Type parameters as constraints

src/compiler/declarationEmitter.ts

@@ -1309,6 +1309,9 @@ namespace ts {
         }
 
         function emitSignatureDeclaration(node: SignatureDeclaration) {
+            const prevEnclosingDeclaration = enclosingDeclaration;
+            enclosingDeclaration = node;
+
             // Construct signature or constructor type write new Signature
             if (node.kind === SyntaxKind.ConstructSignature || node.kind === SyntaxKind.ConstructorType) {
                 write("new ");
@@ -1321,9 +1324,6 @@ namespace ts {
                 write("(");
             }
 
-            const prevEnclosingDeclaration = enclosingDeclaration;
-            enclosingDeclaration = node;
-
             // Parameters
             emitCommaList(node.parameters, emitParameterDeclaration);
 

src/compiler/diagnosticMessages.json

@@ -864,7 +864,7 @@
         "category": "Error",
         "code": 2312
     },
-    "Constraint of a type parameter cannot reference any type parameter from the same type parameter list.": {
+    "Type parameter '{0}' has a circular constraint.": {
         "category": "Error",
         "code": 2313
     },

src/compiler/types.ts

@@ -2045,7 +2045,6 @@ namespace ts {
         resolvedSymbol?: Symbol;          // Cached name resolution result
         flags?: NodeCheckFlags;           // Set of flags specific to Node
         enumMemberValue?: number;         // Constant value of enum member
-        isIllegalTypeReferenceInConstraint?: boolean; // Is type reference in constraint refers to the type parameter from the same list
         isVisible?: boolean;              // Is this node visible
         generatedName?: string;           // Generated name for module, enum, or import declaration
         generatedNames?: Map<string>;     // Generated names table for source file
@@ -2286,6 +2285,7 @@ namespace ts {
         inferUnionTypes: boolean;           // Infer union types for disjoint candidates (otherwise undefinedType)
         inferences: TypeInferences[];       // Inferences made for each type parameter
         inferredTypes: Type[];              // Inferred type for each type parameter
+        mapper?: TypeMapper;                // Type mapper for this inference context
         failedTypeParameterIndex?: number;  // Index of type parameter for which inference failed
         // It is optional because in contextual signature instantiation, nothing fails
     }