Remove diag::type_of_expression_is_ambiguous

This is a diagnostic that is only really emitted as a fallback when
the constraint system isn't able to better diagnose the expression.
It's not particulary helpful for the user, and can be often be
misleading since the underlying issue might not actually be an
ambiguity, and the user may well already have a type annotation. Let's
instead just emit the fallback diagnostic that we emit in all other
cases, asking the user to file a bug.

Author: hamishknight

include/swift/AST/DiagnosticsSema.def

@@ -4749,15 +4749,10 @@ ERROR(could_not_infer_pack_element,none,
 NOTE(note_in_opening_pack_element,none,
       "in inferring pack element #%0 of '%1'", (unsigned,StringRef))
 
-      
-ERROR(type_of_expression_is_ambiguous,none,
-      "type of expression is ambiguous without a type annotation", ())
-
 ERROR(failed_to_produce_diagnostic,none,
       "failed to produce diagnostic for expression; "
       SWIFT_BUG_REPORT_MESSAGE, ())
 
-
 ERROR(missing_protocol,none,
       "missing protocol %0", (Identifier))
 ERROR(nil_literal_broken_proto,none,

lib/Sema/ConstraintSystem.cpp

@@ -4768,6 +4768,21 @@ void ConstraintSystem::diagnoseFailureFor(SyntacticElementTarget target) {
     return;
   }
 
+  if (auto *wrappedVar = target.getAsUninitializedWrappedVar()) {
+    auto *outerWrapper = wrappedVar->getOutermostAttachedPropertyWrapper();
+    Type propertyType = wrappedVar->getInterfaceType();
+    Type wrapperType = outerWrapper->getType();
+
+    // Emit the property wrapper fallback diagnostic
+    wrappedVar->diagnose(diag::property_wrapper_incompatible_property,
+                         propertyType, wrapperType);
+    if (auto nominal = wrapperType->getAnyNominal()) {
+      nominal->diagnose(diag::property_wrapper_declared_here,
+                        nominal->getName());
+    }
+    return;
+  }
+
   if (auto expr = target.getAsExpr()) {
     if (auto *assignment = dyn_cast<AssignExpr>(expr)) {
       if (isa<DiscardAssignmentExpr>(assignment->getDest()))
@@ -4785,33 +4800,12 @@ void ConstraintSystem::diagnoseFailureFor(SyntacticElementTarget target) {
         .highlight(closure->getSourceRange());
       return;
     }
-
-    // If no one could find a problem with this expression or constraint system,
-    // then it must be well-formed... but is ambiguous.  Handle this by
-    // diagnostic various cases that come up.
-    DE.diagnose(expr->getLoc(), diag::type_of_expression_is_ambiguous)
-        .highlight(expr->getSourceRange());
-  } else if (auto *wrappedVar = target.getAsUninitializedWrappedVar()) {
-    auto *outerWrapper = wrappedVar->getOutermostAttachedPropertyWrapper();
-    Type propertyType = wrappedVar->getInterfaceType();
-    Type wrapperType = outerWrapper->getType();
-
-    // Emit the property wrapper fallback diagnostic
-    wrappedVar->diagnose(diag::property_wrapper_incompatible_property,
-                         propertyType, wrapperType);
-    if (auto nominal = wrapperType->getAnyNominal()) {
-      nominal->diagnose(diag::property_wrapper_declared_here,
-                        nominal->getName());
-    }
-  } else if (target.getAsUninitializedVar()) {
-    DE.diagnose(target.getLoc(), diag::failed_to_produce_diagnostic);
-  } else if (target.isForEachPreamble()) {
-    DE.diagnose(target.getLoc(), diag::failed_to_produce_diagnostic);
-  } else {
-    // Emit a poor fallback message.
-    DE.diagnose(target.getAsFunction()->getLoc(),
-                diag::failed_to_produce_diagnostic);
   }
+
+  // Emit a poor fallback message.
+  auto diag = DE.diagnose(target.getLoc(), diag::failed_to_produce_diagnostic);
+  if (auto *expr = target.getAsExpr())
+    diag.highlight(expr->getSourceRange());
 }
 
 bool ConstraintSystem::isDeclUnavailable(const Decl *D,

test/Concurrency/sendable_keypaths.swift

@@ -224,7 +224,7 @@ do {
 
   // TODO(rdar://125948508): This shouldn't be ambiguous (@Sendable version should be preferred)
   func test() -> KeyPath<String, Int> {
-    true ? kp() : kp() // expected-error {{type of expression is ambiguous without a type annotation}}
+    true ? kp() : kp() // expected-error {{failed to produce diagnostic for expression}}
   }
 
