[CSSimplify] Allow conversions between tuple element type and a placeholder

If a placeholder appears on one of the side of tuple element matching
conversion, consider that conversion to be solved, because the actual
error is related to the placeholder.

(cherry picked from commit 1c076f0)

Author: xedin

lib/Sema/CSSimplify.cpp

@@ -6014,6 +6014,12 @@ bool ConstraintSystem::repairFailures(
   }
 
   case ConstraintLocator::TupleElement: {
+    if (lhs->isPlaceholder() || rhs->isPlaceholder()) {
+      recordAnyTypeVarAsPotentialHole(lhs);
+      recordAnyTypeVarAsPotentialHole(rhs);
+      return true;
+    }
+
     if (isExpr<ArrayExpr>(anchor) || isExpr<DictionaryExpr>(anchor)) {
       // If we could record a generic arguments mismatch instead of this fix,
       // don't record a ContextualMismatch here.

test/Constraints/result_builder_diags.swift

@@ -907,3 +907,34 @@ func test_associated_values_dont_block_solver_when_unresolved() {
     }
   }
 }
+
+func test_dependent_member_with_unresolved_base_type() {
+  struct Wrapper<A, T: P> : P {
+  }
+
+  @resultBuilder
+  struct Builder<A> {
+    static func buildBlock(_ value: some P) -> some P {
+      value
+    }
+  }
+
+  func test<A, U: P>(data: KeyPath<A, [(Int, U.T)]>, // expected-note {{in call to function 'test(data:_:)'}}
+                     @Builder<U.T> _: () -> U) -> Wrapper<A, U> { fatalError() }
+
+  struct Value : P {
+    typealias T = (Int, String)
+  }
+
+  struct Test : P {
+    struct T {
+      var values: [(Int, Value.T)]
+    }
+
+    var v: some P {
+      test(data: \T.values) { // expected-error {{generic parameter 'U' could not be inferred}}
+        Value()
+      }
+    }
+  }
+}

test/expr/expressions.swift

@@ -748,8 +748,9 @@ func invalidDictionaryLiteral() {
 
 
 [4].joined(separator: [1])
-// expected-error@-1 {{cannot convert value of type 'Int' to expected element type 'String'}}
-// expected-error@-2 {{cannot convert value of type '[Int]' to expected argument type 'String'}}
+// expected-error@-1 {{no exact matches in call to instance method 'joined'}}
+// expected-note@-2 {{found candidate with type '(String) -> String'}}
+// There is one more note here - candidate requires that 'Int' conform to 'Sequence' (requirement specified as 'Self.Element' : 'Sequence') pointing to Sequence extension
 
 [4].joined(separator: [[[1]]])
 // expected-error@-1 {{cannot convert value of type 'Int' to expected element type 'String'}}