Narrow via discriminant property through optional chain (microsoft#42450)

* Naive attempt at narrowing via discriminant property through optional chain

* Clean up test

Author: andrewbranch

src/compiler/checker.ts

@@ -22599,10 +22599,13 @@ namespace ts {
                 if (propName === undefined) {
                     return type;
                 }
-                const propType = getTypeOfPropertyOfType(type, propName);
+                const includesUndefined = strictNullChecks && maybeTypeOfKind(type, TypeFlags.Undefined);
+                const removeOptional = includesUndefined && isOptionalChain(access);
+                let propType = getTypeOfPropertyOfType(removeOptional ? getTypeWithFacts(type, TypeFacts.NEUndefined) : type, propName);
                 if (!propType) {
                     return type;
                 }
+                propType = removeOptional ? getOptionalType(propType) : propType;
                 const narrowedPropType = narrowType(propType);
                 return filterType(type, t => {
                     const discriminantType = getTypeOfPropertyOrIndexSignature(t, propName);

tests/baselines/reference/controlFlowOptionalChain.symbols

@@ -200,14 +200,14 @@ else {
 >o3 : Symbol(o3, Decl(controlFlowOptionalChain.ts, 58, 13))
 
     o3?.x;
->o3?.x : Symbol(x, Decl(controlFlowOptionalChain.ts, 58, 19), Decl(controlFlowOptionalChain.ts, 58, 41))
+>o3?.x : Symbol(x, Decl(controlFlowOptionalChain.ts, 58, 41))
 >o3 : Symbol(o3, Decl(controlFlowOptionalChain.ts, 58, 13))
->x : Symbol(x, Decl(controlFlowOptionalChain.ts, 58, 19), Decl(controlFlowOptionalChain.ts, 58, 41))
+>x : Symbol(x, Decl(controlFlowOptionalChain.ts, 58, 41))
 
     o3.x;
->o3.x : Symbol(x, Decl(controlFlowOptionalChain.ts, 58, 19), Decl(controlFlowOptionalChain.ts, 58, 41))
+>o3.x : Symbol(x, Decl(controlFlowOptionalChain.ts, 58, 41))
 >o3 : Symbol(o3, Decl(controlFlowOptionalChain.ts, 58, 13))
->x : Symbol(x, Decl(controlFlowOptionalChain.ts, 58, 19), Decl(controlFlowOptionalChain.ts, 58, 41))
+>x : Symbol(x, Decl(controlFlowOptionalChain.ts, 58, 41))
 }
 o3;
 >o3 : Symbol(o3, Decl(controlFlowOptionalChain.ts, 58, 13))

tests/baselines/reference/controlFlowOptionalChain.types

@@ -223,16 +223,16 @@ if (o3?.x === 1) {
 }
 else {
     o3;
->o3 : { x: 1; y: string; } | { x: 2; y: number; } | undefined
+>o3 : { x: 2; y: number; } | undefined
 
     o3?.x;
 >o3?.x : 2 | undefined
->o3 : { x: 1; y: string; } | { x: 2; y: number; } | undefined
+>o3 : { x: 2; y: number; } | undefined
 >x : 2 | undefined
 
     o3.x;
 >o3.x : 2
->o3 : { x: 1; y: string; } | { x: 2; y: number; } | undefined
+>o3 : { x: 2; y: number; } | undefined
 >x : 2
 }
 o3;

tests/baselines/reference/controlFlowOptionalChain2.js

@@ -0,0 +1,30 @@
+//// [controlFlowOptionalChain2.ts]
+type A = {
+  type: 'A';
+  name: string;
+}
+
+type B = {
+  type: 'B';
+}
+
+function funcTwo(arg: A | B | undefined) {
+  if (arg?.type === 'B') {
+    arg; // `B`
+    return;
+  }
+
+  arg;
+  arg?.name;
+}
+
+
+//// [controlFlowOptionalChain2.js]
+function funcTwo(arg) {
+    if ((arg === null || arg === void 0 ? void 0 : arg.type) === 'B') {
+        arg; // `B`
+        return;
+    }
+    arg;
+    arg === null || arg === void 0 ? void 0 : arg.name;
+}

