Add infer T constraint inference rule matching up mapped type templates across check/extends types (microsoft#43649)

Author: weswigham

src/compiler/checker.ts

@@ -12613,6 +12613,19 @@ namespace ts {
                         else if (grandParent.kind === SyntaxKind.TypeParameter && grandParent.parent.kind === SyntaxKind.MappedType) {
                             inferences = append(inferences, keyofConstraintType);
                         }
+                        // When an 'infer T' declaration is the template of a mapped type, and that mapped type if the extends
+                        // clause of a conditional whose check type is also a mapped type, give it the constraint of the template
+                        // of the check type's mapped type
+                        else if (grandParent.kind === SyntaxKind.MappedType && (grandParent as MappedTypeNode).type &&
+                            skipParentheses((grandParent as MappedTypeNode).type!) === declaration.parent && grandParent.parent.kind === SyntaxKind.ConditionalType &&
+                            (grandParent.parent as ConditionalTypeNode).extendsType === grandParent && (grandParent.parent as ConditionalTypeNode).checkType.kind === SyntaxKind.MappedType &&
+                            ((grandParent.parent as ConditionalTypeNode).checkType as MappedTypeNode).type) {
+                            const checkMappedType = (grandParent.parent as ConditionalTypeNode).checkType as MappedTypeNode;
+                            const nodeType = getTypeFromTypeNode(checkMappedType.type!);
+                            inferences = append(inferences, instantiateType(nodeType,
+                                makeUnaryTypeMapper(getDeclaredTypeOfTypeParameter(getSymbolOfNode(checkMappedType.typeParameter)), checkMappedType.typeParameter.constraint ? getTypeFromTypeNode(checkMappedType.typeParameter.constraint) : keyofConstraintType)
+                            ));
+                        }
                     }
                 }
             }

tests/baselines/reference/inferConditionalConstraintMappedMember.js

@@ -0,0 +1,21 @@
+//// [inferConditionalConstraintMappedMember.ts]
+// Return keyof type without string index signature
+type KeysWithoutStringIndex<T> =
+    { [K in keyof T]: string extends K ? never : K } extends { [_ in keyof T]: infer U }
+    ? U
+    : never
+
+// Only "foo" | "bar" as expected, [string] index signature removed
+type test = KeysWithoutStringIndex<{ [index: string]: string; foo: string; bar: 'baz' }>
+// KeysWithoutStringIndex<T> will always be a subset of keyof T, but is reported as unassignable
+export type RemoveIdxSgn<T> = Pick<T, KeysWithoutStringIndex<T>>
+  // ERROR:
+  // Type 'KeysWithoutStringIndex<T>' does not satisfy the constraint 'keyof T'.
+  //  Type 'unknown' is not assignable to type 'keyof T'.(2344)
+
+//// [inferConditionalConstraintMappedMember.js]
+"use strict";
+exports.__esModule = true;
+// ERROR:
+// Type 'KeysWithoutStringIndex<T>' does not satisfy the constraint 'keyof T'.
+//  Type 'unknown' is not assignable to type 'keyof T'.(2344)

