[Diagnostics] When replacing an existential parameter type with a type parameter,

insert 'some' instead of an explicit type parameter.

Replacing 'any' with 'some' allows the code to compile without further
changes, such as naming an explicit type parameter, and is future-proofed
for same-type requirements on primary associated types instead of needing
additional logic to add a 'where' clause.

Author: hborla

lib/Sema/CSDiagnostics.cpp

@@ -3972,8 +3972,11 @@ bool InvalidMemberRefOnExistential::diagnoseAsError() {
 
   // If the base expression is a reference to a function or subscript
   // parameter, offer a fixit that replaces the existential parameter type with
-  // its generic equivalent, e.g. func foo(p: any P) → func foo<T: P>(p: T).
-  // FIXME: Add an option to use 'some' vs. an explicit generic parameter.
+  // its generic equivalent, e.g. func foo(p: any P) → func foo(p: some P).
+  // Replacing 'any' with 'some' allows the code to compile without further
+  // changes, such as naming an explicit type parameter, and is future-proofed
+  // for same-type requirements on primary associated types instead of needing
+  // a where clause.
 
   if (!PD || !PD->getDeclContext()->getAsDecl())
     return true;
@@ -4022,70 +4025,42 @@ bool InvalidMemberRefOnExistential::diagnoseAsError() {
   if (PD->isInOut())
     return true;
 
-  constexpr StringRef GPNamePlaceholder = "<#generic parameter name#>";
-  SourceRange TyReplacementRange;
-  SourceRange RemoveAnyRange;
-  SourceLoc GPDeclLoc;
-  std::string GPDeclStr;
-  {
-    llvm::raw_string_ostream OS(GPDeclStr);
-    auto *const GC = PD->getDeclContext()->getAsDecl()->getAsGenericContext();
-    if (GC->getParsedGenericParams()) {
-      GPDeclLoc = GC->getParsedGenericParams()->getRAngleLoc();
-      OS << ", ";
-    } else {
-      GPDeclLoc =
-          isa<AbstractFunctionDecl>(GC)
-              ? cast<AbstractFunctionDecl>(GC)->getParameters()->getLParenLoc()
-              : cast<SubscriptDecl>(GC)->getIndices()->getLParenLoc();
-      OS << "<";
-    }
-    OS << GPNamePlaceholder << ": ";
-
-    auto *TR = PD->getTypeRepr()->getWithoutParens();
-    if (auto *STR = dyn_cast<SpecifierTypeRepr>(TR)) {
-      TR = STR->getBase()->getWithoutParens();
-    }
-    if (auto *ETR = dyn_cast<ExistentialTypeRepr>(TR)) {
-      TR = ETR->getConstraint();
-      RemoveAnyRange = SourceRange(ETR->getAnyLoc(), TR->getStartLoc());
-      TR = TR->getWithoutParens();
-    }
-    if (auto *MTR = dyn_cast<MetatypeTypeRepr>(TR)) {
-      TR = MTR->getBase();
-
-      // (P & Q).Type -> T.Type
-      // (P).Type -> (T).Type
-      // ((P & Q)).Type -> ((T)).Type
-      if (auto *TTR = dyn_cast<TupleTypeRepr>(TR)) {
-        assert(TTR->isParenType());
-        if (!isa<CompositionTypeRepr>(TTR->getElementType(0))) {
-          TR = TR->getWithoutParens();
-        }
-      }
-    }
-    TyReplacementRange = TR->getSourceRange();
-
-    // Strip any remaining parentheses and print the conformance constraint.
-    TR->getWithoutParens()->print(OS);
-
-    if (!GC->getParsedGenericParams()) {
-      OS << ">";
-    }
+  auto *typeRepr = PD->getTypeRepr()->getWithoutParens();
+  if (auto *STR = dyn_cast<SpecifierTypeRepr>(typeRepr)) {
+    typeRepr = STR->getBase()->getWithoutParens();
   }
 
-  // First, replace the constraint type with the generic parameter type
-  // placeholder.
-  Diag.fixItReplace(TyReplacementRange, GPNamePlaceholder);
+  SourceRange anyRange;
+  TypeRepr *constraintRepr = typeRepr;
+  if (auto *existentialRepr = dyn_cast<ExistentialTypeRepr>(typeRepr)) {
+    constraintRepr = existentialRepr->getConstraint()->getWithoutParens();
+    auto anyStart = existentialRepr->getAnyLoc();
+    auto anyEnd = existentialRepr->getConstraint()->getStartLoc();
+    anyRange = SourceRange(anyStart, anyEnd);
+  }
 
-  // Remove 'any' if needed, using a character-based removal to pick up
-  // whitespaces between it and its constraint repr.
-  if (RemoveAnyRange.isValid()) {
-    Diag.fixItRemoveChars(RemoveAnyRange.Start, RemoveAnyRange.End);
+  bool needsParens = false;
+  while (auto *metatype = dyn_cast<MetatypeTypeRepr>(constraintRepr)) {
+    // The generic equivalent of 'any P.Type' is '(some P).Type'
+    constraintRepr = metatype->getBase()->getWithoutParens();
+    if (isa<SimpleIdentTypeRepr>(constraintRepr))
+      needsParens = !isa<TupleTypeRepr>(metatype->getBase());
   }
 
-  // Finally, insert the generic parameter declaration.
-  Diag.fixItInsert(GPDeclLoc, GPDeclStr);
+  std::string fix;
+  llvm::raw_string_ostream OS(fix);
+  if (needsParens)
+    OS << "(";
+  OS << "some ";
+  constraintRepr->print(OS);
+  if (needsParens)
+    OS << ")";
+
+  // When removing 'any', use a character-based removal to pick up
+  // whitespaces between it and its constraint repr.
+  Diag
+    .fixItReplace(constraintRepr->getSourceRange(), fix)
+    .fixItRemoveChars(anyRange.Start, anyRange.End);
 
   return true;
 }