Don't ignore index signatures in this type constraints

Author: ahejlsberg

src/compiler/checker.ts

@@ -3563,7 +3563,7 @@ namespace ts {
                     context.approximateLength += 6;
                     return createKeywordTypeNode(SyntaxKind.ObjectKeyword);
                 }
-                if (type.flags & TypeFlags.TypeParameter && (type as TypeParameter).isThisType) {
+                if (isThisTypeParameter(type)) {
                     if (context.flags & NodeBuilderFlags.InObjectTypeLiteral) {
                         if (!context.encounteredError && !(context.flags & NodeBuilderFlags.AllowThisInObjectLiteral)) {
                             context.encounteredError = true;
@@ -10193,6 +10193,10 @@ namespace ts {
             return maybeTypeOfKind(type, TypeFlags.InstantiableNonPrimitive | TypeFlags.Index);
         }
 
+        function isThisTypeParameter(type: Type): boolean {
+            return !!(type.flags & TypeFlags.TypeParameter && (<TypeParameter>type).isThisType);
+        }
+
         function getSimplifiedType(type: Type, writing: boolean): Type {
             return type.flags & TypeFlags.IndexedAccess ? getSimplifiedIndexedAccessType(<IndexedAccessType>type, writing) :
                 type.flags & TypeFlags.Conditional ? getSimplifiedConditionalType(<ConditionalType>type, writing) :
@@ -16772,7 +16776,7 @@ namespace ts {
             }
 
             function narrowByInKeyword(type: Type, literal: LiteralExpression, assumeTrue: boolean) {
-                if ((type.flags & (TypeFlags.Union | TypeFlags.Object)) || (type.flags & TypeFlags.TypeParameter && (type as TypeParameter).isThisType)) {
+                if (type.flags & (TypeFlags.Union | TypeFlags.Object) || isThisTypeParameter(type)) {
                     const propName = escapeLeadingUnderscores(literal.text);
                     return filterType(type, t => isTypePresencePossible(t, propName, assumeTrue));
                 }
@@ -20092,7 +20096,7 @@ namespace ts {
                         return anyType;
                     }
                     if (right.escapedText && !checkAndReportErrorForExtendingInterface(node)) {
-                        reportNonexistentProperty(right, leftType.flags & TypeFlags.TypeParameter && (leftType as TypeParameter).isThisType ? apparentType : leftType);
+                        reportNonexistentProperty(right, isThisTypeParameter(leftType) ? apparentType : leftType);
                     }
                     return errorType;
                 }
@@ -20496,7 +20500,7 @@ namespace ts {
 
             const effectiveIndexType = isForInVariableForNumericPropertyNames(indexExpression) ? numberType : indexType;
             const accessFlags = isAssignmentTarget(node) ?
-                AccessFlags.Writing | (isGenericObjectType(objectType) ? AccessFlags.NoIndexSignatures : 0) :
+                AccessFlags.Writing | (isGenericObjectType(objectType) && !isThisTypeParameter(objectType) ? AccessFlags.NoIndexSignatures : 0) :
                 AccessFlags.None;
             const indexedAccessType = getIndexedAccessTypeOrUndefined(objectType, effectiveIndexType, node, accessFlags) || errorType;
             return checkIndexedAccessIndexType(indexedAccessType, node);