Fix #20888 - Compiler spits out implicit conversion technobabble when a return statement doesn't match the return type of a function

compiler/src/dmd/semantic3.d

@@ -933,7 +933,37 @@
                             // if a copy constructor is present, the return type conversion will be handled by it
                             const hasCopyCtor = exp.type.ty == Tstruct && (cast(TypeStruct)exp.type).sym.hasCopyCtor;
                             if (!hasCopyCtor || !exp.isLvalue())
-                                exp = exp.implicitCastTo(sc2, tret);
+                            {
+                                const errors = global.startGagging();
+                                auto implicitlyCastedExp = exp.implicitCastTo(sc2, tret);
+                                global.endGagging(errors);
+
+                                // <https://github.com/dlang/dmd/issues/20888>
+                                if (implicitlyCastedExp.isErrorExp())
+                                {
+                                    auto types = toAutoQualChars(exp.type, tret);
+                                    error(
+                                        exp.loc,
+                                        "return value `%s` of type `%s` does not match return type `%s`"
+                                        ~ ", and cannot be implicitly converted",
+                                        exp.toErrMsg(),
+                                        types[0],
+                                        types[1],
+                                    );
+
+                                    if (const func = exp.type.isFunction_Delegate_PtrToFunction())
+                                        if (func.next.equals(tret))
+                                            errorSupplemental(
+                                                exp.loc,
+                                                "Did you intend to call the %s?",
+                                                (exp.type.isPtrToFunction())
+                                                    ? "function pointer"
+                                                    : exp.type.kind
+                                            );
+                                }
+
+                                exp = implicitlyCastedExp;
+                            }
 
                             exp = exp.optimize(WANTvalue);
 

compiler/test/fail_compilation/diag20888.d

@@ -0,0 +1,71 @@
+/*
+TEST_OUTPUT:
+---
+fail_compilation/diag20888.d(24): Error: return value `callback` of type `int function()` does not match return type `int`, and cannot be implicitly converted
+fail_compilation/diag20888.d(24):        Did you intend to call the function pointer?
+fail_compilation/diag20888.d(29): Error: return value `s` of type `string` does not match return type `int`, and cannot be implicitly converted
+fail_compilation/diag20888.d(34): Error: return value `callback` of type `int delegate()` does not match return type `int`, and cannot be implicitly converted
+fail_compilation/diag20888.d(34):        Did you intend to call the delegate?
+fail_compilation/diag20888.d(39): Error: return value `callback` of type `int delegate()` does not match return type `int`, and cannot be implicitly converted
+fail_compilation/diag20888.d(39):        Did you intend to call the delegate?
+fail_compilation/diag20888.d(44): Error: return value `callback` of type `int delegate()*` does not match return type `int`, and cannot be implicitly converted
+fail_compilation/diag20888.d(49): Error: return value `callback` of type `int delegate()` does not match return type `string`, and cannot be implicitly converted
+fail_compilation/diag20888.d(54): Error: return value `() => 3755` of type `int function() pure nothrow @nogc @safe` does not match return type `int`, and cannot be implicitly converted
+fail_compilation/diag20888.d(54):        Did you intend to call the function pointer?
+fail_compilation/diag20888.d(59): Error: `return` expression expected
+fail_compilation/diag20888.d(64): Error: cannot return non-void from `void` function
+fail_compilation/diag20888.d(70): Error: return value `() => i` of type `int delegate() pure nothrow @nogc @safe` does not match return type `int`, and cannot be implicitly converted
+fail_compilation/diag20888.d(70):        Did you intend to call the delegate?
+---
+*/
+
+int alpha(int function() callback)
+{
+	return callback;
+}
+
+int beta(string s)
+{
+	return s;
+}
+
+int gamma(int delegate() callback)
+{
+	return callback;
+}
+
+int delta(int delegate() callback)
+{
+	return callback;
+}
+
+int epsilon(int delegate()* callback)
+{
+	return callback; // no supplemental yet
+}
+
+string zeta(int delegate() callback)
+{
+	return callback;
+}
+
+int eta()
+{
+	return () => 0xEAB;
+}
+
+int theta()
+{
+	return;
+}
+
+void iota()
+{
+	return 0xEAB;
+}
+
+int kappa()
+{
+	int i = 0xEAB;
+	return () { return i; };
+}

compiler/test/fail_compilation/fail100.d

@@ -1,7 +1,7 @@
 /*
 TEST_OUTPUT:
 ---
-fail_compilation/fail100.d(24): Error: cannot implicitly convert expression `f` of type `Class[]` to `I[]`
+fail_compilation/fail100.d(24): Error: return value `f` of type `Class[]` does not match return type `I[]`, and cannot be implicitly converted
 ---
 */
 

