Fixed Testing

Author: Arthur Ozga

src/compiler/checker.ts

@@ -4763,26 +4763,32 @@ namespace ts {
                 let targetSignatures = getSignaturesOfType(target, kind);
                 let result = Ternary.True;
                 let saveErrorInfo = errorInfo;
-                
+
                 // Because the "abstractness" of a class is the same across all construct signatures
                 // (internally we are checking the corresponding declaration), it is enough to perform 
                 // the check and report an error once over all pairs of source and target construct signatures.
-                let sourceErasedSignature = getErasedSignature(s);
-                let targetErasedSignature = getErasedSignature(t);
+                let sourceSig = sourceSignatures[0];
+                // Note that in an extends-clause, targetSignatures is stripped, so the check never proceeds.
+                let targetSig = targetSignatures[0];
 
-                let sourceReturnType = sourceErasedSignature && getReturnTypeOfSignature(sourceErasedSignature);
-                let targetReturnType = targetErasedSignature && getReturnTypeOfSignature(targetErasedSignature);
+                if (sourceSig && targetSig) {
+                    let sourceErasedSignature = getErasedSignature(sourceSig);
+                    let targetErasedSignature = getErasedSignature(targetSig);
 
-                let sourceReturnDecl = sourceReturnType && sourceReturnType.symbol && getDeclarationOfKind(sourceReturnType.symbol, SyntaxKind.ClassDeclaration);
-                let targetReturnDecl = targetReturnType && targetReturnType.symbol && getDeclarationOfKind(targetReturnType.symbol, SyntaxKind.ClassDeclaration);
-                let sourceIsAbstract = sourceReturnDecl && sourceReturnDecl.flags & NodeFlags.Abstract;
-                let targetIsAbstract = targetReturnDecl && targetReturnDecl.flags & NodeFlags.Abstract;
+                    let sourceReturnType = sourceErasedSignature && getReturnTypeOfSignature(sourceErasedSignature);
+                    let targetReturnType = targetErasedSignature && getReturnTypeOfSignature(targetErasedSignature);
 
-                if (sourceIsAbstract && !targetIsAbstract) {
-                    if (reportErrors) {
-                        reportError(Diagnostics.Cannot_assign_an_abstract_constructor_type_to_a_non_abstract_constructor_type);
+                    let sourceReturnDecl = sourceReturnType && sourceReturnType.symbol && getDeclarationOfKind(sourceReturnType.symbol, SyntaxKind.ClassDeclaration);
+                    let targetReturnDecl = targetReturnType && targetReturnType.symbol && getDeclarationOfKind(targetReturnType.symbol, SyntaxKind.ClassDeclaration);
+                    let sourceIsAbstract = sourceReturnDecl && sourceReturnDecl.flags & NodeFlags.Abstract;
+                    let targetIsAbstract = targetReturnDecl && targetReturnDecl.flags & NodeFlags.Abstract;
+
+                    if (sourceIsAbstract && !targetIsAbstract) {
+                        if (reportErrors) {
+                            reportError(Diagnostics.Cannot_assign_an_abstract_constructor_type_to_a_non_abstract_constructor_type);
+                        }
+                        return Ternary.False;
                     }
-                    result = Ternary.False;
                 }
 
                 outer: for (let t of targetSignatures) {