 5, 21))
+>O : Symbol(O, Decl(observableInferenceCanBeMade.ts, 5, 21))
+>ObservableInput : Symbol(ObservableInput, Decl(observableInferenceCanBeMade.ts, 9, 1))
+>T : Symbol(T, Decl(observableInferenceCanBeMade.ts, 5, 57))
+>T : Symbol(T, Decl(observableInferenceCanBeMade.ts, 5, 57))
+
+interface Subscribable<T> {
+>Subscribable : Symbol(Subscribable, Decl(observableInferenceCanBeMade.ts, 5, 73))
+>T : Symbol(T, Decl(observableInferenceCanBeMade.ts, 7, 23))
+
+    subscribe(next?: (value: T) => void, error?: (error: any) => void, complete?: () => void): void;
+>subscribe : Symbol(Subscribable.subscribe, Decl(observableInferenceCanBeMade.ts, 7, 27))
+>next : Symbol(next, Decl(observableInferenceCanBeMade.ts, 8, 14))
+>value : Symbol(value, Decl(observableInferenceCanBeMade.ts, 8, 22))
+>T : Symbol(T, Decl(observableInferenceCanBeMade.ts, 7, 23))
+>error : Symbol(error, Decl(observableInferenceCanBeMade.ts, 8, 40))
+>error : Symbol(error, Decl(observableInferenceCanBeMade.ts, 8, 50))
+>complete : Symbol(complete, Decl(observableInferenceCanBeMade.ts, 8, 70))
+}
+type ObservableInput<T> = Subscribable<T> | Subscribable<never>;
+>ObservableInput : Symbol(ObservableInput, Decl(observableInferenceCanBeMade.ts, 9, 1))
+>T : Symbol(T, Decl(observableInferenceCanBeMade.ts, 10, 21))
+>Subscribable : Symbol(Subscribable, Decl(observableInferenceCanBeMade.ts, 5, 73))
+>T : Symbol(T, Decl(observableInferenceCanBeMade.ts, 10, 21))
+>Subscribable : Symbol(Subscribable, Decl(observableInferenceCanBeMade.ts, 5, 73))
+
+
+declare class Observable<T> implements Subscribable<T> {
+>Observable : Symbol(Observable, Decl(observableInferenceCanBeMade.ts, 10, 64))
+>T : Symbol(T, Decl(observableInferenceCanBeMade.ts, 13, 25))
+>Subscribable : Symbol(Subscribable, Decl(observableInferenceCanBeMade.ts, 5, 73))
+>T : Symbol(T, Decl(observableInferenceCanBeMade.ts, 13, 25))
+
+    subscribe(next?: (value: T) => void, error?: (error: any) => void, complete?: () => void): void;
+>subscribe : Symbol(Observable.subscribe, Decl(observableInferenceCanBeMade.ts, 13, 56))
+>next : Symbol(next, Decl(observableInferenceCanBeMade.ts, 14, 14))
+>value : Symbol(value, Decl(observableInferenceCanBeMade.ts, 14, 22))
+>T : Symbol(T, Decl(observableInferenceCanBeMade.ts, 13, 25))
+>error : Symbol(error, Decl(observableInferenceCanBeMade.ts, 14, 40))
+>error : Symbol(error, Decl(observableInferenceCanBeMade.ts, 14, 50))
+>complete : Symbol(complete, Decl(observableInferenceCanBeMade.ts, 14, 70))
+}
+
+function asObservable(input: string | ObservableInput<string>): Observable<string> {
+>asObservable : Symbol(asObservable, Decl(observableInferenceCanBeMade.ts, 15, 1))
+>input : Symbol(input, Decl(observableInferenceCanBeMade.ts, 17, 22))
+>ObservableInput : Symbol(ObservableInput, Decl(observableInferenceCanBeMade.ts, 9, 1))
+>Observable : Symbol(Observable, Decl(observableInferenceCanBeMade.ts, 10, 64))
+
+    return typeof input === 'string' ? of(input) : from(input)
+>input : Symbol(input, Decl(observableInferenceCanBeMade.ts, 17, 22))
+>of : Symbol(of, Decl(observableInferenceCanBeMade.ts, 0, 0))
+>input : Symbol(input, Decl(observableInferenceCanBeMade.ts, 17, 22))
+>from : Symbol(from, Decl(observableInferenceCanBeMade.ts, 2, 44))
+>input : Symbol(input, Decl(observableInferenceCanBeMade.ts, 17, 22))
+}
+

