Narrow down non-const enity name expressions within element access expressions

src/compiler/checker.ts

@@ -2834,8 +2834,12 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
                     return findAncestor(errorBindingElement, isBindingElement) !== findAncestor(declaration, isBindingElement) ||
                         declaration.pos < errorBindingElement.pos;
                 }
+                const rootDeclaration = getRootDeclaration(declaration) as Declaration;
+                if (rootDeclaration.kind !== SyntaxKind.VariableDeclaration) {
+                    return true;
+                }
                 // or it might be illegal if usage happens before parent variable is declared (eg var [a] = a)
-                return isBlockScopedNameDeclaredBeforeUse(getAncestor(declaration, SyntaxKind.VariableDeclaration) as Declaration, usage);
+                return isBlockScopedNameDeclaredBeforeUse(rootDeclaration, usage);
             }
             else if (declaration.kind === SyntaxKind.VariableDeclaration) {
                 // still might be illegal if usage is in the initializer of the variable declaration (eg var a = a)
@@ -26404,29 +26408,8 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
 
     function tryGetNameFromEntityNameExpression(node: EntityNameOrEntityNameExpression) {
         const symbol = resolveEntityName(node, SymbolFlags.Value, /*ignoreErrors*/ true);
-        if (!symbol || !(isConstantVariable(symbol) || (symbol.flags & SymbolFlags.EnumMember))) return undefined;
-
-        const declaration = symbol.valueDeclaration;
-        if (declaration === undefined) return undefined;
-
-        const type = tryGetTypeFromEffectiveTypeNode(declaration);
-        if (type) {
-            const name = tryGetNameFromType(type);
-            if (name !== undefined) {
-                return name;
-            }
-        }
-        if (hasOnlyExpressionInitializer(declaration) && isBlockScopedNameDeclaredBeforeUse(declaration, node)) {
-            const initializer = getEffectiveInitializer(declaration);
-            if (initializer) {
-                const initializerType = isBindingPattern(declaration.parent) ? getTypeForBindingElement(declaration as BindingElement) : getTypeOfExpression(initializer);
-                return initializerType && tryGetNameFromType(initializerType);
-            }
-            if (isEnumMember(declaration)) {
-                return getTextOfPropertyName(declaration.name);
-            }
-        }
-        return undefined;
+        if (!symbol || symbol.valueDeclaration && !isBlockScopedNameDeclaredBeforeUse(symbol.valueDeclaration, node)) return undefined;
+        return tryGetNameFromType(getTypeOfExpression(node as Expression));
     }
 
     function containsMatchingReference(source: Node, target: Node) {

tests/baselines/reference/typeGuardNarrowsIndexedAccessOfKnownProperty13.symbols

@@ -0,0 +1,30 @@
+//// [tests/cases/compiler/typeGuardNarrowsIndexedAccessOfKnownProperty13.ts] ////
+
+=== typeGuardNarrowsIndexedAccessOfKnownProperty13.ts ===
+interface Data {
+>Data : Symbol(Data, Decl(typeGuardNarrowsIndexedAccessOfKnownProperty13.ts, 0, 0))
+
+  a?: number;
+>a : Symbol(Data.a, Decl(typeGuardNarrowsIndexedAccessOfKnownProperty13.ts, 0, 16))
+}
+
+declare const data: Data;
+>data : Symbol(data, Decl(typeGuardNarrowsIndexedAccessOfKnownProperty13.ts, 4, 13))
+>Data : Symbol(Data, Decl(typeGuardNarrowsIndexedAccessOfKnownProperty13.ts, 0, 0))
+
+let key = "a" as const;
+>key : Symbol(key, Decl(typeGuardNarrowsIndexedAccessOfKnownProperty13.ts, 6, 3))
+>const : Symbol(const)
+
+if (data.a !== undefined) {
+>data.a : Symbol(Data.a, Decl(typeGuardNarrowsIndexedAccessOfKnownProperty13.ts, 0, 16))
+>data : Symbol(data, Decl(typeGuardNarrowsIndexedAccessOfKnownProperty13.ts, 4, 13))
+>a : Symbol(Data.a, Decl(typeGuardNarrowsIndexedAccessOfKnownProperty13.ts, 0, 16))
+>undefined : Symbol(undefined)
+
+  const a = data[key];
+>a : Symbol(a, Decl(typeGuardNarrowsIndexedAccessOfKnownProperty13.ts, 9, 7))
+>data : Symbol(data, Decl(typeGuardNarrowsIndexedAccessOfKnownProperty13.ts, 4, 13))
+>key : Symbol(key, Decl(typeGuardNarrowsIndexedAccessOfKnownProperty13.ts, 6, 3))
+}
+

tests/baselines/reference/typeGuardNarrowsIndexedAccessOfKnownProperty13.types

@@ -0,0 +1,30 @@
+//// [tests/cases/compiler/typeGuardNarrowsIndexedAccessOfKnownProperty13.ts] ////
+
+=== typeGuardNarrowsIndexedAccessOfKnownProperty13.ts ===
+interface Data {
+  a?: number;
+>a : number | undefined
+}
+
+declare const data: Data;
+>data : Data
+
+let key = "a" as const;
+>key : "a"
+>"a" as const : "a"
+>"a" : "a"
+
+if (data.a !== undefined) {
+>data.a !== undefined : boolean
+>data.a : number | undefined
+>data : Data
+>a : number | undefined
+>undefined : undefined
+
+  const a = data[key];
+>a : number
+>data[key] : number
+>data : Data
+>key : "a"
+}
+

tests/cases/compiler/typeGuardNarrowsIndexedAccessOfKnownProperty13.ts

@@ -0,0 +1,14 @@
+// @strict: true
+// @noEmit: true
+
+interface Data {
+  a?: number;
+}
+
+declare const data: Data;
+
+let key = "a" as const;
+
+if (data.a !== undefined) {
+  const a = data[key];
+}