Cherry-pick PR #33426 into release-3.6

Component commits:
4ae62c3 getConstraintDeclaration gets the first declaration with a constraint, rather than just the first declaration

b1ad54b Add type annotation

2232f5e Update comment

src/compiler/checker.ts

@@ -8940,9 +8940,8 @@ namespace ts {
             return undefined;
         }
 
-        function getConstraintDeclaration(type: TypeParameter) {
-            const decl = type.symbol && getDeclarationOfKind<TypeParameterDeclaration>(type.symbol, SyntaxKind.TypeParameter);
-            return decl && getEffectiveConstraintOfTypeParameter(decl);
+        function getConstraintDeclaration(type: TypeParameter): TypeNode | undefined {
+            return mapDefined(filter(type.symbol && type.symbol.declarations, isTypeParameterDeclaration), getEffectiveConstraintOfTypeParameter)[0];
         }
 
         function getInferredTypeParameterConstraint(typeParameter: TypeParameter) {
@@ -29185,11 +29184,10 @@ namespace ts {
                     const constraint = getEffectiveConstraintOfTypeParameter(source);
                     const sourceConstraint = constraint && getTypeFromTypeNode(constraint);
                     const targetConstraint = getConstraintOfTypeParameter(target);
-                    if (sourceConstraint) {
-                        // relax check if later interface augmentation has no constraint
-                        if (!targetConstraint || !isTypeIdenticalTo(sourceConstraint, targetConstraint)) {
-                            return false;
-                        }
+                    // relax check if later interface augmentation has no constraint, it's more broad and is OK to merge with
+                    // a more constrained interface (this could be generalized to a full heirarchy check, but that's maybe overkill)
+                    if (sourceConstraint && targetConstraint && !isTypeIdenticalTo(sourceConstraint, targetConstraint)) {
+                        return false;
                     }
 
                     // If the type parameter node has a default and it is not identical to the default

tests/baselines/reference/interfaceMergedUnconstrainedNoErrorIrrespectiveOfOrder.js

@@ -0,0 +1,28 @@
+//// [tests/cases/compiler/interfaceMergedUnconstrainedNoErrorIrrespectiveOfOrder.ts] ////
+
+//// [working.ts]
+// minmal samples from #33395
+export namespace ns {
+    interface Function<T extends (...args: any) => any> {
+        throttle(): Function<T>;
+    }
+    interface Function<T> {
+        unary(): Function<() => ReturnType<T>>;
+    }
+}
+//// [regression.ts]
+export namespace ns {
+    interface Function<T> {
+        unary(): Function<() => ReturnType<T>>;
+    }
+    interface Function<T extends (...args: any) => any> {
+        throttle(): Function<T>;
+    }
+}
+
+//// [working.js]
+"use strict";
+exports.__esModule = true;
+//// [regression.js]
+"use strict";
+exports.__esModule = true;

tests/baselines/reference/interfaceMergedUnconstrainedNoErrorIrrespectiveOfOrder.symbols

