[CSDiagnostics] Diagnose attempts to specialize with invalid number of generic arguments

```swift
struct Test<T, U> {}
_ = Test<Int>() // error
```

Author: xedin

lib/Sema/CSDiagnostics.cpp

@@ -9167,3 +9167,9 @@ bool ConcreteTypeSpecialization::diagnoseAsError() {
   emitDiagnostic(diag::not_a_generic_type, ConcreteType);
   return true;
 }
+
+bool InvalidTypeSpecializationArity::diagnoseAsError() {
+  emitDiagnostic(diag::type_parameter_count_mismatch, D->getBaseIdentifier(),
+                 NumParams, NumArgs, NumArgs < NumParams, HasParameterPack);
+  return true;
+}

lib/Sema/CSDiagnostics.h

@@ -3069,6 +3069,30 @@ class ConcreteTypeSpecialization final : public FailureDiagnostic {
   bool diagnoseAsError() override;
 };
 
+/// Diagnose attempts to specialize with invalid number of generic arguments:
+///
+/// \code
+/// struct Test<T, U> {}
+///
+/// _ = Test<Int>() // error
+/// \endcode
+class InvalidTypeSpecializationArity final : public FailureDiagnostic {
+  ValueDecl *D;
+  unsigned NumParams;
+  unsigned NumArgs;
+  bool HasParameterPack;
+
+public:
+  InvalidTypeSpecializationArity(const Solution &solution, ValueDecl *decl,
+                                 unsigned numParams, unsigned numArgs,
+                                 bool hasParameterPack,
+                                 ConstraintLocator *locator)
+      : FailureDiagnostic(solution, locator), D(decl), NumParams(numParams),
+        NumArgs(numArgs), HasParameterPack(hasParameterPack) {}
+
+  bool diagnoseAsError() override;
+};
+
 } // end namespace constraints
 } // end namespace swift
 

lib/Sema/CSFix.cpp

@@ -2845,7 +2845,9 @@ AllowConcreteTypeSpecialization::create(ConstraintSystem &cs, Type concreteTy,
 
 bool IgnoreGenericSpecializationArityMismatch::diagnose(
     const Solution &solution, bool asNote) const {
-  return false;
+  InvalidTypeSpecializationArity failure(solution, D, NumParams, NumArgs,
+                                         HasParameterPack, getLocator());
+  return failure.diagnose(asNote);
 }
 
 IgnoreGenericSpecializationArityMismatch *