Narrow type constraints

src/compiler/checker.ts

@@ -7211,7 +7211,14 @@ namespace ts {
                         return getUnionType(assignableConstituents);
                     }
                 }
-
+                if (originalType.flags & TypeFlags.TypeParameter) {
+                    const originalTypeParameter = <TypeParameter>originalType;
+                    if (originalTypeParameter.constraint && isTypeAssignableTo(narrowedTypeCandidate, originalTypeParameter.constraint)) {
+                        const narrowedTypeParameter = cloneTypeParameter(originalTypeParameter);
+                        narrowedTypeParameter.constraint = narrowedTypeCandidate;
+                        return narrowedTypeParameter;
+                    }
+                }
                 if (isTypeAssignableTo(narrowedTypeCandidate, originalType)) {
                     // Narrow to the target type if it's assignable to the current type
                     return narrowedTypeCandidate;

tests/baselines/reference/narrowByTypeConstraint.js

@@ -0,0 +1,19 @@
+//// [narrowByTypeConstraint.ts]
+interface Base { b }
+interface Derived extends Base { d }
+declare function isDerived(b: Base): b is Derived;
+function f<T extends Base>(x: T, y: T) {
+	return isDerived(x) && isDerived(y) && x.d === y.d; 
+}
+function g(z: Base, ka: Base) {
+	return isDerived(z) && isDerived(ka) && z.d === ka.d;
+}
+
+
+//// [narrowByTypeConstraint.js]
+function f(x, y) {
+    return isDerived(x) && isDerived(y) && x.d === y.d;
+}
+function g(z, ka) {
+    return isDerived(z) && isDerived(ka) && z.d === ka.d;
+}

tests/baselines/reference/narrowByTypeConstraint.symbols

@@ -0,0 +1,58 @@
+=== tests/cases/compiler/narrowByTypeConstraint.ts ===
+interface Base { b }
+>Base : Symbol(Base, Decl(narrowByTypeConstraint.ts, 0, 0))
+>b : Symbol(b, Decl(narrowByTypeConstraint.ts, 0, 16))
+
+interface Derived extends Base { d }
+>Derived : Symbol(Derived, Decl(narrowByTypeConstraint.ts, 0, 20))
+>Base : Symbol(Base, Decl(narrowByTypeConstraint.ts, 0, 0))
+>d : Symbol(d, Decl(narrowByTypeConstraint.ts, 1, 32))
+
+declare function isDerived(b: Base): b is Derived;
+>isDerived : Symbol(isDerived, Decl(narrowByTypeConstraint.ts, 1, 36))
+>b : Symbol(b, Decl(narrowByTypeConstraint.ts, 2, 27))
+>Base : Symbol(Base, Decl(narrowByTypeConstraint.ts, 0, 0))
+>b : Symbol(b, Decl(narrowByTypeConstraint.ts, 2, 27))
+>Derived : Symbol(Derived, Decl(narrowByTypeConstraint.ts, 0, 20))
+
+function f<T extends Base>(x: T, y: T) {
+>f : Symbol(f, Decl(narrowByTypeConstraint.ts, 2, 50))
+>T : Symbol(T, Decl(narrowByTypeConstraint.ts, 3, 11))
+>Base : Symbol(Base, Decl(narrowByTypeConstraint.ts, 0, 0))
+>x : Symbol(x, Decl(narrowByTypeConstraint.ts, 3, 27))
+>T : Symbol(T, Decl(narrowByTypeConstraint.ts, 3, 11))
+>y : Symbol(y, Decl(narrowByTypeConstraint.ts, 3, 32))
+>T : Symbol(T, Decl(narrowByTypeConstraint.ts, 3, 11))
+
+	return isDerived(x) && isDerived(y) && x.d === y.d; 
+>isDerived : Symbol(isDerived, Decl(narrowByTypeConstraint.ts, 1, 36))
+>x : Symbol(x, Decl(narrowByTypeConstraint.ts, 3, 27))
+>isDerived : Symbol(isDerived, Decl(narrowByTypeConstraint.ts, 1, 36))
+>y : Symbol(y, Decl(narrowByTypeConstraint.ts, 3, 32))
+>x.d : Symbol(Derived.d, Decl(narrowByTypeConstraint.ts, 1, 32))
+>x : Symbol(x, Decl(narrowByTypeConstraint.ts, 3, 27))
+>d : Symbol(Derived.d, Decl(narrowByTypeConstraint.ts, 1, 32))
+>y.d : Symbol(Derived.d, Decl(narrowByTypeConstraint.ts, 1, 32))
+>y : Symbol(y, Decl(narrowByTypeConstraint.ts, 3, 32))
+>d : Symbol(Derived.d, Decl(narrowByTypeConstraint.ts, 1, 32))
+}
+function g(z: Base, ka: Base) {
+>g : Symbol(g, Decl(narrowByTypeConstraint.ts, 5, 1))
+>z : Symbol(z, Decl(narrowByTypeConstraint.ts, 6, 11))
+>Base : Symbol(Base, Decl(narrowByTypeConstraint.ts, 0, 0))
+>ka : Symbol(ka, Decl(narrowByTypeConstraint.ts, 6, 19))
+>Base : Symbol(Base, Decl(narrowByTypeConstraint.ts, 0, 0))
+
+	return isDerived(z) && isDerived(ka) && z.d === ka.d;
+>isDerived : Symbol(isDerived, Decl(narrowByTypeConstraint.ts, 1, 36))
+>z : Symbol(z, Decl(narrowByTypeConstraint.ts, 6, 11))
+>isDerived : Symbol(isDerived, Decl(narrowByTypeConstraint.ts, 1, 36))
+>ka : Symbol(ka, Decl(narrowByTypeConstraint.ts, 6, 19))
+>z.d : Symbol(Derived.d, Decl(narrowByTypeConstraint.ts, 1, 32))
+>z : Symbol(z, Decl(narrowByTypeConstraint.ts, 6, 11))
+>d : Symbol(Derived.d, Decl(narrowByTypeConstraint.ts, 1, 32))
+>ka.d : Symbol(Derived.d, Decl(narrowByTypeConstraint.ts, 1, 32))
+>ka : Symbol(ka, Decl(narrowByTypeConstraint.ts, 6, 19))
+>d : Symbol(Derived.d, Decl(narrowByTypeConstraint.ts, 1, 32))
+}
+

