Still generate signatures in SkipContextSensitive mode just to match on return types (#25937)

* Still generate signatures in SkipContextSensitive mode just to match on return types

* Add cache for context-free type of a signature node

* Accept post-merge baseline

Author: weswigham

src/compiler/checker.ts

@@ -10369,7 +10369,10 @@ namespace ts {
                     return true;
                 }
             }
+            return hasContextSensitiveReturnExpression(node);
+        }
 
+        function hasContextSensitiveReturnExpression(node: FunctionLikeDeclaration) {
             // TODO(anhans): A block should be context-sensitive if it has a context-sensitive return value.
             const body = node.body!;
             return body.kind === SyntaxKind.Block ? false : isContextSensitive(body);
@@ -20704,6 +20707,16 @@ namespace ts {
 
             // The identityMapper object is used to indicate that function expressions are wildcards
             if (checkMode === CheckMode.SkipContextSensitive && isContextSensitive(node)) {
+                // Skip parameters, return signature with return type that retains noncontextual parts so inferences can still be drawn in an early stage
+                if (!getEffectiveReturnTypeNode(node) && hasContextSensitiveReturnExpression(node)) {
+                    const links = getNodeLinks(node);
+                    if (links.contextFreeType) {
+                        return links.contextFreeType;
+                    }
+                    const returnType = getReturnTypeFromBody(node, checkMode);
+                    const singleReturnSignature = createSignature(undefined, undefined, undefined, emptyArray, returnType, /*resolvedTypePredicate*/ undefined, 0, /*hasRestParameter*/ false, /*hasLiteralTypes*/ false);
+                    return links.contextFreeType = createAnonymousType(node.symbol, emptySymbols, [singleReturnSignature], emptyArray, undefined, undefined);
+                }
                 return anyFunctionType;
             }
 

src/compiler/types.ts

@@ -3667,6 +3667,7 @@ namespace ts {
         superCall?: SuperCall;  // Cached first super-call found in the constructor. Used in checking whether super is called before this-accessing
         switchTypes?: Type[];             // Cached array of switch case expression types
         jsxNamespace?: Symbol | false;          // Resolved jsx namespace symbol for this node
+        contextFreeType?: Type;          // Cached context-free type used by the first pass of inference; used when a function's return is partially contextually sensitive
     }
 
     export const enum TypeFlags {

tests/baselines/reference/badInferenceLowerPriorityThanGoodInference.js

@@ -0,0 +1,21 @@
+//// [badInferenceLowerPriorityThanGoodInference.ts]
+interface Foo<A> {
+    a: A;
+    b: (x: A) => void;
+}
+
+declare function canYouInferThis<A>(fn: () => Foo<A>): A;
+
+const result = canYouInferThis(() => ({
+    a: { BLAH: 33 },
+    b: x => { }
+}))
+
+result.BLAH;
+
+//// [badInferenceLowerPriorityThanGoodInference.js]
+var result = canYouInferThis(function () { return ({
+    a: { BLAH: 33 },
+    b: function (x) { }
+}); });
+result.BLAH;

