fix issue 17933 - Warn in case of implicit truncating float conversion

src/ddmd/expression.d

@@ -6016,12 +6016,11 @@ extern (C++) abstract class BinExp : Expression
 
         // T opAssign floating yields a floating. Prevent truncating conversions (float to int).
         // See issue 3841.
-        // Should we also prevent double to float (type.isfloating() && type.size() < t2.size()) ?
         if (op == TOKaddass || op == TOKminass ||
             op == TOKmulass || op == TOKdivass || op == TOKmodass ||
             op == TOKpowass)
         {
-            if ((type.isintegral() && t2.isfloating()))
+            if ((type.isintegral() && t2.isfloating()) || (type.isfloating() && type.size() < t2.size()))
             {
                 warning("%s %s %s is performing truncating conversion", type.toChars(), Token.toChars(op), t2.toChars());
             }

src/ddmd/expressionsem.d

@@ -5396,6 +5396,7 @@ private extern (C++) final class ExpressionSemanticVisitor : Visitor
             assert(exp.e1.type);
         }
         Type t1 = exp.e1.type.toBasetype();
+        Type oldt2;
 
         /* Run this.e2 semantic.
          * Different from other binary expressions, the analysis of e2
@@ -5414,7 +5415,10 @@ private extern (C++) final class ExpressionSemanticVisitor : Visitor
             }
             if (e2x.checkValue())
                 return setError();
+
             exp.e2 = e2x;
+            if (e2x.type)
+                oldt2 = e2x.type;
         }
 
         /* Rewrite tuple assignment as a tuple of assignments.
@@ -6144,6 +6148,13 @@ private extern (C++) final class ExpressionSemanticVisitor : Visitor
         exp.e2 = e2x;
         t2 = exp.e2.type.toBasetype();
 
+        if (oldt2 && oldt2.isfloating() && exp.e1.type.isfloating() &&
+            exp.e1.type.size() < oldt2.size())
+        {
+            exp.warning("%s = %s is performing truncating conversion",
+                exp.e1.type.toChars(), oldt2.toChars());
+        }
+
         /* Look for array operations
          */
         if ((t2.ty == Tarray || t2.ty == Tsarray) && isArrayOpValid(exp.e2))

src/ddmd/statementsem.d

@@ -2928,6 +2928,13 @@ else
                  */
             }
 
+            if (rs && rs.exp && tret && tret.isfloating() && rs.exp.type.isfloating() &&
+                tret.size() < rs.exp.type.size())
+            {
+                rs.warning("return %s from %s function is performing truncating conversion",
+                    rs.exp.type.toChars(), tret.toChars());
+            }
+
             // handle NRVO
             if (fd.nrvo_can && rs.exp.op == TOKvar)
             {

test/fail_compilation/warn17933.d

@@ -0,0 +1,24 @@
+// REQUIRED_ARGS: -w
+
+/*
+TEST_OUTPUT:
+---
+fail_compilation/warn17933.d(14): Warning: float = double is performing truncating conversion
+fail_compilation/warn17933.d(15): Warning: float += double is performing truncating conversion
+fail_compilation/warn17933.d(22): Warning: return double from float function is performing truncating conversion
+---
+*/
+
+void test17933()
+{
+    float f;
+    double d;
+    f = d;
+    f += 0.1;
+}
+
+float testReturn(float f)
+{
+    return f * 0.5;
+}
+