@@ -0,0 +1,51 @@
+=== tests/cases/compiler/working.ts ===
+// minmal samples from #33395
+export namespace ns {
+>ns : Symbol(ns, Decl(working.ts, 0, 0))
+
+    interface Function<T extends (...args: any) => any> {
+>Function : Symbol(Function, Decl(working.ts, 1, 21), Decl(working.ts, 4, 5))
+>T : Symbol(T, Decl(working.ts, 2, 23), Decl(working.ts, 5, 23))
+>args : Symbol(args, Decl(working.ts, 2, 34))
+
+        throttle(): Function<T>;
+>throttle : Symbol(Function.throttle, Decl(working.ts, 2, 57))
+>Function : Symbol(Function, Decl(working.ts, 1, 21), Decl(working.ts, 4, 5))
+>T : Symbol(T, Decl(working.ts, 2, 23), Decl(working.ts, 5, 23))
+    }
+    interface Function<T> {
+>Function : Symbol(Function, Decl(working.ts, 1, 21), Decl(working.ts, 4, 5))
+>T : Symbol(T, Decl(working.ts, 2, 23), Decl(working.ts, 5, 23))
+
+        unary(): Function<() => ReturnType<T>>;
+>unary : Symbol(Function.unary, Decl(working.ts, 5, 27))
+>Function : Symbol(Function, Decl(working.ts, 1, 21), Decl(working.ts, 4, 5))
+>ReturnType : Symbol(ReturnType, Decl(lib.es5.d.ts, --, --))
+>T : Symbol(T, Decl(working.ts, 2, 23), Decl(working.ts, 5, 23))
+    }
+}
+=== tests/cases/compiler/regression.ts ===
+export namespace ns {
+>ns : Symbol(ns, Decl(regression.ts, 0, 0))
+
+    interface Function<T> {
+>Function : Symbol(Function, Decl(regression.ts, 0, 21), Decl(regression.ts, 3, 5))
+>T : Symbol(T, Decl(regression.ts, 1, 23), Decl(regression.ts, 4, 23))
+
+        unary(): Function<() => ReturnType<T>>;
+>unary : Symbol(Function.unary, Decl(regression.ts, 1, 27))
+>Function : Symbol(Function, Decl(regression.ts, 0, 21), Decl(regression.ts, 3, 5))
+>ReturnType : Symbol(ReturnType, Decl(lib.es5.d.ts, --, --))
+>T : Symbol(T, Decl(regression.ts, 1, 23), Decl(regression.ts, 4, 23))
+    }
+    interface Function<T extends (...args: any) => any> {
+>Function : Symbol(Function, Decl(regression.ts, 0, 21), Decl(regression.ts, 3, 5))
+>T : Symbol(T, Decl(regression.ts, 1, 23), Decl(regression.ts, 4, 23))
+>args : Symbol(args, Decl(regression.ts, 4, 34))
+
+        throttle(): Function<T>;
+>throttle : Symbol(Function.throttle, Decl(regression.ts, 4, 57))
+>Function : Symbol(Function, Decl(regression.ts, 0, 21), Decl(regression.ts, 3, 5))
+>T : Symbol(T, Decl(regression.ts, 1, 23), Decl(regression.ts, 4, 23))
+    }
+}

tests/baselines/reference/interfaceMergedUnconstrainedNoErrorIrrespectiveOfOrder.types

@@ -0,0 +1,27 @@
+=== tests/cases/compiler/working.ts ===
+// minmal samples from #33395
+export namespace ns {
+    interface Function<T extends (...args: any) => any> {
+>args : any
+
+        throttle(): Function<T>;
+>throttle : () => Function<T>
+    }
+    interface Function<T> {
+        unary(): Function<() => ReturnType<T>>;
+>unary : () => Function<() => ReturnType<T>>
+    }
+}
+=== tests/cases/compiler/regression.ts ===
+export namespace ns {
+    interface Function<T> {
+        unary(): Function<() => ReturnType<T>>;
+>unary : () => Function<() => ReturnType<T>>
+    }
+    interface Function<T extends (...args: any) => any> {
+>args : any
+
+        throttle(): Function<T>;
+>throttle : () => Function<T>
+    }
+}

tests/baselines/reference/twoGenericInterfacesWithDifferentConstraints.errors.txt