tests/baselines/reference/badInferenceLowerPriorityThanGoodInference.symbols

@@ -0,0 +1,42 @@
+=== tests/cases/compiler/badInferenceLowerPriorityThanGoodInference.ts ===
+interface Foo<A> {
+>Foo : Symbol(Foo, Decl(badInferenceLowerPriorityThanGoodInference.ts, 0, 0))
+>A : Symbol(A, Decl(badInferenceLowerPriorityThanGoodInference.ts, 0, 14))
+
+    a: A;
+>a : Symbol(Foo.a, Decl(badInferenceLowerPriorityThanGoodInference.ts, 0, 18))
+>A : Symbol(A, Decl(badInferenceLowerPriorityThanGoodInference.ts, 0, 14))
+
+    b: (x: A) => void;
+>b : Symbol(Foo.b, Decl(badInferenceLowerPriorityThanGoodInference.ts, 1, 9))
+>x : Symbol(x, Decl(badInferenceLowerPriorityThanGoodInference.ts, 2, 8))
+>A : Symbol(A, Decl(badInferenceLowerPriorityThanGoodInference.ts, 0, 14))
+}
+
+declare function canYouInferThis<A>(fn: () => Foo<A>): A;
+>canYouInferThis : Symbol(canYouInferThis, Decl(badInferenceLowerPriorityThanGoodInference.ts, 3, 1))
+>A : Symbol(A, Decl(badInferenceLowerPriorityThanGoodInference.ts, 5, 33))
+>fn : Symbol(fn, Decl(badInferenceLowerPriorityThanGoodInference.ts, 5, 36))
+>Foo : Symbol(Foo, Decl(badInferenceLowerPriorityThanGoodInference.ts, 0, 0))
+>A : Symbol(A, Decl(badInferenceLowerPriorityThanGoodInference.ts, 5, 33))
+>A : Symbol(A, Decl(badInferenceLowerPriorityThanGoodInference.ts, 5, 33))
+
+const result = canYouInferThis(() => ({
+>result : Symbol(result, Decl(badInferenceLowerPriorityThanGoodInference.ts, 7, 5))
+>canYouInferThis : Symbol(canYouInferThis, Decl(badInferenceLowerPriorityThanGoodInference.ts, 3, 1))
+
+    a: { BLAH: 33 },
+>a : Symbol(a, Decl(badInferenceLowerPriorityThanGoodInference.ts, 7, 39))
+>BLAH : Symbol(BLAH, Decl(badInferenceLowerPriorityThanGoodInference.ts, 8, 8))
+
+    b: x => { }
+>b : Symbol(b, Decl(badInferenceLowerPriorityThanGoodInference.ts, 8, 20))
+>x : Symbol(x, Decl(badInferenceLowerPriorityThanGoodInference.ts, 9, 6))
+
+}))
+
+result.BLAH;
+>result.BLAH : Symbol(BLAH, Decl(badInferenceLowerPriorityThanGoodInference.ts, 8, 8))
+>result : Symbol(result, Decl(badInferenceLowerPriorityThanGoodInference.ts, 7, 5))
+>BLAH : Symbol(BLAH, Decl(badInferenceLowerPriorityThanGoodInference.ts, 8, 8))
+

tests/baselines/reference/badInferenceLowerPriorityThanGoodInference.types

@@ -0,0 +1,40 @@
+=== tests/cases/compiler/badInferenceLowerPriorityThanGoodInference.ts ===
+interface Foo<A> {
+    a: A;
+>a : A
+
+    b: (x: A) => void;
+>b : (x: A) => void
+>x : A
+}
+
+declare function canYouInferThis<A>(fn: () => Foo<A>): A;
+>canYouInferThis : <A>(fn: () => Foo<A>) => A
+>fn : () => Foo<A>
+
+const result = canYouInferThis(() => ({
+>result : { BLAH: number; }
+>canYouInferThis(() => ({    a: { BLAH: 33 },    b: x => { }})) : { BLAH: number; }
+>canYouInferThis : <A>(fn: () => Foo<A>) => A
+>() => ({    a: { BLAH: 33 },    b: x => { }}) : () => { a: { BLAH: number; }; b: (x: { BLAH: number; }) => void; }
+>({    a: { BLAH: 33 },    b: x => { }}) : { a: { BLAH: number; }; b: (x: { BLAH: number; }) => void; }
+>{    a: { BLAH: 33 },    b: x => { }} : { a: { BLAH: number; }; b: (x: { BLAH: number; }) => void; }
+
+    a: { BLAH: 33 },
+>a : { BLAH: number; }
+>{ BLAH: 33 } : { BLAH: number; }
+>BLAH : number
+>33 : 33
+
+    b: x => { }
+>b : (x: { BLAH: number; }) => void
+>x => { } : (x: { BLAH: number; }) => void
+>x : { BLAH: number; }
+
+}))
+
+result.BLAH;
+>result.BLAH : number
+>result : { BLAH: number; }
+>BLAH : number
+

tests/cases/compiler/badInferenceLowerPriorityThanGoodInference.ts

@@ -0,0 +1,13 @@
+interface Foo<A> {
+    a: A;
+    b: (x: A) => void;
+}
+
+declare function canYouInferThis<A>(fn: () => Foo<A>): A;
+
+const result = canYouInferThis(() => ({
+    a: { BLAH: 33 },
+    b: x => { }
+}))
+
+result.BLAH;