compiler/test/fail_compilation/fail13498.d

@@ -1,7 +1,7 @@
 /*
 TEST_OUTPUT:
 ---
-fail_compilation/fail13498.d(11): Error: cannot implicitly convert expression `"foo"` of type `string` to `int`
+fail_compilation/fail13498.d(11): Error: return value `"foo"` of type `string` does not match return type `int`, and cannot be implicitly converted
 fail_compilation/fail13498.d(16): Error: template instance `fail13498.foo!()` error instantiating
 ---
 */

compiler/test/fail_compilation/fail20073.d

@@ -3,7 +3,7 @@
 TEST_OUTPUT:
 ---
 fail_compilation/fail20073.d(20): Error: cannot implicitly convert expression `s` of type `S` to `string`
-fail_compilation/fail20073.d(21): Error: cannot implicitly convert expression `s` of type `S` to `string`
+fail_compilation/fail20073.d(21): Error: return value `s` of type `S` does not match return type `string`, and cannot be implicitly converted
 ---
 */
 

compiler/test/fail_compilation/fail20376.d

@@ -3,7 +3,7 @@
 /*
 TEST_OUTPUT:
 ---
-fail_compilation/fail20376.d(17): Error: cannot implicitly convert expression `Foo()` of type `Foo` to `ubyte`
+fail_compilation/fail20376.d(17): Error: return value `Foo()` of type `Foo` does not match return type `ubyte`, and cannot be implicitly converted
 ---
 */
 

compiler/test/fail_compilation/noreturn2.d

@@ -55,7 +55,7 @@ NR returns()
 /+
 TEST_OUTPUT:
 ---
-fail_compilation/noreturn2.d(64): Error: cannot implicitly convert expression `1` of type `int` to `noreturn`
+fail_compilation/noreturn2.d(64): Error: return value `1` of type `int` does not match return type `noreturn`, and cannot be implicitly converted
 ---
 +/
 

compiler/test/fail_compilation/shared.d

@@ -96,7 +96,7 @@ fail_compilation/shared.d(2216):        return value `getSharedObject()` is not
 fail_compilation/shared.d(2222): Error: direct access to shared `a` is not allowed, see `core.atomic`
 fail_compilation/shared.d(2220): Error: function `shared.test_inference_4` function returns `shared` but cannot be inferred `ref`
 fail_compilation/shared.d(2222):        cannot implicitly convert `a` of type `shared(const(Object))` to `object.Object`
-fail_compilation/shared.d(2222): Error: cannot implicitly convert expression `a` of type `shared(const(Object))` to `object.Object`
+fail_compilation/shared.d(2222): Error: return value `a` of type `shared(const(Object))` does not match return type `object.Object`, and cannot be implicitly converted
 ---
  */
 

compiler/test/fail_compilation/test14538.d

@@ -2,7 +2,7 @@
 /*
 TEST_OUTPUT:
 ---
-fail_compilation/test14538.d(18): Error: cannot implicitly convert expression `x ? cast(uint)this.fCells[x].code : 32u` of type `uint` to `Cell`
+fail_compilation/test14538.d(18): Error: return value `x ? cast(uint)this.fCells[x].code : 32u` of type `uint` does not match return type `Cell`, and cannot be implicitly converted
 ---
 */
 

compiler/test/fail_compilation/testInference.d

@@ -1,7 +1,7 @@
 /*
 TEST_OUTPUT:
 ---
-fail_compilation/testInference.d(24): Error: cannot implicitly convert expression `this.a` of type `inout(A8998)` to `immutable(A8998)`
+fail_compilation/testInference.d(24): Error: return value `this.a` of type `inout(A8998)` does not match return type `immutable(A8998)`, and cannot be implicitly converted
 ---
 */
 
