sema: require witness for _const decls to be also _const

Author: nkcsgexi

include/swift/AST/Decl.h

@@ -4583,6 +4583,7 @@ class AbstractStorageDecl : public ValueDecl {
   void setStatic(bool IsStatic) {
     Bits.AbstractStorageDecl.IsStatic = IsStatic;
   }
+  bool isCompileTimeConst() const;
 
   /// \returns the way 'static'/'class' should be spelled for this declaration.
   StaticSpellingKind getCorrectStaticSpelling() const;

include/swift/AST/DiagnosticsSema.def

@@ -2323,6 +2323,9 @@ WARNING(warn_protocol_witness_optionality,none,
 NOTE(protocol_witness_static_conflict,none,
      "candidate operates on %select{a type|an instance}0, not "
      "%select{an instance|a type}0 as required", (bool))
+NOTE(protocol_witness_const_conflict,none,
+     "candidate operates as %select{const|non-const}0, not "
+     "%select{non-const|const}0 as required", (bool))
 NOTE(protocol_witness_prefix_postfix_conflict,none,
      "candidate is %select{|prefix, |postfix, }1not "
      "%select{prefix|postfix}0 as required", (bool, unsigned))

lib/AST/Decl.cpp

@@ -763,6 +763,10 @@ void AbstractFunctionDecl::setSingleExpressionBody(Expr *NewBody) {
   getBody()->setLastElement(NewBody);
 }
 
+bool AbstractStorageDecl::isCompileTimeConst() const {
+  return getAttrs().hasAttribute<CompileTimeConstAttr>();
+}
+
 bool AbstractStorageDecl::isTransparent() const {
   return getAttrs().hasAttribute<TransparentAttr>();
 }

lib/Sema/TypeCheckProtocol.cpp

@@ -627,6 +627,11 @@ swift::matchWitness(
     if (reqASD->isStatic() != witnessASD->isStatic())
       return RequirementMatch(witness, MatchKind::StaticNonStaticConflict);
 
+    // Check that the compile-time constness matches.
+    if (reqASD->isCompileTimeConst() && !witnessASD->isCompileTimeConst()) {
+      return RequirementMatch(witness, MatchKind::CompileTimeConstConflict);
+    }
+
     // If the requirement is settable and the witness is not, reject it.
     if (reqASD->isSettable(req->getDeclContext()) &&
         !witnessASD->isSettable(witness->getDeclContext()))
@@ -2566,7 +2571,16 @@ diagnoseMatch(ModuleDecl *module, NormalProtocolConformance *conformance,
                          diag);
     break;
   }
-
+  case MatchKind::CompileTimeConstConflict: {
+    auto witness = match.Witness;
+    auto missing = !witness->getAttrs().getAttribute<CompileTimeConstAttr>();
+    auto diag = diags.diagnose(witness, diag::protocol_witness_const_conflict,
+                   missing);
+    if (missing) {
+      diag.fixItInsert(witness->getAttributeInsertionLoc(true), "_const");
+    }
+    break;
+  }
   case MatchKind::StaticNonStaticConflict: {
     auto witness = match.Witness;
     auto diag = diags.diagnose(witness, diag::protocol_witness_static_conflict,

lib/Sema/TypeCheckProtocol.h

@@ -282,6 +282,9 @@ enum class MatchKind : uint8_t {
   /// The witness did not match because it is an enum case with
   /// associated values.
   EnumCaseWithAssociatedValues,
+
+  /// The witness did not match due to _const/non-_const differences.
+  CompileTimeConstConflict,
 };
 
 /// Describes the kind of optional adjustment performed when
@@ -507,6 +510,7 @@ struct RequirementMatch {
     case MatchKind::TypeConflict:
     case MatchKind::MissingRequirement:
     case MatchKind::StaticNonStaticConflict:
+    case MatchKind::CompileTimeConstConflict:
     case MatchKind::SettableConflict:
     case MatchKind::PrefixNonPrefixConflict:
     case MatchKind::PostfixNonPostfixConflict:
@@ -542,6 +546,7 @@ struct RequirementMatch {
     case MatchKind::TypeConflict:
     case MatchKind::MissingRequirement:
     case MatchKind::StaticNonStaticConflict:
+    case MatchKind::CompileTimeConstConflict:
     case MatchKind::SettableConflict:
     case MatchKind::PrefixNonPrefixConflict:
     case MatchKind::PostfixNonPostfixConflict:
@@ -576,6 +581,7 @@ struct RequirementMatch {
     case MatchKind::Circularity:
     case MatchKind::KindConflict:
     case MatchKind::StaticNonStaticConflict:
+    case MatchKind::CompileTimeConstConflict:
     case MatchKind::SettableConflict:
     case MatchKind::PrefixNonPrefixConflict:
     case MatchKind::PostfixNonPostfixConflict:

test/Sema/const_pass_as_arguments.swift

@@ -31,3 +31,15 @@ func main_member(_ u: Utils, _ i: Int, _ d: Double, _ s: String) {
 	u.takeStringConst("\(d)") // expected-error {{expect a compile-time constant literal}}
 	u.takeStringConst(s) // expected-error {{expect a compile-time constant literal}}
 }
+
+protocol ConstFan {
+	static _const var v: String { get }  // expected-note {{protocol requires property 'v' with type 'String'; do you want to add a stub?}}
+}
+
+class ConstFanClass1: ConstFan { // expected-error {{type 'ConstFanClass1' does not conform to protocol 'ConstFan'}}
+	static var v: String = "" // expected-note {{candidate operates as non-const, not const as required}}
+}
+
+class ConstFanClassCorrect: ConstFan {
+	static _const var v: String = ""
+}