tests/baselines/reference/controlFlowOptionalChain2.symbols

@@ -0,0 +1,44 @@
+=== tests/cases/conformance/controlFlow/controlFlowOptionalChain2.ts ===
+type A = {
+>A : Symbol(A, Decl(controlFlowOptionalChain2.ts, 0, 0))
+
+  type: 'A';
+>type : Symbol(type, Decl(controlFlowOptionalChain2.ts, 0, 10))
+
+  name: string;
+>name : Symbol(name, Decl(controlFlowOptionalChain2.ts, 1, 12))
+}
+
+type B = {
+>B : Symbol(B, Decl(controlFlowOptionalChain2.ts, 3, 1))
+
+  type: 'B';
+>type : Symbol(type, Decl(controlFlowOptionalChain2.ts, 5, 10))
+}
+
+function funcTwo(arg: A | B | undefined) {
+>funcTwo : Symbol(funcTwo, Decl(controlFlowOptionalChain2.ts, 7, 1))
+>arg : Symbol(arg, Decl(controlFlowOptionalChain2.ts, 9, 17))
+>A : Symbol(A, Decl(controlFlowOptionalChain2.ts, 0, 0))
+>B : Symbol(B, Decl(controlFlowOptionalChain2.ts, 3, 1))
+
+  if (arg?.type === 'B') {
+>arg?.type : Symbol(type, Decl(controlFlowOptionalChain2.ts, 0, 10), Decl(controlFlowOptionalChain2.ts, 5, 10))
+>arg : Symbol(arg, Decl(controlFlowOptionalChain2.ts, 9, 17))
+>type : Symbol(type, Decl(controlFlowOptionalChain2.ts, 0, 10), Decl(controlFlowOptionalChain2.ts, 5, 10))
+
+    arg; // `B`
+>arg : Symbol(arg, Decl(controlFlowOptionalChain2.ts, 9, 17))
+
+    return;
+  }
+
+  arg;
+>arg : Symbol(arg, Decl(controlFlowOptionalChain2.ts, 9, 17))
+
+  arg?.name;
+>arg?.name : Symbol(name, Decl(controlFlowOptionalChain2.ts, 1, 12))
+>arg : Symbol(arg, Decl(controlFlowOptionalChain2.ts, 9, 17))
+>name : Symbol(name, Decl(controlFlowOptionalChain2.ts, 1, 12))
+}
+

tests/baselines/reference/controlFlowOptionalChain2.types

@@ -0,0 +1,44 @@
+=== tests/cases/conformance/controlFlow/controlFlowOptionalChain2.ts ===
+type A = {
+>A : A
+
+  type: 'A';
+>type : "A"
+
+  name: string;
+>name : string
+}
+
+type B = {
+>B : B
+
+  type: 'B';
+>type : "B"
+}
+
+function funcTwo(arg: A | B | undefined) {
+>funcTwo : (arg: A | B | undefined) => void
+>arg : A | B | undefined
+
+  if (arg?.type === 'B') {
+>arg?.type === 'B' : boolean
+>arg?.type : "A" | "B" | undefined
+>arg : A | B | undefined
+>type : "A" | "B" | undefined
+>'B' : "B"
+
+    arg; // `B`
+>arg : B
+
+    return;
+  }
+
+  arg;
+>arg : A | undefined
+
+  arg?.name;
+>arg?.name : string | undefined
+>arg : A | undefined
+>name : string | undefined
+}
+

tests/cases/conformance/controlFlow/controlFlowOptionalChain2.ts

@@ -0,0 +1,20 @@
+// @strictNullChecks: true
+
+type A = {
+  type: 'A';
+  name: string;
+}
+
+type B = {
+  type: 'B';
+}
+
+function funcTwo(arg: A | B | undefined) {
+  if (arg?.type === 'B') {
+    arg; // `B`
+    return;
+  }
+
+  arg;
+  arg?.name;
+}