tests/baselines/reference/narrowByTypeConstraint.types

@@ -0,0 +1,68 @@
+=== tests/cases/compiler/narrowByTypeConstraint.ts ===
+interface Base { b }
+>Base : Base
+>b : any
+
+interface Derived extends Base { d }
+>Derived : Derived
+>Base : Base
+>d : any
+
+declare function isDerived(b: Base): b is Derived;
+>isDerived : (b: Base) => b is Derived
+>b : Base
+>Base : Base
+>b : any
+>Derived : Derived
+
+function f<T extends Base>(x: T, y: T) {
+>f : <T extends Base>(x: T, y: T) => boolean
+>T : T
+>Base : Base
+>x : T
+>T : T
+>y : T
+>T : T
+
+	return isDerived(x) && isDerived(y) && x.d === y.d; 
+>isDerived(x) && isDerived(y) && x.d === y.d : boolean
+>isDerived(x) && isDerived(y) : boolean
+>isDerived(x) : boolean
+>isDerived : (b: Base) => b is Derived
+>x : T
+>isDerived(y) : boolean
+>isDerived : (b: Base) => b is Derived
+>y : T
+>x.d === y.d : boolean
+>x.d : any
+>x : T
+>d : any
+>y.d : any
+>y : T
+>d : any
+}
+function g(z: Base, ka: Base) {
+>g : (z: Base, ka: Base) => boolean
+>z : Base
+>Base : Base
+>ka : Base
+>Base : Base
+
+	return isDerived(z) && isDerived(ka) && z.d === ka.d;
+>isDerived(z) && isDerived(ka) && z.d === ka.d : boolean
+>isDerived(z) && isDerived(ka) : boolean
+>isDerived(z) : boolean
+>isDerived : (b: Base) => b is Derived
+>z : Base
+>isDerived(ka) : boolean
+>isDerived : (b: Base) => b is Derived
+>ka : Base
+>z.d === ka.d : boolean
+>z.d : any
+>z : Derived
+>d : any
+>ka.d : any
+>ka : Derived
+>d : any
+}
+

tests/cases/compiler/narrowByTypeConstraint.ts

@@ -0,0 +1,9 @@
+interface Base { b }
+interface Derived extends Base { d }
+declare function isDerived(b: Base): b is Derived;
+function f<T extends Base>(x: T, y: T) {
+	return isDerived(x) && isDerived(y) && x.d === y.d; 
+}
+function g(z: Base, ka: Base) {
+	return isDerived(z) && isDerived(ka) && z.d === ka.d;
+}