tests/baselines/reference/observableInferenceCanBeMade.types

@@ -0,0 +1,57 @@
+=== tests/cases/compiler/observableInferenceCanBeMade.ts ===
+// Repro from #33131
+
+declare function of<T>(a: T): Observable<T>;
+>of : <T>(a: T) => Observable<T>
+>a : T
+
+declare function from<O extends ObservableInput<any>>(input: O): Observable<ObservedValueOf<O>>;
+>from : <O extends ObservableInput<any>>(input: O) => Observable<ObservedValueOf<O>>
+>input : O
+
+type ObservedValueOf<O> = O extends ObservableInput<infer T> ? T : never;
+>ObservedValueOf : ObservedValueOf<O>
+
+interface Subscribable<T> {
+    subscribe(next?: (value: T) => void, error?: (error: any) => void, complete?: () => void): void;
+>subscribe : (next?: ((value: T) => void) | undefined, error?: ((error: any) => void) | undefined, complete?: (() => void) | undefined) => void
+>next : ((value: T) => void) | undefined
+>value : T
+>error : ((error: any) => void) | undefined
+>error : any
+>complete : (() => void) | undefined
+}
+type ObservableInput<T> = Subscribable<T> | Subscribable<never>;
+>ObservableInput : ObservableInput<T>
+
+
+declare class Observable<T> implements Subscribable<T> {
+>Observable : Observable<T>
+
+    subscribe(next?: (value: T) => void, error?: (error: any) => void, complete?: () => void): void;
+>subscribe : (next?: ((value: T) => void) | undefined, error?: ((error: any) => void) | undefined, complete?: (() => void) | undefined) => void
+>next : ((value: T) => void) | undefined
+>value : T
+>error : ((error: any) => void) | undefined
+>error : any
+>complete : (() => void) | undefined
+}
+
+function asObservable(input: string | ObservableInput<string>): Observable<string> {
+>asObservable : (input: string | Subscribable<never> | Subscribable<string>) => Observable<string>
+>input : string | Subscribable<never> | Subscribable<string>
+
+    return typeof input === 'string' ? of(input) : from(input)
+>typeof input === 'string' ? of(input) : from(input) : Observable<string>
+>typeof input === 'string' : boolean
+>typeof input : "string" | "number" | "bigint" | "boolean" | "symbol" | "undefined" | "object" | "function"
+>input : string | Subscribable<never> | Subscribable<string>
+>'string' : "string"
+>of(input) : Observable<string>
+>of : <T>(a: T) => Observable<T>
+>input : string
+>from(input) : Observable<string>
+>from : <O extends ObservableInput<any>>(input: O) => Observable<ObservedValueOf<O>>
+>input : ObservableInput<string>
+}
+

tests/baselines/reference/unionAndIntersectionInference2.types

@@ -20,7 +20,7 @@ var e1: number | string | boolean;
 >e1 : string | number | boolean
 
 f1(a1); // string
->f1(a1) : unknown
+>f1(a1) : string
 >f1 : <T>(x: string | T) => T
 >a1 : string
 

tests/baselines/reference/unionTypeInference.types

@@ -104,8 +104,8 @@ const c4 = f3(b);  // true
 >b : boolean
 
 const c5 = f3("abc");  // never
->c5 : unknown
->f3("abc") : unknown
+>c5 : "abc"
+>f3("abc") : "abc"
 >f3 : <T>(x: string | false | T) => T
 >"abc" : "abc"
 

tests/cases/compiler/observableInferenceCanBeMade.ts

@@ -0,0 +1,22 @@
+// @strict: true
+
+// Repro from #33131
+
+declare function of<T>(a: T): Observable<T>;
+declare function from<O extends ObservableInput<any>>(input: O): Observable<ObservedValueOf<O>>;
+
+type ObservedValueOf<O> = O extends ObservableInput<infer T> ? T : never;
+
+interface Subscribable<T> {
+    subscribe(next?: (value: T) => void, error?: (error: any) => void, complete?: () => void): void;
+}
+type ObservableInput<T> = Subscribable<T> | Subscribable<never>;
+
+
+declare class Observable<T> implements Subscribable<T> {
+    subscribe(next?: (value: T) => void, error?: (error: any) => void, complete?: () => void): void;
+}
+
+function asObservable(input: string | ObservableInput<string>): Observable<string> {
+    return typeof input === 'string' ? of(input) : from(input)
+}