[Diagnostics] Detect and diagnose type mismatches related to function builder result

Generic requirement failures are already covered but general type
mismatches have to be handled separately.

Resolves: rdar://problem/65413640

Author: xedin

lib/Sema/CSDiagnostics.cpp

@@ -2186,6 +2186,11 @@ bool ContextualFailure::diagnoseAsError() {
     return true;
   }
 
+  case ConstraintLocator::FunctionBuilderBodyResult: {
+    diagnostic = *getDiagnosticFor(CTP_Initialization, toType);
+    break;
+  }
+
   default:
     return false;
   }

lib/Sema/CSSimplify.cpp

@@ -4420,6 +4420,12 @@ bool ConstraintSystem::repairFailures(
                           getConstraintLocator(anchor, path));
   }
 
+  case ConstraintLocator::FunctionBuilderBodyResult: {
+    conversionsOrFixes.push_back(ContextualMismatch::create(
+        *this, lhs, rhs, getConstraintLocator(locator)));
+    break;
+  }
+
   default:
     break;
   }

lib/Sema/ConstraintSystem.cpp

@@ -3853,6 +3853,11 @@ void constraints::simplifyLocator(ASTNode &anchor,
       break;
     }
 
+    case ConstraintLocator::FunctionBuilderBodyResult: {
+      path = path.slice(1);
+      break;
+    }
+
     default:
       // FIXME: Lots of other cases to handle.
       break;

test/Constraints/function_builder_diags.swift

@@ -613,4 +613,8 @@ struct MyView {
     case . // expected-error {{expected ':' after 'case'}}
     } // expected-error {{expected identifier after '.' expression}}
   }
+
+  @TupleBuilder var invalidConversion: Int { // expected-error {{cannot convert value of type 'String' to specified type 'Int'}}
+    ""
+  }
 }