[ConstraintSystem] Allow fixing missing conformance failures for

`Void` and uninhabited types.

Author: hborla

lib/Sema/CSSimplify.cpp

@@ -3971,11 +3971,6 @@ ConstraintSystem::SolutionKind ConstraintSystem::simplifyConformsToConstraint(
       return recordFix(fix) ? SolutionKind::Error : SolutionKind::Solved;
     }
 
-    // Let's not try to fix missing conformance for Void
-    // or Never because that doesn't really make sense.
-    if (type->isVoid() || type->isUninhabited())
-      return SolutionKind::Error;
-
     if (path.back().is<LocatorPathElt::AnyRequirement>()) {
       // If this is a requirement associated with `Self` which is bound
       // to `Any`, let's consider this "too incorrect" to continue.

test/Constraints/rdar44770297.swift

@@ -4,8 +4,8 @@ protocol P {
   associatedtype A
 }
 
-func foo<T: P>(_: () throws -> T) -> T.A? { // expected-note {{in call to function 'foo'}}
+func foo<T: P>(_: () throws -> T) -> T.A? { // expected-note {{where 'T' = 'Never'}}
   fatalError()
 }
 
-let _ = foo() {fatalError()} & nil // expected-error {{generic parameter 'T' could not be inferred}}
+let _ = foo() {fatalError()} & nil // expected-error {{global function 'foo' requires that 'Never' conform to 'P'}}

test/type/opaque.swift

@@ -172,8 +172,9 @@ func recursion(x: Int) -> some P {
   return recursion(x: x - 1)
 }
 
-// FIXME: We need to emit a better diagnostic than the failure to convert Never to opaque.
-func noReturnStmts() -> some P { fatalError() } // expected-error{{cannot convert return expression of type 'Never' to return type 'some P'}} expected-error{{no return statements}}
+func noReturnStmts() -> some P { // expected-note {{opaque return type declared here}}
+    fatalError() // expected-error{{return type of global function 'noReturnStmts()' requires that 'Never' conform to 'P'}}
+}
 
 func mismatchedReturnTypes(_ x: Bool, _ y: Int, _ z: String) -> some P { // expected-error{{do not have matching underlying types}}
   if x {