[error] Improve diagnostic for concatenation to a string reference.

Fix #225

Author: pfusik

Sema.fu

@@ -1144,6 +1144,8 @@ public class FuSema
 			CheckLValue(left);
 			if (left.Type.Id == FuId.StringStorageType)
 				Coerce(right, this.Host.Program.System.StringPtrType);
+			else if (left.Type.Id == FuId.StringPtrType)
+				ReportError(left, $"'{left}' is 'string', concatenation requires 'string()'");
 			else {
 				Coerce(left, this.Host.Program.System.DoubleType);
 				Coerce(right, left.Type);

libfut.cpp

@@ -5715,6 +5715,8 @@ std::shared_ptr<FuExpr> FuSema::visitBinaryExpr(std::shared_ptr<FuBinaryExpr> ex
 		checkLValue(left.get());
 		if (left->type->id == FuId::stringStorageType)
 			coerce(right.get(), this->host->program->system->stringPtrType.get());
+		else if (left->type->id == FuId::stringPtrType)
+			reportError(left.get(), std::format("'{}' is 'string', concatenation requires 'string()'", left->toString()));
 		else {
 			coerce(left.get(), this->host->program->system->doubleType.get());
 			coerce(right.get(), left->type.get());

libfut.cs

@@ -5848,6 +5848,8 @@ FuExpr VisitBinaryExpr(FuBinaryExpr expr)
 				CheckLValue(left);
 				if (left.Type.Id == FuId.StringStorageType)
 					Coerce(right, this.Host.Program.System.StringPtrType);
+				else if (left.Type.Id == FuId.StringPtrType)
+					ReportError(left, $"'{left}' is 'string', concatenation requires 'string()'");
 				else {
 					Coerce(left, this.Host.Program.System.DoubleType);
 					Coerce(right, left.Type);

libfut.js

@@ -6172,6 +6172,8 @@ export class FuSema
 			this.#checkLValue(left);
 			if (left.type.id == FuId.STRING_STORAGE_TYPE)
 				this.#coerce(right, this.#host.program.system.stringPtrType);
+			else if (left.type.id == FuId.STRING_PTR_TYPE)
+				this.#reportError(left, `'${left}' is 'string', concatenation requires 'string()'`);
 			else {
 				this.#coerce(left, this.#host.program.system.doubleType);
 				this.#coerce(right, left.type);

test/error/OpAddAssignStringPtr.fu

@@ -0,0 +1,9 @@
+public class Test
+{
+	public static bool Run()
+	{
+		string s = "";
+		s += "foo"; //ERROR: 's' is 'string', concatenation requires 'string()'
+		return true;
+	}
+}