   func forward<T>(_ v: T) -> T { v }
@@ -249,7 +249,7 @@ do {
 
   // TODO(rdar://125948508): This shouldn't be ambiguous (@Sendable version should be preferred)
   func fnRet(cond: Bool) -> () -> Void {
-    cond ? Test.fn : Test.otherFn // expected-error {{type of expression is ambiguous without a type annotation}}
+    cond ? Test.fn : Test.otherFn // expected-error {{failed to produce diagnostic for expression}}
   }
 
   func forward<T>(_: T) -> T {

test/Constraints/keypath.swift

@@ -290,7 +290,7 @@ func test_invalid_argument_to_keypath_subscript() {
   // The diagnostic should point out that `ambiguous` is indeed ambiguous and that `5` is not a valid argument
   // for a key path subscript.
   ambiguous {
-    // expected-error@-1 {{type of expression is ambiguous without a type annotation}}
+    // expected-error@-1 {{failed to produce diagnostic for expression}}
     $0[keyPath: 5]
   }
 

test/Constraints/operator.swift

@@ -285,7 +285,7 @@ func rdar60727310() {
 // FIXME: Bad diagnostic.
 func f_54877(_ e: Error) {
   func foo<T>(_ a: T, _ op: ((T, T) -> Bool)) {}
-  foo(e, ==) // expected-error {{type of expression is ambiguous without a type annotation}}
+  foo(e, ==) // expected-error {{failed to produce diagnostic for expression}}
 }
 
 // rdar://problem/62054241 - Swift compiler crashes when passing < as the sort function in sorted(by:) and the type of the array is not comparable

test/Constraints/parameterized_existentials.swift

@@ -8,7 +8,7 @@ protocol P<A> {
 
 func f1(x: any P) -> any P<Int> {
   // FIXME: Bad diagnostic
-  return x // expected-error {{type of expression is ambiguous without a type annotation}}
+  return x // expected-error {{failed to produce diagnostic for expression}}
 }
 
 func f2(x: any P<Int>) -> any P {
@@ -52,4 +52,4 @@ func h3(x: (any P<Int>)?) -> (any P<String>)? {
 
 func generic1<T>(x: any P<T>) -> T {
   return x.f()
-}
+}

test/Constraints/rdar85263844_swift6.swift

@@ -29,5 +29,5 @@ extension S4 where T == (outer: Int, y: Int) {
 
 public func rdar85263844_2(_ x: [Int]) -> S4<(outer: Int, y: Int)> {
   // FIXME: Bad error message.
-  S4(x.map { (inner: $0, y: $0) }) // expected-error {{type of expression is ambiguous without a type annotation}}
+  S4(x.map { (inner: $0, y: $0) }) // expected-error {{failed to produce diagnostic for expression}}
 }

test/Constraints/variadic_generic_constraints.swift

@@ -41,7 +41,7 @@ takesAnyObject()
 takesAnyObject(C(), C(), C())
 
 // FIXME: Bad diagnostic
-takesAnyObject(C(), S(), C())  // expected-error {{type of expression is ambiguous without a type annotation}}
+takesAnyObject(C(), S(), C())  // expected-error {{failed to produce diagnostic for expression}}
 
 // Same-type requirements
 

test/Interop/Cxx/foreign-reference/not-any-object.swift

@@ -22,5 +22,5 @@ import Test;
 public func test(_ _: AnyObject) {}
 
 // TODO: make this a better error.
-test(Empty.create()) // expected-error {{type of expression is ambiguous without a type annotation}}
+test(Empty.create()) // expected-error {{failed to produce diagnostic for expression}}
 test([Empty.create()][0]) // expected-error {{argument type 'Any' expected to be an instance of a class or class-constrained type}}

test/Interop/Cxx/namespace/import-as-member-typechecker.swift

@@ -21,4 +21,4 @@ func takesDeepNestedStruct(_ s: MyNS.MyDeepNS.DeepNestedStruct) {
 }
 
 MyNS.method() // expected-error {{type 'MyNS' has no member 'method'}}
-MyNS.nestedMethod() // expected-error {{type of expression is ambiguous without a type annotation}}
+MyNS.nestedMethod() // expected-error {{failed to produce diagnostic for expression}}