Handle -Wtruncation for link-time jr values

Fixes #2028

src/link/patch.cpp

@@ -585,12 +585,15 @@ static void applyFilePatches(Section &section, Section &dataSection) {
 				rpnErrorAt(patch, "PC has no value outside of a section");
 				dataSection.data[offset] = 0;
 			} else {
+				// A `jr` is *encoded* in ROM as a 1-byte (8-bit) offset, so here `typeSize == 8`,
+				// but the object's *value* size is a 16-bit absolute address, so we pass 16 here.
+				checkPatchSize(patch, value, 16);
 				// Offset is relative to the byte *after* the operand
 				// PC as operand to `jr` is lower than reference PC by 2
 				uint16_t address = patch.pcSection->org + patch.pcOffset + 2;
-				// The truncation of `value - address` is intentional, since
-				// a low ROM0 address may `jr` to a high HRAM one.
-				int16_t jumpOffset = value - address;
+				// The 16-bit truncation of `value - address` is intentional, since
+				// a low ROM0 address may `jr` backwards to a high HRAM one.
+				int16_t jumpOffset = static_cast<int16_t>(value - address);
 
 				if (jumpOffset < -128 || jumpOffset > 127) {
 					firstErrorAt(

test/link/jr-truncation/a.asm

@@ -0,0 +1,2 @@
+SECTION "jr", ROM0
+	jr CONSTANT

test/link/jr-truncation/b.asm

@@ -0,0 +1 @@
+EXPORT DEF CONSTANT EQU $1234_0078

test/link/jr-truncation/out.err

@@ -0,0 +1,2 @@
+warning: Value $12340078 is not 16-bit [-Wtruncation]
+    at jr-truncation/a.asm(2)

test/link/jr-truncation/ref.out.sym

@@ -0,0 +1,2 @@
+; File generated by rgblink
+12340078 CONSTANT

test/link/test.sh

@@ -201,6 +201,17 @@ tryCmpRom "$test"/ref.out.bin
 tryDiff "$test"/ref.out.sym "$outtemp2"
 evaluateTest
 
+test="jr-truncation"
+startTest
+"$RGBASM" -o "$otemp" "$test"/a.asm
+"$RGBASM" -o "$outtemp" "$test"/b.asm
+continueTest
+rgblinkQuiet -o "$gbtemp" -n "$outtemp2" "$otemp" "$outtemp" 2>"$outtemp3"
+tryDiff "$test"/out.err "$outtemp3"
+tryCmpRom "$test"/ref.out.bin
+tryDiff "$test"/ref.out.sym "$outtemp2"
+evaluateTest
+
 test="high-low"
 startTest
 "$RGBASM" -o "$otemp" "$test"/a.asm