Don't allow overloading by opaque types with different constraints.

The mangling unfortunately doesn't take the constraints into account, leading to
SR-10725 | rdar://problem/50987172. Changing the mangling at this point is hazardous, so it's probably best
to disallow this for now.

Author: jckarter

lib/AST/Decl.cpp

@@ -2369,7 +2369,7 @@ static Type mapSignatureFunctionType(ASTContext &ctx, Type type,
     // Functions and subscripts cannot overload differing only in opaque return
     // types. Replace the opaque type with `Any`.
     if (auto opaque = type->getAs<OpaqueTypeArchetypeType>()) {
-      type = opaque->getExistentialType();
+      type = ProtocolCompositionType::get(ctx, {}, /*hasAnyObject*/ false);
     }
 
     return mapSignatureParamType(ctx, type);

test/type/opaque.swift

@@ -272,30 +272,31 @@ func associatedTypeIdentity() {
   sameType(gary(doug()).r_out(), gary(candace()).r_out()) // expected-error{{}}
 }
 
-func redeclaration() -> some P { return 0 } // expected-note{{previously declared}}
+func redeclaration() -> some P { return 0 } // expected-note 2{{previously declared}}
 func redeclaration() -> some P { return 0 } // expected-error{{redeclaration}}
-func redeclaration() -> some Q { return 0 }
+func redeclaration() -> some Q { return 0 } // expected-error{{redeclaration}}
 func redeclaration() -> P { return 0 }
+func redeclaration() -> Any { return 0 }
 
 var redeclaredProp: some P { return 0 } // expected-note 3{{previously declared}}
 var redeclaredProp: some P { return 0 } // expected-error{{redeclaration}}
 var redeclaredProp: some Q { return 0 } // expected-error{{redeclaration}}
 var redeclaredProp: P { return 0 } // expected-error{{redeclaration}}
 
 struct RedeclarationTest {
-  func redeclaration() -> some P { return 0 } // expected-note{{previously declared}}
+  func redeclaration() -> some P { return 0 } // expected-note 2{{previously declared}}
   func redeclaration() -> some P { return 0 } // expected-error{{redeclaration}}
-  func redeclaration() -> some Q { return 0 }
+  func redeclaration() -> some Q { return 0 } // expected-error{{redeclaration}}
   func redeclaration() -> P { return 0 }
 
   var redeclaredProp: some P { return 0 } // expected-note 3{{previously declared}}
   var redeclaredProp: some P { return 0 } // expected-error{{redeclaration}}
   var redeclaredProp: some Q { return 0 } // expected-error{{redeclaration}}
   var redeclaredProp: P { return 0 } // expected-error{{redeclaration}}
 
-  subscript(redeclared _: Int) -> some P { return 0 } // expected-note{{previously declared}}
+  subscript(redeclared _: Int) -> some P { return 0 } // expected-note 2{{previously declared}}
   subscript(redeclared _: Int) -> some P { return 0 } // expected-error{{redeclaration}}
-  subscript(redeclared _: Int) -> some Q { return 0 }
+  subscript(redeclared _: Int) -> some Q { return 0 } // expected-error{{redeclaration}}
   subscript(redeclared _: Int) -> P { return 0 }
 }