@@ -4,11 +4,9 @@ tests/cases/conformance/interfaces/declarationMerging/twoGenericInterfacesWithDi
 tests/cases/conformance/interfaces/declarationMerging/twoGenericInterfacesWithDifferentConstraints.ts(14,15): error TS2428: All declarations of 'B' must have identical type parameters.
 tests/cases/conformance/interfaces/declarationMerging/twoGenericInterfacesWithDifferentConstraints.ts(32,22): error TS2428: All declarations of 'A' must have identical type parameters.
 tests/cases/conformance/interfaces/declarationMerging/twoGenericInterfacesWithDifferentConstraints.ts(38,22): error TS2428: All declarations of 'A' must have identical type parameters.
-tests/cases/conformance/interfaces/declarationMerging/twoGenericInterfacesWithDifferentConstraints.ts(53,11): error TS2428: All declarations of 'C' must have identical type parameters.
-tests/cases/conformance/interfaces/declarationMerging/twoGenericInterfacesWithDifferentConstraints.ts(57,11): error TS2428: All declarations of 'C' must have identical type parameters.
 
 
-==== tests/cases/conformance/interfaces/declarationMerging/twoGenericInterfacesWithDifferentConstraints.ts (8 errors) ====
+==== tests/cases/conformance/interfaces/declarationMerging/twoGenericInterfacesWithDifferentConstraints.ts (6 errors) ====
     interface A<T extends Date> {
               ~
 !!! error TS2428: All declarations of 'A' must have identical type parameters.
@@ -74,14 +72,10 @@ tests/cases/conformance/interfaces/declarationMerging/twoGenericInterfacesWithDi
     }
 
     interface C<T> {
-              ~
-!!! error TS2428: All declarations of 'C' must have identical type parameters.
       x: T;
     }
 
-    interface C<T extends number> { // error
-              ~
-!!! error TS2428: All declarations of 'C' must have identical type parameters.
+    interface C<T extends number> { // ok
       y: T;
     }
 

tests/baselines/reference/twoGenericInterfacesWithDifferentConstraints.js

@@ -55,7 +55,7 @@ interface C<T> {
   x: T;
 }
 
-interface C<T extends number> { // error
+interface C<T extends number> { // ok
   y: T;
 }
 

tests/baselines/reference/twoGenericInterfacesWithDifferentConstraints.symbols

@@ -137,7 +137,7 @@ interface C<T> {
 >T : Symbol(T, Decl(twoGenericInterfacesWithDifferentConstraints.ts, 52, 12), Decl(twoGenericInterfacesWithDifferentConstraints.ts, 56, 12))
 }
 
-interface C<T extends number> { // error
+interface C<T extends number> { // ok
 >C : Symbol(C, Decl(twoGenericInterfacesWithDifferentConstraints.ts, 50, 1), Decl(twoGenericInterfacesWithDifferentConstraints.ts, 54, 1))
 >T : Symbol(T, Decl(twoGenericInterfacesWithDifferentConstraints.ts, 52, 12), Decl(twoGenericInterfacesWithDifferentConstraints.ts, 56, 12))
 

tests/baselines/reference/twoGenericInterfacesWithDifferentConstraints.types

@@ -70,7 +70,7 @@ interface C<T> {
 >x : T
 }
 
-interface C<T extends number> { // error
+interface C<T extends number> { // ok
   y: T;
 >y : T
 }

tests/cases/compiler/interfaceMergedUnconstrainedNoErrorIrrespectiveOfOrder.ts

@@ -0,0 +1,19 @@
+// @filename: working.ts
+// minmal samples from #33395
+export namespace ns {
+    interface Function<T extends (...args: any) => any> {
+        throttle(): Function<T>;
+    }
+    interface Function<T> {
+        unary(): Function<() => ReturnType<T>>;
+    }
+}
+// @filename: regression.ts
+export namespace ns {
+    interface Function<T> {
+        unary(): Function<() => ReturnType<T>>;
+    }
+    interface Function<T extends (...args: any) => any> {
+        throttle(): Function<T>;
+    }
+}

tests/cases/conformance/interfaces/declarationMerging/twoGenericInterfacesWithDifferentConstraints.ts

@@ -54,7 +54,7 @@ interface C<T> {
   x: T;
 }
 
-interface C<T extends number> { // error
+interface C<T extends number> { // ok
   y: T;
 }