analyzer: a .u (declared or inferred) function can't return a borrowed parameter or a value derived from one; fixtures

2 files changed, 9 insertions, 1 deletions

diff --git a/src/compiler/analyzer.zc b/src/compiler/analyzer.zc
index 839c1ef..55a96bb 100644
--- a/src/compiler/analyzer.zc
+++ b/src/compiler/analyzer.zc
@@ -1644,6 +1644,7 @@ int assigncheck(int k, int i, int rc) {
 // the function has no .u (its callers compiled after this point see it). an m
 // object is lent, not given, so returning it is an error, and so is a plain word
 // from an owned function that was called before its body.
+int ret_borrowed;
 int retcheck(int c) {
  int i;
  if (isptr(xty) || xty == tword) {
@@ -1670,9 +1671,16 @@ int retcheck(int c) {
 	if ((isptr(xty) || isagg(xty)) && ((xaf && xaf[2] == 4) || (xbor >= 0 && lafact[xbor] && lafact[xbor][2] == 4))) { cerr("an array literal cannot escape its function through return"); return 0; }
 	i = -1;
 	if (c >= C_VAR && tracked(c - C_VAR)) i = c - C_VAR;
+	// a value that comes from a parameter the caller still owns (not a .d one,
+	// which is tracked) is a borrow: it can't become the caller's owned result.
+	if (c != C_FRESH && i < 0 && isptr(xty) && effect_mask(c)) {
+		if (fret) { cerr("a borrowed parameter cannot be returned as an owned result"); return 0; }
+		ret_borrowed = 1;
+	}
 	if (i >= 0 && lstate[lroot(i)] == S_DEAD) { cerrn(i, "used after it was given away"); return 0; }
 	if (i >= 0 && (lstate[i] == S_D || (lstate[i] == S_M && lown[i])) && (fret || finfer)) lstate[i] = S_U;	// returning an owned object (a d object, or a local that owns one) gives it up, as u*x does
 	if (!fret && finfer && (c == C_FRESH || (i >= 0 && lstate[i] == S_U))) {
+		if (ret_borrowed) cerr("this function also returns a borrowed parameter, so its result can't be owned");
 		if (sflag[finsym] & F_EARLY) serrn(sname[finsym], "is called before its body, so its owned return can't be known there; declare the return type with .u");
 		ssig[finsym] = ssig[finsym] | 1;	// the return is owned: callers read it from the signature
 		sflag[finsym] = sflag[finsym] | F_RETOWN;
diff --git a/src/compiler/decl.zc b/src/compiler/decl.zc
index 6bc8245..e2c3613 100644
--- a/src/compiler/decl.zc
+++ b/src/compiler/decl.zc
@@ -435,7 +435,7 @@ int fndef(int chk, int rty, int ret, int n) {
 	fixall(sfix[s], here); sfix[s] = 0;
 	fn_begin(chk, ret);
 	lt = lt_begin(sval[s], sname[s], lfl); xc_fn = sname[s];
-	finsym = s; finfer = !ret;
+	finsym = s; finfer = !ret; ret_borrowed = 0;
  effect_current = effect_record(s); rt.fill_w(effect_current, 0, 50); effect_current[0] = 1; if (!chk || stubgen) effect_current[0] = 2; effect_current[1] = np;
  i = 0; while (i < np) { if (pt[i] == 2) { effect_current[2 + i] = EF_CONSUME; effect_current[26 + i] = EF_CONSUME; } i++; }
 	if (typed && explicit_rty) fty = rty;