Error on forward references for property initializers

The error only appears when a property initializer references another
property before its definition. References to outer variables, etc are
still allowed.

Author: sandersn

src/compiler/checker.ts

@@ -641,7 +641,7 @@ namespace ts {
                 }
                 // declaration is after usage
                 // can be legal if usage is deferred (i.e. inside function or in initializer of instance property)
-                if (isUsedInFunctionOrNonStaticProperty(usage)) {
+                if (isUsedInFunctionOrInstanceProperty(usage)) {
                     return true;
                 }
                 const sourceFiles = host.getSourceFiles();
@@ -668,10 +668,12 @@ namespace ts {
             }
 
 
-            // declaration is after usage
-            // can be legal if usage is deferred (i.e. inside function or in initializer of instance property)
+            // declaration is after usage, but it can still be legal if usage is deferred:
+            // 1. inside a function
+            // 2. inside an instance property initializer, a reference to a non-instance property
             const container = getEnclosingBlockScopeContainer(declaration);
-            return isUsedInFunctionOrNonStaticProperty(usage, container);
+            const isInstanceProperty = declaration.kind === SyntaxKind.PropertyDeclaration && !(getModifierFlags(declaration) & ModifierFlags.Static);
+            return isUsedInFunctionOrInstanceProperty(usage, isInstanceProperty, container);
 
             function isImmediatelyUsedInInitializerOfBlockScopedVariable(declaration: VariableDeclaration, usage: Node): boolean {
                 const container = getEnclosingBlockScopeContainer(declaration);
@@ -700,7 +702,7 @@ namespace ts {
                 return false;
             }
 
-            function isUsedInFunctionOrNonStaticProperty(usage: Node, container?: Node): boolean {
+            function isUsedInFunctionOrInstanceProperty(usage: Node, isDeclarationInstanceProperty?: boolean, container?: Node): boolean {
                 let current = usage;
                 while (current) {
                     if (current === container) {
@@ -711,13 +713,13 @@ namespace ts {
                         return true;
                     }
 
-                    const initializerOfNonStaticProperty = current.parent &&
+                    const initializerOfInstanceProperty = current.parent &&
                         current.parent.kind === SyntaxKind.PropertyDeclaration &&
                         (getModifierFlags(current.parent) & ModifierFlags.Static) === 0 &&
                         (<PropertyDeclaration>current.parent).initializer === current;
 
-                    if (initializerOfNonStaticProperty) {
-                        return true;
+                    if (initializerOfInstanceProperty) {
+                        return !isDeclarationInstanceProperty;
                     }
 
                     current = current.parent;
@@ -986,10 +988,10 @@ namespace ts {
                 //              interface bar {}
                 //          }
                 //      const foo/*1*/: foo/*2*/.bar;
-                // The foo at /*1*/ and /*2*/ will share same symbol with two meaning
-                // block - scope variable and namespace module. However, only when we
+                // The foo at /*1*/ and /*2*/ will share same symbol with two meanings:
+                // block-scoped variable and namespace module. However, only when we
                 // try to resolve name in /*1*/ which is used in variable position,
-                // we want to check for block- scoped
+                // we want to check for block-scoped
                 if (meaning & SymbolFlags.BlockScopedVariable) {
                     const exportOrLocalSymbol = getExportSymbolOfValueSymbolIfExported(result);
                     if (exportOrLocalSymbol.flags & SymbolFlags.BlockScopedVariable) {
@@ -1013,7 +1015,7 @@ namespace ts {
                 return false;
             }
 
-            const container = getThisContainer(errorLocation, /* includeArrowFunctions */ true);
+            const container = getThisContainer(errorLocation, /*includeArrowFunctions*/ true);
             let location = container;
             while (location) {
                 if (isClassLike(location.parent)) {
@@ -12543,6 +12545,16 @@ namespace ts {
             }
         }
 
+        function isInPropertyInitializer(node: Node): boolean {
+            while (node) {
+                if (node.parent && node.parent.kind === SyntaxKind.PropertyDeclaration && (node.parent as PropertyDeclaration).initializer === node) {
+                    return true;
+                }
+                node = node.parent;
+            }
+            return false;
+        }
+
         function checkPropertyAccessExpressionOrQualifiedName(node: PropertyAccessExpression | QualifiedName, left: Expression | QualifiedName, right: Identifier) {
             const type = checkNonNullExpression(left);
             if (isTypeAny(type) || type === silentNeverType) {
@@ -12565,6 +12577,11 @@ namespace ts {
                 }
                 return unknownType;
             }
+            if (prop.valueDeclaration &&
+                isInPropertyInitializer(node) &&
+                !isBlockScopedNameDeclaredBeforeUse(prop.valueDeclaration, right)) {
+                error(right, Diagnostics.Block_scoped_variable_0_used_before_its_declaration, right.text);
+            }
 
             markPropertyAsReferenced(prop);