tests/baselines/reference/inferConditionalConstraintMappedMember.symbols

@@ -0,0 +1,40 @@
+=== tests/cases/compiler/inferConditionalConstraintMappedMember.ts ===
+// Return keyof type without string index signature
+type KeysWithoutStringIndex<T> =
+>KeysWithoutStringIndex : Symbol(KeysWithoutStringIndex, Decl(inferConditionalConstraintMappedMember.ts, 0, 0))
+>T : Symbol(T, Decl(inferConditionalConstraintMappedMember.ts, 1, 28))
+
+    { [K in keyof T]: string extends K ? never : K } extends { [_ in keyof T]: infer U }
+>K : Symbol(K, Decl(inferConditionalConstraintMappedMember.ts, 2, 7))
+>T : Symbol(T, Decl(inferConditionalConstraintMappedMember.ts, 1, 28))
+>K : Symbol(K, Decl(inferConditionalConstraintMappedMember.ts, 2, 7))
+>K : Symbol(K, Decl(inferConditionalConstraintMappedMember.ts, 2, 7))
+>_ : Symbol(_, Decl(inferConditionalConstraintMappedMember.ts, 2, 64))
+>T : Symbol(T, Decl(inferConditionalConstraintMappedMember.ts, 1, 28))
+>U : Symbol(U, Decl(inferConditionalConstraintMappedMember.ts, 2, 84))
+
+    ? U
+>U : Symbol(U, Decl(inferConditionalConstraintMappedMember.ts, 2, 84))
+
+    : never
+
+// Only "foo" | "bar" as expected, [string] index signature removed
+type test = KeysWithoutStringIndex<{ [index: string]: string; foo: string; bar: 'baz' }>
+>test : Symbol(test, Decl(inferConditionalConstraintMappedMember.ts, 4, 11))
+>KeysWithoutStringIndex : Symbol(KeysWithoutStringIndex, Decl(inferConditionalConstraintMappedMember.ts, 0, 0))
+>index : Symbol(index, Decl(inferConditionalConstraintMappedMember.ts, 7, 38))
+>foo : Symbol(foo, Decl(inferConditionalConstraintMappedMember.ts, 7, 61))
+>bar : Symbol(bar, Decl(inferConditionalConstraintMappedMember.ts, 7, 74))
+
+// KeysWithoutStringIndex<T> will always be a subset of keyof T, but is reported as unassignable
+export type RemoveIdxSgn<T> = Pick<T, KeysWithoutStringIndex<T>>
+>RemoveIdxSgn : Symbol(RemoveIdxSgn, Decl(inferConditionalConstraintMappedMember.ts, 7, 88))
+>T : Symbol(T, Decl(inferConditionalConstraintMappedMember.ts, 9, 25))
+>Pick : Symbol(Pick, Decl(lib.es5.d.ts, --, --))
+>T : Symbol(T, Decl(inferConditionalConstraintMappedMember.ts, 9, 25))
+>KeysWithoutStringIndex : Symbol(KeysWithoutStringIndex, Decl(inferConditionalConstraintMappedMember.ts, 0, 0))
+>T : Symbol(T, Decl(inferConditionalConstraintMappedMember.ts, 9, 25))
+
+  // ERROR:
+  // Type 'KeysWithoutStringIndex<T>' does not satisfy the constraint 'keyof T'.
+  //  Type 'unknown' is not assignable to type 'keyof T'.(2344)

tests/baselines/reference/inferConditionalConstraintMappedMember.types

@@ -0,0 +1,23 @@
+=== tests/cases/compiler/inferConditionalConstraintMappedMember.ts ===
+// Return keyof type without string index signature
+type KeysWithoutStringIndex<T> =
+>KeysWithoutStringIndex : KeysWithoutStringIndex<T>
+
+    { [K in keyof T]: string extends K ? never : K } extends { [_ in keyof T]: infer U }
+    ? U
+    : never
+
+// Only "foo" | "bar" as expected, [string] index signature removed
+type test = KeysWithoutStringIndex<{ [index: string]: string; foo: string; bar: 'baz' }>
+>test : never
+>index : string
+>foo : string
+>bar : "baz"
+
+// KeysWithoutStringIndex<T> will always be a subset of keyof T, but is reported as unassignable
+export type RemoveIdxSgn<T> = Pick<T, KeysWithoutStringIndex<T>>
+>RemoveIdxSgn : RemoveIdxSgn<T>
+
+  // ERROR:
+  // Type 'KeysWithoutStringIndex<T>' does not satisfy the constraint 'keyof T'.
+  //  Type 'unknown' is not assignable to type 'keyof T'.(2344)

tests/cases/compiler/inferConditionalConstraintMappedMember.ts

@@ -0,0 +1,13 @@
+// Return keyof type without string index signature
+type KeysWithoutStringIndex<T> =
+    { [K in keyof T]: string extends K ? never : K } extends { [_ in keyof T]: infer U }
+    ? U
+    : never
+
+// Only "foo" | "bar" as expected, [string] index signature removed
+type test = KeysWithoutStringIndex<{ [index: string]: string; foo: string; bar: 'baz' }>
+// KeysWithoutStringIndex<T> will always be a subset of keyof T, but is reported as unassignable
+export type RemoveIdxSgn<T> = Pick<T, KeysWithoutStringIndex<T>>
+  // ERROR:
+  // Type 'KeysWithoutStringIndex<T>' does not satisfy the constraint 'keyof T'.
+  //  Type 'unknown' is not assignable to type 'keyof T'.(2344)