Merge pull request #12843 from Microsoft/mappedTypeModifiers3

Additional tweak to mapped type property modifier propagation

Author: ahejlsberg

src/compiler/checker.ts

@@ -4590,12 +4590,22 @@ namespace ts {
 
         function getModifiersTypeFromMappedType(type: MappedType) {
             if (!type.modifiersType) {
-                // If the mapped type was declared as { [P in keyof T]: X } or as { [P in K]: X }, where
-                // K is constrained to 'K extends keyof T', then we will copy property modifiers from T.
-                const declaredType = <MappedType>getTypeFromMappedTypeNode(type.declaration);
-                const constraint = getConstraintTypeFromMappedType(declaredType);
-                const extendedConstraint = constraint.flags & TypeFlags.TypeParameter ? getConstraintOfTypeParameter(<TypeParameter>constraint) : constraint;
-                type.modifiersType = extendedConstraint.flags & TypeFlags.Index ? instantiateType((<IndexType>extendedConstraint).type, type.mapper || identityMapper) : emptyObjectType;
+                const constraintDeclaration = type.declaration.typeParameter.constraint;
+                if (constraintDeclaration.kind === SyntaxKind.TypeOperator) {
+                    // If the constraint declaration is a 'keyof T' node, the modifiers type is T. We check
+                    // AST nodes here because, when T is a non-generic type, the logic below eagerly resolves
+                    // 'keyof T' to a literal union type and we can't recover T from that type.
+                    type.modifiersType = instantiateType(getTypeFromTypeNode((<TypeOperatorNode>constraintDeclaration).type), type.mapper || identityMapper);
+                }
+                else {
+                    // Otherwise, get the declared constraint type, and if the constraint type is a type parameter,
+                    // get the constraint of that type parameter. If the resulting type is an indexed type 'keyof T',
+                    // the modifiers type is T. Otherwise, the modifiers type is {}.
+                    const declaredType = <MappedType>getTypeFromMappedTypeNode(type.declaration);
+                    const constraint = getConstraintTypeFromMappedType(declaredType);
+                    const extendedConstraint = constraint.flags & TypeFlags.TypeParameter ? getConstraintOfTypeParameter(<TypeParameter>constraint) : constraint;
+                    type.modifiersType = extendedConstraint.flags & TypeFlags.Index ? instantiateType((<IndexType>extendedConstraint).type, type.mapper || identityMapper) : emptyObjectType;
+                }
             }
             return type.modifiersType;
         }

tests/baselines/reference/mappedTypeModifiers.js

@@ -19,19 +19,22 @@ var v01: Pick<Pick<T, keyof T>, keyof T>;
 var v02: TP;
 var v02: { [P in keyof T]?: T[P] };
 var v02: Partial<T>;
-var v02: Pick<TP, keyof T>;
+var v02: { [P in keyof TP]: TP[P] }
+var v02: Pick<TP, keyof TP>;
 
 var v03: TR;
 var v03: { readonly [P in keyof T]: T[P] };
 var v03: Readonly<T>;
-var v03: Pick<TR, keyof T>;
+var v03: { [P in keyof TR]: TR[P] }
+var v03: Pick<TR, keyof TR>;
 
 var v04: TPR;
 var v04: { readonly [P in keyof T]?: T[P] };
 var v04: Partial<TR>;
 var v04: Readonly<TP>;
 var v04: Partial<Readonly<T>>;
 var v04: Readonly<Partial<T>>;
+var v04: { [P in keyof TPR]: TPR[P] }
 var v04: Pick<TPR, keyof T>;
 
 type Boxified<T> = { [P in keyof T]: { x: T[P] } };
@@ -55,20 +58,23 @@ var b01: Pick<Pick<B, keyof B>, keyof B>;
 var b02: BP;
 var b02: { [P in keyof B]?: B[P] };
 var b02: Partial<B>;
-var b02: Pick<BP, keyof B>;
+var b02: { [P in keyof BP]: BP[P] }
+var b02: Pick<BP, keyof BP>;
 
 var b03: BR;
 var b03: { readonly [P in keyof B]: B[P] };
 var b03: Readonly<B>;
-var b03: Pick<BR, keyof B>;
+var b03: { [P in keyof BR]: BR[P] }
+var b03: Pick<BR, keyof BR>;
 
 var b04: BPR;
 var b04: { readonly [P in keyof B]?: B[P] };
 var b04: Partial<BR>;
 var b04: Readonly<BP>;
 var b04: Partial<Readonly<B>>;
 var b04: Readonly<Partial<B>>;
-var b04: Pick<BPR, keyof B>;
+var b04: { [P in keyof BPR]: BPR[P] }
+var b04: Pick<BPR, keyof BPR>;
 
 //// [mappedTypeModifiers.js]
 var v00;
@@ -84,6 +90,8 @@ var v02;
 var v02;
 var v02;
 var v02;
+var v02;
+var v03;
 var v03;
 var v03;
 var v03;
@@ -95,6 +103,7 @@ var v04;
 var v04;
 var v04;
 var v04;
+var v04;
 var b00;
 var b00;
 var b00;
@@ -108,6 +117,8 @@ var b02;
 var b02;
 var b02;
 var b02;
+var b02;
+var b03;
 var b03;
 var b03;
 var b03;
@@ -119,3 +130,4 @@ var b04;
 var b04;
 var b04;
 var b04;
+var b04;