[CSDiangostics] Adjust phrasing of invalid specialization diagnostic

Instead of "a generic function" which is too abstract and not always
correct, let's actually point to a kind of the declaration being referenced.

Author: xedin

include/swift/AST/DiagnosticsSema.def

@@ -4607,7 +4607,7 @@ NOTE(duplicated_key_declared_here, none,
 
 // Generic specializations
 ERROR(cannot_explicitly_specialize_function,none,
-      "cannot explicitly specialize a%select{| generic}0 function", (bool))
+      "cannot explicitly specialize %kind0", (ValueDecl *))
 ERROR(not_a_generic_type,none,
       "cannot specialize non-generic type %0", (Type))
 ERROR(not_a_generic_macro,none,

lib/Sema/CSDiagnostics.cpp

@@ -9366,8 +9366,7 @@ bool ConcreteTypeSpecialization::diagnoseAsError() {
 }
 
 bool InvalidFunctionSpecialization::diagnoseAsError() {
-  emitDiagnostic(diag::cannot_explicitly_specialize_function,
-                 bool(Decl->getAsGenericContext()));
+  emitDiagnostic(diag::cannot_explicitly_specialize_function, Decl);
   emitDiagnosticAt(Decl, diag::decl_declared_here, Decl);
   return true;
 }

test/Parse/generic_disambiguation.swift

@@ -42,8 +42,8 @@ _ = a>(b)
 _ = a > (b)
 
 generic<Int>(0)
-// expected-swift5-warning@-1{{cannot explicitly specialize a generic function}}
-// expected-swift6-error@-2 {{cannot explicitly specialize a generic function}}
+// expected-swift5-warning@-1{{cannot explicitly specialize global function 'generic'}}
+// expected-swift6-error@-2 {{cannot explicitly specialize global function 'generic'}}
 
 A<B>.c()
 A<A<B>>.c()

test/Sema/generic_specialization.swift

@@ -16,21 +16,21 @@ extension Int {
 // https://github.com/swiftlang/swift/issues/74857
 func test(i: Int) {
   let _ = i.foo<Int>()
-  // expected-swift5-warning@-1 {{cannot explicitly specialize a generic function}}
-  // expected-swift6-error@-2 {{cannot explicitly specialize a generic function}}
+  // expected-swift5-warning@-1 {{cannot explicitly specialize instance method 'foo()'}}
+  // expected-swift6-error@-2 {{cannot explicitly specialize instance method 'foo()'}}
 
   let _ = i.gen<Int>()
-  // expected-swift5-warning@-1 {{cannot explicitly specialize a generic function}}
-  // expected-swift6-error@-2 {{cannot explicitly specialize a generic function}}
+  // expected-swift5-warning@-1 {{cannot explicitly specialize instance method 'gen()'}}
+  // expected-swift6-error@-2 {{cannot explicitly specialize instance method 'gen()'}}
   // expected-error@-3 {{generic parameter 'T' could not be inferred}}
 
   let _ = 0.foo<Int>()
-  // expected-swift5-warning@-1 {{cannot explicitly specialize a generic function}}
-  // expected-swift6-error@-2 {{cannot explicitly specialize a generic function}}
+  // expected-swift5-warning@-1 {{cannot explicitly specialize instance method 'foo()'}}
+  // expected-swift6-error@-2 {{cannot explicitly specialize instance method 'foo()'}}
 
   let _ = i.gen<Int>
-  // expected-swift5-warning@-1 {{cannot explicitly specialize a generic function}}
-  // expected-swift6-error@-2 {{cannot explicitly specialize a generic function}}
+  // expected-swift5-warning@-1 {{cannot explicitly specialize instance method 'gen()'}}
+  // expected-swift6-error@-2 {{cannot explicitly specialize instance method 'gen()'}}
   // expected-error@-3 {{generic parameter 'T' could not be inferred}}
   let _ = i.bar<Int>
   // expected-swift5-error@-1 {{cannot specialize non-generic type 'Int'}}
@@ -45,8 +45,8 @@ extension Bool {
 }
 
 let _: () -> Bool = false.foo<Int>
-// expected-swift5-warning@-1 {{cannot explicitly specialize a generic function}}
-// expected-swift6-error@-2 {{cannot explicitly specialize a generic function}}
+// expected-swift5-warning@-1 {{cannot explicitly specialize instance method 'foo()'}}
+// expected-swift6-error@-2 {{cannot explicitly specialize instance method 'foo()'}}
 
 func foo(_ x: Int) {
   _ = {
@@ -62,6 +62,6 @@ do {
   }
 
   _ = Test.init<Int>({ (_: Int) -> Void in })
-  // expected-swift5-warning@-1 {{cannot explicitly specialize a generic function}}
-  // expected-swift6-error@-2 {{cannot explicitly specialize a generic function}}
+  // expected-swift5-warning@-1 {{cannot explicitly specialize initializer 'init(_:)'}}
+  // expected-swift6-error@-2 {{cannot explicitly specialize initializer 'init(_:)'}}
 }