@@ -28,10 +28,10 @@ class C8998
 /*
 TEST_OUTPUT:
 ---
-fail_compilation/testInference.d(39): Error: cannot implicitly convert expression `s` of type `const(char[])` to `string`
-fail_compilation/testInference.d(44): Error: cannot implicitly convert expression `a` of type `int[]` to `immutable(int[])`
-fail_compilation/testInference.d(49): Error: cannot implicitly convert expression `a` of type `int[]` to `immutable(int[])`
-fail_compilation/testInference.d(54): Error: cannot implicitly convert expression `a` of type `int[]` to `immutable(int[])`
+fail_compilation/testInference.d(39): Error: return value `s` of type `const(char[])` does not match return type `string`, and cannot be implicitly converted
+fail_compilation/testInference.d(44): Error: return value `a` of type `int[]` does not match return type `immutable(int[])`, and cannot be implicitly converted
+fail_compilation/testInference.d(49): Error: return value `a` of type `int[]` does not match return type `immutable(int[])`, and cannot be implicitly converted
+fail_compilation/testInference.d(54): Error: return value `a` of type `int[]` does not match return type `immutable(int[])`, and cannot be implicitly converted
 ---
 */
 string foo(in char[] s) pure
@@ -58,18 +58,18 @@ immutable(int[]) x3(immutable(int[]) org) /*pure*/
 /*
 TEST_OUTPUT:
 ---
-fail_compilation/testInference.d(94): Error: cannot implicitly convert expression `c` of type `testInference.C1` to `immutable(C1)`
-fail_compilation/testInference.d(95): Error: cannot implicitly convert expression `c` of type `testInference.C1` to `immutable(C1)`
-fail_compilation/testInference.d(96): Error: cannot implicitly convert expression `c` of type `testInference.C3` to `immutable(C3)`
-fail_compilation/testInference.d(97): Error: cannot implicitly convert expression `c` of type `testInference.C3` to `immutable(C3)`
+fail_compilation/testInference.d(94): Error: return value `c` of type `testInference.C1` does not match return type `immutable(C1)`, and cannot be implicitly converted
+fail_compilation/testInference.d(95): Error: return value `c` of type `testInference.C1` does not match return type `immutable(C1)`, and cannot be implicitly converted
+fail_compilation/testInference.d(96): Error: return value `c` of type `testInference.C3` does not match return type `immutable(C3)`, and cannot be implicitly converted
+fail_compilation/testInference.d(97): Error: return value `c` of type `testInference.C3` does not match return type `immutable(C3)`, and cannot be implicitly converted
 fail_compilation/testInference.d(100): Error: undefined identifier `X1`, did you mean function `x1`?
-fail_compilation/testInference.d(106): Error: cannot implicitly convert expression `s` of type `S1` to `immutable(S1)`
-fail_compilation/testInference.d(109): Error: cannot implicitly convert expression `a` of type `int*[]` to `immutable(int*[])`
-fail_compilation/testInference.d(110): Error: cannot implicitly convert expression `a` of type `const(int)*[]` to `immutable(int*[])`
-fail_compilation/testInference.d(114): Error: cannot implicitly convert expression `s` of type `S2` to `immutable(S2)`
-fail_compilation/testInference.d(115): Error: cannot implicitly convert expression `s` of type `S2` to `immutable(S2)`
-fail_compilation/testInference.d(116): Error: cannot implicitly convert expression `s` of type `S2` to `immutable(S2)`
-fail_compilation/testInference.d(118): Error: cannot implicitly convert expression `a` of type `const(int)*[]` to `immutable(int*[])`
+fail_compilation/testInference.d(106): Error: return value `s` of type `S1` does not match return type `immutable(S1)`, and cannot be implicitly converted
+fail_compilation/testInference.d(109): Error: return value `a` of type `int*[]` does not match return type `immutable(int*[])`, and cannot be implicitly converted
+fail_compilation/testInference.d(110): Error: return value `a` of type `const(int)*[]` does not match return type `immutable(int*[])`, and cannot be implicitly converted
+fail_compilation/testInference.d(114): Error: return value `s` of type `S2` does not match return type `immutable(S2)`, and cannot be implicitly converted
+fail_compilation/testInference.d(115): Error: return value `s` of type `S2` does not match return type `immutable(S2)`, and cannot be implicitly converted
+fail_compilation/testInference.d(116): Error: return value `s` of type `S2` does not match return type `immutable(S2)`, and cannot be implicitly converted
+fail_compilation/testInference.d(118): Error: return value `a` of type `const(int)*[]` does not match return type `immutable(int*[])`, and cannot be implicitly converted
 ---
 */
 immutable(Object) get(inout int*) pure
@@ -122,7 +122,7 @@ immutable(int*[]) bar2c(              S2 prm) pure { immutable(int)*[] a; return
 /*
 TEST_OUTPUT:
 ---
-fail_compilation/testInference.d(134): Error: cannot implicitly convert expression `f10063(cast(inout(void*))p)` of type `inout(void)*` to `immutable(void)*`
+fail_compilation/testInference.d(134): Error: return value `f10063(cast(inout(void*))p)` of type `inout(void)*` does not match return type `immutable(void)*`, and cannot be implicitly converted
 ---
 */
 inout(void)* f10063(inout void* p) pure