[CSDiagnostics] Use correct overload locator when requirement belongs to explicit init ref

Previously the logic to determine path to the selected overload
in such case was simplistic and only checked the name to be
of constructor, but `ConstructorMember` path is formed only if
member reference is a constructor delegation e.g. `self.init` or
`super.init` in an initializer context.

Author: xedin

lib/Sema/CSDiagnostics.cpp

@@ -134,6 +134,7 @@ ProtocolConformance *RequirementFailure::getConformanceForConditionalReq(
 
 ValueDecl *RequirementFailure::getDeclRef() const {
   auto &cs = getConstraintSystem();
+  auto &TC = getTypeChecker();
 
   auto *anchor = getRawAnchor();
   auto *locator = cs.getConstraintLocator(anchor);
@@ -156,7 +157,7 @@ ValueDecl *RequirementFailure::getDeclRef() const {
   } else if (auto *UDE = dyn_cast<UnresolvedDotExpr>(anchor)) {
     ConstraintLocatorBuilder member(locator);
 
-    if (UDE->getName().isSimpleName(DeclBaseName::createConstructor())) {
+    if (TC.getSelfForInitDelegationInConstructor(getDC(), UDE)) {
       member = member.withPathElement(PathEltKind::ConstructorMember);
     } else {
       member = member.withPathElement(PathEltKind::Member);

test/Constraints/generics.swift

@@ -677,3 +677,13 @@ protocol C {
 protocol D {
   associatedtype Foo where Foo: String // expected-error {{type 'Self.Foo' constrained to non-protocol, non-class type 'String'}}
 }
+
+func member_ref_with_explicit_init() {
+  struct S<T: P> { // expected-note {{where 'T' = 'Int'}}
+    init(_: T) {}
+    init(_: T, _ other: Int = 42) {}
+  }
+
+  _ = S.init(42)
+  // expected-error@-1 {{generic struct 'S' requires that 'Int' conform to 'P'}}
+}